CONTACT LENS PRACTICE [1, 3 ed.] 9780702066603

Table of contents :
CONTENTS
PART 1 Introduction
1 History 3
2 Anterior Eye 10
3 Visual Optics 28
PART 2 Soft Contact Lenses
4 Soft Lens Materials
5 Soft Lens Manufacture
6 Soft Lens Optics 68
7 Soft Lens Measurement 73
8 Soft Lens Design and Fitting 68
9 Soft Toric Lens Design and Fitting
10 Soft Lens Care Systems 103
PART 3 Rigid Contact Lenses
11 Rigid Lens Materials 115
12 Rigid Lens Manufacture 123
13 Rigid Lens Optics 130
14 Rigid Lens Measurement 136
15 Rigid Lens Design and Fitting 143
16 Rigid Toric Lens Design and Fitting 156
17 Rigid Lens Care Systems 163
PART 4 Lens Replacement Modalities
18 Daily Disposable Soft Lenses 167
19 Reusable Soft Lenses 175
20 Planned Replacement Rigid Lenses 187
PART 5 Special Lenses and Fitting Considerations
21 Scleral Lenses 195
22 Tinted Lenses 204
23 Presbyopia 214
24 Extended Wear 231
25 Sport 246
26 Keratoconus 251
27 High Ametropia 263
28 Babies and Children 268
29 Therapeutic Applications 275
30 Post-refractive Surgery 282
31 Post-keratoplasty 287
32 Orthokeratology 296
33 Myopia Control 306
34 Diabetes 314
PART 6 Patient Examination and Management
35 History Taking 323
36 Diagnostic Instruments 327
37 Preliminary Examination 346
38 Patient Education 356
39 Aftercare 364
40 Complications 385
41 Digital Imaging 410
42 Compliance 420
43 Practice Management 427
Appendices

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Co nt act Le ns Pract ice

To Suzanne, Zoe and Bruce

Co nt act Le ns Pract ice T h i r d Ed i t i o n

EDITED BY

Nat han Efro n AC, DSc (Manche ste r), PhD, BScO p tom (Me lb ourne ), FACO , FAAO , FIACLE, FCCLSA Profe ssor Eme ritus, School of O p tome try, Q ue e nsland Unive rsity of Te chnolog y, Brisb ane , Australia

EDINBURGH

LONDON

NEW YORK OXFORD

PHILADELPHIA ST LOUIS SYDNEY TORONTO iii

© 2018 Elsevier Ltd. All rights reserved. First published 2002 Reprinted 2005 Second edition 2010 Reprinted 2013 T ird edition 2018 T e right o Nathan E ron to be identif ed as editor o this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. No part o this publication may be reproduced or transmitted in any orm or by any means, electronic or mechanical, including photocopying, recording, or any in ormation storage and retrieval system, without permission in writing rom the publisher. Details on how to seek permission, urther in ormation about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be ound at our website: www.elsevier.com/permissions. T is book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this f eld are constantly changing. As new research and experience broaden our understanding, changes in research methods, pro essional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any in ormation, methods, compounds, or experiments described herein. In using such in ormation or methods they should be mind ul o their own sa ety and the sa ety o others, including parties or whom they have a pro essional responsibility. With respect to any drug or pharmaceutical products identif ed, readers are advised to check the most current in ormation provided (i) on procedures eatured or (ii) by the manu acturer o each product to be administered, to veri y the recommended dose or ormula, the method and duration o administration, and contraindications. It is the responsibility o practitioners, relying on their own experience and knowledge o their patients, to make diagnoses, to determine dosages and the best treatment or each individual patient, and to take all appropriate sa ety precautions. o the ullest extent o the law, neither the Publisher nor the authors, contributors, or editors, assume any liability or any injury and/or damage to persons or property as a matter o products liability, negligence or otherwise, or rom any use or operation o any methods, products, instructions, or ideas contained in the material herein. ISBN 978-0-7020-6660-3

Executive Content Strategist: Russell Gabbedy Content Development Specialist: Sam Crowe Project Manager: Dr. Atiyaah Muskaan Design: Matthew Limbert Illustration Manager: Karen Giacomucci Grading scale artwork: Terry R. Tarrant Marketing Manager: Melissa Fogarty

Printed in China Last digit is print number: 9 8 7 6 5 4 3 2 1

CO NTENTS

Contrib uting Authors

13 Rig id Le ns O p tics 130

vii

W NEIL CHARMAN

Pre ace to the Third Ed ition Trib ute s

ix

14 Rig id Le ns Me asure me nt 136

x

KLAUS EHRMANN

Acknowle d g e me nts

15 Rig id Le ns De sig n and Fitting

xi

143

GRAEME YO UNG

PART

16 Rig id Toric Le ns De sig n and Fitting

1 Int ro d uct io n

RICHARD G LINDSAY

1 History 3

17 Rig id Le ns Care Syste ms 163

NATHAN EFRO N

2 Ante rior Eye

156

PHILIP B MO RGAN

10

JO HN G LAWRENSO N

PART

3 Visual O p tics 28

4 Le ns Re p lace me nt Mo d alit ie s

W NEIL CHARMAN

18 Daily Disp osab le So t Le nse s 167 PART

NATHAN EFRO N

2 So ft Co nt act Le nse s

19 Re usab le So t Le nse s 175

4 So t Le ns Mate rials 45

JO E TANNER | NATHAN EFRO N

CARO LE MALDO NADO -CO DINA

5 So t Le ns Manu acture

20 Planne d Re p lace me nt Rig id Le nse s 187 61

CRAIG A WO O DS

NATHAN EFRO N

6 So t Le ns O p tics 68

PART

W NEIL CHARMAN

7 So t Le ns Me asure me nt 73

NATHAN EFRO N

8 So t Le ns De sig n and Fitting

86

22 Tinte d Le nse s 204

GRAEME YO UNG

NATHAN EFRO N | SUZANNE E EFRO N

9 So t Toric Le ns De sig n and Fitting

95

RICHARD G LINDSAY

PHILIP B MO RGAN

24 Exte nd e d We ar 231 NO EL A BRENNAN | M-L CHANTAL CO LES

3 Rig id Co nt act Le nse s

11 Rig id Le ns Mate rials 115 NATHAN EFRO N

NATHAN EFRO N

23 Pre sb yop ia 214 JO HN MEYLER | DAVID RUSTO N

10 So t Le ns Care Syste ms 103

12 Rig id Le ns Manu acture

Co nsid e rat io ns

21 Scle ral Le nse s 195

KLAUS EHRMANN

PART

5 Sp e cial Le nse s and Fit t ing

25 Sp ort 246 NATHAN EFRO N

26 Ke ratoconus 251 LAURA E DO WNIE | RICHARD G LINDSAY

123

27 Hig h Ame trop ia 263 JO SEPH T BARR

v

vi

CO NTENTS

28 Bab ie s and Child re n 268 CINDY TRO MANS | HELEN WILSO N

29 The rap e utic Ap p lications 275 NATHAN EFRO N | SUZANNE E EFRO N

30 Post-re ractive Surg e ry 282 SUZANNE E EFRO N

BARRY A WEISSMAN

32 O rthoke ratolog y 296 PAUL GIFFO RD

33 Myop ia Control 306 PADMAJA SANKARIDURG | BRIEN A HO LDEN

34 Diab e te s 314

ADRIAN S BRUCE | MILTO N M HO M

42 Comp liance

420

NATHAN EFRO N

43 Practice Manag e me nt 427

Ap p e nd ice s A Contact Le ns De sig n and Sp e cif cations 438 B Contact Le ns Tole rance s 440 C Ve rte x Distance Corre ction 441 D Corne al Curvature – Corne al Powe r

CLARE O ’DO NNELL

Conve rsion

6 Pat ie nt Examinat io n and Manag e me nt

35 History Taking

410

NIZAR K HIRJI

31 Post-ke ratop lasty 287

PART

41 Dig ital Imag ing

323

443

E Exte nd e d Ke ratome te r Rang e Conve rsion 445 F So t Le ns Ave rag e Thickne ss 446 G So t Le ns O xyg e n Pe r ormance

447

JAMES S W WO LFFSO HN

36 Diag nostic Instrume nts 327 LYNDO N W JO NES | SRUTHI SRINIVASAN | ALISO N NG | MARC SCHULZE

37 Pre liminary Examination 346 ADRIAN S BRUCE

38 Patie nt Ed ucation 356 SARAH L MO RGAN

39 A te rcare

364

LO RETTA B SZCZO TKA-FLYNN | NATHAN EFRO N

40 Comp lications 385 NATHAN EFRO N

H Constant Ed g e Cle arance Rig id Le ns De sig ns

449

I So t Toric Le ns Misalig nme nt De monstrator 450

J Dry-e ye Q ue stionnaire

451

K E ron Grad ing Scale s or Contact Le ns Comp lications

453

L Scle ral Le ns Fit Scale s 456

Ind e x

459

CO NTRIBUTING AUTHO RS

J o se p h T Barr, O D, MS, FAAO

Emeritus Pro essor, College o Optometry, T e Ohio State University, Columbus, Ohio, USA 27 High Ametropia

No e l A Bre nnan, MScO p t o m, PhD, FAAO , FCLSA

Clinical Research Fellow and Global Plat orm Lead, Myopia Control, Johnson & Johnson Vision Care Inc., Jacksonville, Florida, USA 24 Extended Wear

Ad rian S Bruce , BScO p t o m, PhD, FAAO , FVCO

Lead Optometrist, Australian College o Optometry, Melbourne, Victoria, Australia; Senior Fellow, Department o Optometry and Vision Sciences, University o Melbourne, Parkville, Victoria, Australia 37 Preliminary Examination 41 Digital Imaging

M-L Chant al Co le s, BS, O D

Optometrist, Johnson & Johnson Vision Care Inc., Jacksonville, Florida, USA 24 Extended Wear

W Ne il Charman, BSc, PhD, DSc, FO p t So cAm, FCO p t o m(Ho n)

Emeritus Pro essor, T e University o Manchester, Manchester, UK 3 Visual Optics 6 Sof Lens Optics 13 Rigid Lens Optics

Laura E Do w nie , PhD, BO p t o m, PGCe rt O cThe r, FACO , FAAO , Dip Mus(Prac), AMusA Lecturer and NHMRC ranslating Research Into Practice ( RIP) Fellow, Department o Optometry and Vision Sciences, T e University o Melbourne, Parkville, Victoria, Australia 26 Keratoconus

Nat han Efro n, AC, DSc, PhD, BScO p t o m, FACO , FAAO , FIACLE, FCCLSA Pro essor Emeritus, School o Optometry, Queensland University o echnology, Brisbane, Queensland, Australia 1 History 5 Sof Lens Manu acture 11 Rigid Lens Materials 12 Rigid Lens Manu acture 18 Daily Disposable Sof Lenses 19 Reusable Sof Lenses 21 Scleral Lenses 22 inted Lenses 25 Sport 29 T erapeutic Applications 39 Af ercare 40 Complications 42 Compliance

Suzanne E Efro n, BSc(Ho ns), MPhil, PGCe rt O cThe r

Locum Optometrist, Broadbeach, Queensland, Australia 22 inted Lenses 29 T erapeutic Applications 30 Post-re ractive Surgery

Klaus Ehrmann

Director – echnology, Brien Holden Vision Institute, University o New South Wales, Sydney, Australia 7 Sof Lens Measurement 14 Rigid Lens Measurement

Paul Giffo rd , PhD, MSc, BSc(Ho ns), MCO p t o m, FBCLA, FIACLE, FAAO

Private Practice, Brisbane, Queensland, and Adjunct Senior Lecturer, University o New South Wales, Sydney, Australia 32 Orthokeratology

Nizar K Hirji, BSc, PhD, MBA, FCO p t o m, FAAO , FIMg t

Optometrist and Principal Consultant, Hirji Associates, Birmingham, UK; Visiting Pro essor o Optometry, University o Manchester, Manchester, UK; Visiting Pro essor o Optometry, City University, London, UK 43 Practice Management

Brie n A Ho ld e n, O AM, PhD, DSc(Ho n), BAp p Sc, LO Sc (d e ce ase d )

Founding Chie Executive O cer, Brien Holden Vision Institute, University o New South Wales, Sydney, Australia 33 Myopia Control

Milt o n M Ho m, O D, FAAO FACAAI(Sc) Private Practice, Azusa, Cali ornia, USA 41 Digital Imaging

Lynd o n W J o ne s, PhD, FCO p t o m, Dip CLP, Dip O rt h, FAAO , FIACLE, FBCLA

University Research Chair, Pro essor, School o Optometry and Vision Science, and Director, Centre or Contact Lens Research, University o Waterloo, Waterloo, Ontario, Canada 36 Diagnostic Instruments

J o hn G Law re nso n, BSc, PhD, MCO p t o m

Pro essor o Clinical Visual Science, City, University o London, London, UK 2 Anterior Eye

Richard G Lind say, BScO p t o m, MBA, FAAO , FCLSA, FVCO Private Practice, East Melbourne, Victoria, Australia 9 Sof oric Lens Design and Fitting 16 Rigid oric Lens Design and Fitting 26 Keratoconus

vii

viii

CO NTRIBUTING AUTHO RS

Caro le Mald o nad o -Co d ina, BSc(Ho ns), MSc, PhD, MCO p t o m, FAAO , FBCLA

Senior Lecturer in Optometry, T e University o Manchester, Manchester, UK 4 Sof Lens Materials

J o hn Me yle r, BSc(Ho ns), FCO p t o m, Dip CLP

Lo re t t a B Szczo t ka-Flynn, O D, PhD, FAAO

Pro essor, Department o Ophthalmology and Visual Science, Case Western Reserve University; Director, Contact Lens Service, University Hospitals Case Medical Center, Cleveland, Ohio, USA 39 Af ercare

Senior Director, Global Pro essional Af airs, Johnson & Johnson Vision Care Companies, Wokingham, Berkshire, UK 23 Presbyopia

J o e Tanne r, BO p t o m

Philip B Mo rg an, BSc(Ho ns), PhD, MCO p t o m, FAAO , FBCLA

Cind y Tro mans, BSc(Ho ns), PhD, MCO p t o m, Dip (Tp )IP, FEAO O

Pro essor o Optometry and Director, Eurolens Research, T e University o Manchester, Manchester, UK 10 Sof Lens Care Systems 17 Rigid Lens Care Systems

Consultant Optometrist, Manchester Royal Eye Hospital; Honorary Clinical Lecturer, Department o Ophthalmology, T e University o Manchester, Manchester, UK 28 Babies and Children

Sarah L Mo rg an, BSc(Ho ns), MPhil, MCO p t o m, FAAO , FBCLA

Barry A We issman, O D, PhD, FAAO

Staf Development Consultant, Manchester, UK; Vision Sciences Fellow in Optometry, T e University o Manchester, Manchester, UK 38 Patient Education

Aliso n Ng , PhD, MCO p t o m

Post Doctoral Fellow, Centre or Contact Lens Research, University o Waterloo, Waterloo, Ontario, Canada 36 Diagnostic Instruments

Clare O ’Do nne ll, BSc(Ho ns), MBA, PhD, MCO p t o m, FAAO , FBCLA

Head o Eye Sciences, Optegra Manchester Eye Hospital, Didsbury; Reader, Aston University, Birmingham, UK 34 Diabetes

David Rust o n, BSc, FCO p t o m, Dip CLP, FAAO , FIACLE Director, Global Pro essional Af airs, Johnson & Johnson Vision Care Companies, Wokingham, Berkshire, UK 23 Presbyopia

Pad maja Sankarid urg , BO p t o m, MIP, PhD

Associate Pro essor, Program Leader – Myopia, Manager, Intellectual Property, Brien Holden Vision Institute, University o New South Wales, Sydney, Australia 33 Myopia Control

Marc Schulze , PhD, Dip lIng (AO ), FAAO

Clinical Scientist, Centre or Contact Lens Research, University o Waterloo, Waterloo, Ontario, Canada 36 Diagnostic Instruments

Srut hi Srinivasan, PhD, BS O p t o m, FAAO

Clinical Research Manager and Senior Clinical Scientist, Centre or Contact Lens Research, University o Waterloo, Waterloo, Ontario, Canada 36 Diagnostic Instruments

Pro essional Services Manager, CooperVision Australia and New Zealand 19 Reusable Sof Lenses

Pro essor o Optometry, Southern Cali ornia College o Optometry at Marshall B Ketchum University, Fullerton, Cali ornia, USA; Emeritus Pro essor o Ophthalmology, Stein Eye Institute, David Gef en School o Medicine at UCLA, Los Angeles Cali ornia, USA 31 Post-keratoplasty

He le n Wilso n, BSc(Ho ns), MCO p t o m, Dip Tp (IP), Dip O C, Dip Glauc Principal Optometrist, Manchester Royal Eye Hospital, Manchester, UK. 28 Babies and Children

J ame s S W Wo lffso hn, BSc(Ho ns), PGCe rt HE, PGDip Ad vClinO p t o m, MBA, PhD, FCO p t o m, FHEA, FSB, FAAO , FIACLE, FBCLA Pro essor and Deputy Executive Dean, School o Li e and Health Sciences, Aston University, Birmingham, UK 35 History aking

Craig A Wo o d s, BSc(Ho ns), PhD, MCO p t o m, Dip CLP, PGCe rt O cThe r, FAAO , FACO , FBCLA

Pro essor, Head o Clinical Partnerships, Deakin Optometry, School o Medicine, Deakin University, Geelong, Australia 20 Planned Replacement Rigid Lenses

Grae me Yo ung , BSc, MPhil, PhD, FCO p t o m, Dip CLP, FAAO

Director, Visioncare Research, Farnham, Surrey; Honorary Pro essor, School o Li e and Health Sciences, Aston University, Birmingham, UK 8 Sof Lens Design and Fitting 15 Rigid Lens Design and Fitting

PREFACE TO THE THIRD EDITIO N

T is book strives to achieve the ‘middle ground’ among contact lens textbooks. It is not intended to be a brie clinical manual o contact lens tting; nor is it intended to be a weighty tome with extensive research coverage. Like its predecessors, this third edition o Contact Lens Practice seeks to be a comprehensive, easily accessible book that provides in ormation o immediate relevance to contact lens practitioners, underpinned by wellounded evidence and expert clinical insight by the authors o the various chapters, each o whom is an expert in the area covered. T is new edition is not just a cosmetic make-over. T ere have been extensive revisions to most chapters, many o which have been written by authors who are new or this edition. T ere is also a new chapter on myopia control – an area o

considerable interest at present in view o the current myopia epidemic (especially in Asia), and the potential or tting contact lenses that can arrest myopia progression to a certain degree. T e chapter on daily disposable lenses has been updated and expanded, which is particularly important given that this modality now represents nearly one-third o contact lenses prescribed worldwide. I hope that students using this book nd it to be a valuable guide to their studies and acquisition o knowledge in the science and art o contact lens tting, and I trust that this work will be a valuable companion to practitioners in their ef orts to satis y the needs o those patients tted with contact lenses. Professor Nathan Efron AC

ix

TRIBUTES

Here we pay tribute to two contributors to Contact Lens Practice who have passed away since the second edition o this book was published.

Keith Edwards, who wrote the chapter on History Taking in the rst two editions o this book, lost a long- ought battle with cancer in 2014. Keith was an inspirational educator, clinician and researcher who had an impact internationally in the eld o contact lenses and intraocular lenses. Following his Optometry degree at City University, he worked in private practice and served as secretary o the London Re raction Hospital and examinations advisor at the College o Optometrists. He was an inaugural director o Optometric Educators Ltd and later worked or Madden and Layman, which was acquired by Bausch & Lomb in the late 1980s. He expanded his role rom UK Pro essional Services to Director o Global Clinical Development or Surgical at Bausch & Lomb, which took him to the US, where his nal job was as Vice-President o Clinical and Regulatory A airs at LENSAR.

x

Brien Holden, who co-authored the chapter in this book on Myopia Control, passed away suddenly in 2015. He was Chie Executive O cer o the Brien Holden Vision Institute and Proessor at the School o Optometry and Vision Science at the University o New South Wales, Australia. Pro essor Holden was a global leader in eye care and vision research, and an internationally renowned and awarded scientist and humanitarian. He was widely acknowledged as the most inf uential optometrist o our generation. His career was spent inspiring scientists and health-care pro essionals around the world with his dream o ‘vision or everyone, everywhere’. Pro essor Holden was the recipient o seven honorary doctorates rom universities around the world, and was awarded an Order o Australia Medal or his work in eye health and vision science.

ACKNO WLEDGEMENTS

I am grate ul to the contributing authors o this third edition o Contact Lens Practice. All have worked diligently to update their chapters, or write new chapters, to bring the latest clinically relevant in ormation to the ore. I continue to enjoy the strong support o the long-standing publisher o all o my books – Elsevier. In particular, I am grateul to Russell Gabbedy (Commissioning Editor) and Alexandra Mortimer (Development Editor) or their encouragement and support during the planning and production o this book. T anks also to Samuel Crowe, or assisting e ciently with various aspects o production. Editing a book o this size and scope is a substantial undertaking, and in this regard I wish to o er special thanks to my lovely wi e, Suzanne, who has served as a ‘virtual co-editor’ by way o

spending many long hours assisting me in assembling, editing, organizing and proo reading the contributed material. She has done a wonder ul job. I really could not have completed this task without her assistance. I also thank Suzanne or co-authoring Chapters 22 and 29 with me, and or revising and authoring Chapter 30. Let me also pay tribute to the photographers and illustrators, many o whom were not contributing authors o this book, or their extraordinary skills and insights in creating such antastic imagery. I also thank them or giving me permission to use this material in the book. I apologize i I have made any errors in attribution; please let me know i I have erred in this regard, and I shall correct this at the f rst reprinting opportunity.

xi

This pa ge inte ntiona lly le ft bla nk

PART

1

Int ro d uct io n

PART O UTLINE 1 History 3 Nathan E ron 2 Ante rior Eye 10 John G Lawre nson 3 Visual O p tics 28 W Ne il Charman

This pa ge inte ntiona lly le ft bla nk

1

Hist o ry NATHAN EFRO N

Int ro d uct io n We canno t co nt inue t he se b rilliant succe sse s in t he fut ure , unle ss we co nt inue t o le arn fro m t he p ast . Calvin Coolid g e , inaug ural US p re sid e ntial ad d re ss, 1923

Coolidge was re erring to the successes o a nation, but his sentiment could apply to any eld o endeavour, including contact lens practice. As we continue to ride on the crest o a huge wave o exciting developments in the 21st century, we would not wish to lose sight o the past. Hence the inclusion in this book o this brie historical overview. Outlined below in chronological order (allowing or some historical overlaps) is the development o contact lenses, rom the earliest theories to present-day technology. Each heading, which represents a major achievement, is annotated with a year that is considered to be especially signi cant to that development. T ese dates are based on various sources o in ormation, such as dates o patents, published papers and anecdotal reports. It is recognized, there ore, that some o the dates cited are open to debate, but they are nevertheless presented to provide a reasonable chronological perspective. 

snugly into the orbital rim (Young, 1801) (Figs. 1.3 and 1.4). A microscope eyepiece was tted into the base o the eyecup, thus orming a similar system to that used by Descartes. Young’s invention was somewhat more practical in that it could be held in place with a headband and blinking was possible; however, he did not intend this device to be used or the correction o re ractive errors. In a ootnote in his treatise on light in the 1845 edition o the Encyclopedia Metropolitana, Sir John Herschel suggested two possible methods o correcting ‘very bad cases o irregular cornea’: (1) ‘applying to the cornea a spherical capsule o glass

Early The o rie s (1508–1887) Although contact lenses were not tted until the late 19th century, a number o scholars had earlier given thought to the possibility o applying an optical device directly to the eyeball to correct vision. Virtually all o these suggestions were impractical. Many contact lens historians point to Leonardo da Vinci’s Codex o the Eye, Manual D, written in 1508, as having introduced the optical principle underlying the contact lens. Indeed, da Vinci described a method o directly altering corneal power – by immersing the eye in a bowl o water (Fig. 1.1). O course, a contact lens corrects vision by altering corneal power. However, da Vinci was primarily interested in learning the mechanisms o accommodation o the eye (Heitz and Enoch, 1987) and did not re er to a mechanism or device or correcting vision. In 1636, René Descartes described a glass uid- lled tube that was to be placed in direct contact with the cornea (Fig. 1.2). T e end o the tube was made o clear glass, the shape o which would determine the optical correction. O course, such a device is impractical as blinking is not possible; nevertheless, the principle o directly neutralizing corneal power used by Descartes is consistent with the principles underlying modern contact lens design (Enoch, 1956). As part o a series o experiments concerning the mechanisms o accommodation, T omas Young, in 1801, constructed a device that was essentially a uid- lled eyecup that tted

Fig . 1.1

Id e a o Le onard o d a Vinci to alte r corne al p owe r.

Fig . 1.2

Fluid -f lle d tub e d e scrib e d b y Re né De scarte s.

Fig . 1.3

Eye cup d e sig n o Thomas Young .

3

4

PART 1

Fig . 1.4

Thomas Young .

Int ro d uct io n

Fig . 1.6

Ad ol Gaston Eug e ne Fick.

Fig . 1.7

Eug è ne Kalt.

Fig . 1.5 ‘Animal je lly’ sand wiche d b e twe e n a ‘sp he rical cap sule o g lass’ (contact le ns) and corne a, as p rop ose d b y Sir Jo hn He rsche l.

lled with animal jelly’ (Fig. 1.5), or (2) ‘taking a mould o the cornea and impressing it on some transparent medium’ (Herschel, 1845). Although it seems that Herschel did not attempt to conduct such trials, his latter suggestion was ultimately adopted some 40 years later by a number o inventors, working independently and unbeknown to each other, who were all apparently unaware o the writings o Herschel. 

Glass Scle ral Le nse s (1888) T ere was a great deal o activity in contact lens research in the late 1880s, which has led to debate as to who should be given credit or being the rst to t a contact lens. Adol Gaston Eugene Fick (Fig. 1.6), a German ophthalmologist working in Zurich, appears to have been the rst to describe the process o abricating and tting contact lenses in 1888; speci cally, he described the tting o a ocal scleral contact shells rst on rabbits, then on himsel and nally on a small group o volunteer patients (E ron and Pearson, 1988). In their textbook dated 1910, Müller and Müller, who were manu acturers o ocular prostheses, described the tting in 1887 o a partially transparent protective glass shell to a patient re erred to them by Dr Edwin T eodor Sämisch (Müller and Müller, 1910). Pearson (2009) asserts that the tting was done by Albert C Müller-Uri. Fick’s work was published in the journal Archiv ür Augenheilkunde in March 1888, and must be accorded historical precedence over later anecdotal textbook accounts. French ophthalmologist Eugène Kalt (Fig. 1.7) tted two keratoconic patients with a ocal glass scleral shells and obtained

a signi cant improvement in vision. A report o this work, presented to the Paris Academy o Medicine on 20 March, 1888 by Kalt’s senior medical colleague, Pro essor Photinos Panas, acknowledges and there ore e ectively con rms that the work o Fick occurred earlier (Pearson, 1989). Credit or tting the rst powered contact lens must be given to August Müller (Fig. 1.8) (no relation to Müller and Müller, mentioned above), who conducted his work while he was a medical student at Kiel University in Germany (Pearson and E ron, 1989). In his inaugural dissertation presented to the Faculty o Medicine in 1889, Müller described the correction o his own high myopia with a powered scleral contact lens. Paradoxically, Müller subsequently lost interest in ophthalmology and went on to practise as an orthopaedic specialist.

1

Hist o ry

5

T e Rohm and Haas company introduced transparent plastic (polymethyl methacrylate: PMMA) into the USA in 1936, and in the same year Feinbloom (1936) described a scleral lens consisting o an opaque plastic haptic portion and a clear glass centre. Soon a er, scleral lenses were abricated entirely rom PMMA using lathing techniques. T e earliest report o the tting o PMMA lenses appears to have been made by T ier in 1939. T ese lenses were said to be ‘about hal the weight o ordinary glass, unbreakable and quicker to manu acture’. T ey did not provoke any irritation, but the optical zone needed to be repolished every 6 months (Pearson, 2015). A key rationale or using PMMA or the manu acture o contact lenses was that this material was considered to be biologically inert in the eye. T is view was ormed by military medical o cers who examined the eyes o pilots who su ered eye injuries during World War II as a result o ragments rom shattered cockpit windscreens (as would occur during aerial dog ghts) becoming permanently embedded in the eye. T ese eyes remained unreactive or years a er such accidents. Other advantages o PMMA included its light weight, break resistance and being easy to lathe and polish.  Fig . 1.8

Aug ust Mülle r. (Courte sy of Richard Pe arson.)

T e lenses worn by Müller were made by an optical engineer, Karl Otto Himmler (1841–1903), whose rm enjoyed, until the outbreak o World War II, an international reputation or the manu acture o microscopes and their accessories. Himmler must there ore be acknowledged as the rst manu acturer o optically ground contact lenses (Pearson, 2007). Little development occurred in the 50 years subsequent to these early clinical trials. Improvements in methods o scleral lens tting were described by clinicians such as Dallos, who emphasized the importance o designing the lens to acilitate tear ow beneath the lens (Dallos, 1936). Dallos also went on to develop techniques or taking impressions o the human eye and grinding the lenses rom these impressions. 

Plast ic Scle ral Le nse s (1936) Carl Zeiss o Jena, Germany applied or a patent that proposed the manu acture o contact lenses rom ‘cellon, celluloid or an organic substance with similar mechanical and optical properties’, which was eventually issued in 1923 (Pearson, 2015). Cellon is cellulose acetate and celluloid is cellulose nitrate plasticized with camphor; there ore, this is a re erence to a lens made o a plastic material. T is was also the rst mention o the manu acture o contact lenses by moulding. T e Zeiss patents envisaged that contact lenses made rom plastic materials would be less expensive, have some exibility that would improve the t, be ‘unbreakable’ and o er ocular protection (Pearson, 2015). It appears that in Germany there may have been some largely unsuccess ul attempts to t plastic lenses rom around 1930. It was reported in that year that Zeiss contact lenses moulded rom cellon and celluloid lacked the degree o polish achieved with glass lenses and were unstable owing to the in uences o humidity and temperature. More serious ndings were that they put a ‘tourniquet’ on the conjunctiva in the region o the limbus and caused extensive corneal erosion. T ese un avourable results were possibly due to the act that they were made with a single back scleral radius o 12 mm (Pearson, 2015).

Plast ic Co rne al Le nse s (1948) T e development o corneal lenses – or rigid lenses, as they are re erred to today – began as the result o an error in the laboratory o optical technician Kevin uohy. During the lathing o a PMMA scleral lens, its haptic and corneal portions separated. uohy became curious as to whether the corneal portion could be worn, so he polished the edge, placed it in his own eye and ound that the lens could be tolerated (Bra , 1983). Further trials were conducted, leading to the development o the rigid contact lens (rigid lenses were previously re erred to as ‘hard’ lenses i they were manu actured rom PMMA). uohy led a patent or his invention in February 1948. So began an era o popularization o the contact lens. T e spherical uohy lens design su ered rom two main drawbacks: considerable apical bearing, which caused central corneal abrasion and oedema, and excessive edge li , which made the lens easy to dislodge. It was soon realized that these problems could be overcome by altering the peripheral curvature o the posterior lens sur ace, heralding the development o multicurve and aspheric designs, which remain in widespread use today, albeit with superior gas-permeable materials (PMMA is now virtually obsolete). 

Silico ne Elast o me r Le nse s (1965) Silicone rubber orms a unique category amongst contact lens materials. It is a ‘so lens’ in terms o its physical behaviour and lenses are abricated rom this material in the orm o a so lens. Unlike all other so lens materials, silicone elastomer does not contain water and in this respect is analogous to a hard lens material. Silicone elastomer is highly permeable to oxygen and carbon dioxide and there ore provides minimal inter erence to corneal respiration; however, it is di cult to manu acture and its sur ace is hydrophobic and must be treated to allow com ortable wear. T e considerable di culties involved in enhancing sur ace wettability have limited the clinical application o this lens, and ew advances have been made since it was originally tted. T e precise date o silicone elastomer lenses becoming commercially available is unclear. T ere was some patent activity in the mid 1960s to early 1970s, and Mandell (1988) claims

6

PART 1

Int ro d uct io n

to have personally observed ten patients who were wearing such lenses in 1965, noting very poor clinical results. 

So ft Le nse s (1972) Possibly the greatest understatement that can be ound in the literature pertaining to contact lens development is the nal sentence o a paper entitled ‘Hydrophilic gels or biological use’, published in Nature on 9 January, 1960, by Wichterle and Lim (1960): ‘Promising results have also been obtained in experiments in other cases, or example, in manu acturing contact lenses, arteries, etc.’ Initial attempts by Otto Wichterle (Fig. 1.9) to produce so lenses abricated rom hydroxyethyl methacrylate (HEMA), and manu actured using cast moulding, met with limited success. Unable to attract support rom the Institute o Macromolecular Research in Czechoslovakia (now the Czech Republic) where he worked, and indeed discouraged by his superiors, Wichterle was orced to conduct urther secret experiments in his own home. Working with a children’s mechanical construction kit, Wichterle developed the spin-casting technique (Fig. 1.10) and

eventually managed to persuade his peers to conduct urther trials at the Institute. He claims to have produced ‘the rst suitable contact lenses’ in late 1961 (Wichterle, 1978), which presumably approximates to the rst occasion when a so lens was actually worn on a human eye. T e patent to develop so contact lenses commercially was subsequently acquired by Bausch & Lomb in the USA, who introduced so lenses into the world market in 1972. Lenses manu actured rom HEMA were an immediate market success, primarily by virtue o their superior com ort and enhanced biocompatibility. However, clinical experience and laboratory studies indicated that the poor physiological response o the anterior eye during wear o the early thick HEMA lenses could be enhanced by making so lenses more permeable to oxygen – speci cally by making them thinner and o a higher water content. Much o the research and development in contact lenses up to the present time has been concerned with the development o materials and lens designs that optimize biocompatibility, primarily by enhancing corneal oxygenation and minimizing absorption o proteins, lipids and other tear constituents (McMahon and Zadnik, 2000). 

Rig id Gas-p e rme ab le Le nse s (1974)

Fig . 1.9

O tto Wichte rle . (Courte sy of De b b ie Swe e ne y.)

In most respects, PMMA is considered to be an ideal contact lens material; however, its single drawback is its impermeability to gases that are exchanged at the corneal sur ace as part o aerobic metabolism. Speci cally, oxygen is prevented rom moving rom the atmosphere into the cornea, and carbon dioxide ef ux into the atmosphere is impeded. T is drawback has been the major driving orce in the development o rigid lens materials that are permeable to gases. One o the rst rigid gas-permeable materials to be tried was cellulose acetate butyrate, which a orded some oxygen permeability but was subject to warpage. In 1974, Norman Gaylord managed to incorporate silicone into the basic PMMA structure, heralding the introduction o a new amily o contact lens polymers known as silicone acrylates (Gaylord, 1974). Subsequently, other ingredients such as styrene and uorine have been incorporated into rigid materials in attempts to enhance material biocompatibility urther. 

Disp o sab le Le nse s (1988)

Fig . 1.10 The p rototyp e sp in-casting machine b uilt at home b y Wichte rle using his son’s toy Me ccano construction se t.

In the early days o so lens development, patients would typically use the same pair o lenses until the lenses became too uncom ortable to wear, caused severe eye reactions, or were damaged or lost. It became apparent that lens deposition and spoilation over time were major impediments to success ul long-term lens wear. Although regular lens replacement was an obvious solution to some o these problems, the high unit cost o lenses proved to be a signi cant disincentive. In the early 1980s, orward-thinking practitioners – notably Klas Nilsson o Gothenburg, Sweden – convinced patients o the bene ts o replacing lenses on a regular basis (6-monthly in Nilsson’s case) and began prescribing lenses in this way. A subsequent landmark scienti c publication co-authored by Nilsson – known as the ‘Gothenburg study’ (Holden et al., 1985) – unequivocally proved the bene ts o regular lens replacement. So was born the concept o regular lens replacement, albeit relatively expensive or the patient at the time.

1

I regular lens replacement were to become the norm, something had to be done about lens cost. A group o Danish clinicians and engineers, led by ophthalmologist Michael Bay, developed a moulding process so that low-cost, multiple individual lens packs could be produced (Mertz, 1997). T is product – known as ‘Danalens’ – was released into the Scandinavian market in 1984 and must be recognized as the rst truly disposable lens. However, the initial manu acturing process was crude and numerous problems with the lenses and packaging were reported (Benjamin et al., 1985; Bergmanson et al., 1987). T e pharmaceutical giant Johnson & Johnson, which had not previously been involved in the contact lens business, purchased the Danalens technology in 1984 and completely overhauled the lens polymer ormulation, packaging system and moulding technology (Mertz, 1997). T e result was the Acuvue lens, an inexpensive weekly-replacement extendedwear lens, which was released in the USA in June 1988, and worldwide shortly therea er. T e success o this lens elevated Johnson & Johnson to a leadership position in the contact lens market. All other major contact lens companies ollowed suit, and today the majority o so lenses prescribed worldwide (85%) are designed to be replaced monthly or more requently (Morgan et al., 2015). 

Daily Disp o sab le Le nse s (1994) T e ultimate requency with which lenses can be replaced is daily. A Scottish company, Award (which was acquired by Bausch & Lomb in 1996), developed a manu acturing technique whereby the male hal o the mould that ormed the lens became the lens packaging. T is technique urther reduced the unit cost o a lens, making daily disposability a viable proposition. T e ‘Premier’ daily disposable lens was launched in the UK in 1994. Johnson & Johnson released the ‘1-Day Acuvue’ daily disposable lens into western regions o the USA around the same time, leading to an ongoing dispute as to which company (Award or Johnson & Johnson) was the rst to release a daily disposable contact lens into the market (Meyler and Ruston, 2006). CIBA Vision entered the daily disposable lens market in 1997 with a product called ‘Dailies’. 

Silico ne Hyd ro g e l Le nse s (1998) T e allure o a so contact lens made rom a material with a phenomenally high oxygen per ormance never escaped the contact lens industry. T e development o such a lens would be critical to solving hypoxic lens-related problems, which severely limit the clinical utility o contact lenses, especially or extended wear. Silicone elastomers were the obvious answer, but, or reasons outlined above, success ul lenses could never be produced rom this material. Polymer scientists in the contact lens industry had long recognized that many o the problems associated with silicone elastomers or contact lens abrication could theoretically be overcome by creating a silicone–hydrogel hybrid. A er more than a decade o intensive research and development, two spherical-design silicone hydrogel lenses were introduced into the market in 1998: Focus Night & Day (CIBA Vision) and Purevision (Bausch & Lomb). T e introduction o these lenses is considered by many to be the most signi cant advance in contact lens material technology since the development o HEMA by Wichterle in the 1960s. Within a decade o these products entering the market, all major contact lens

Hist o ry

7

manu acturers had introduced silicone hydrogel lenses; this lens type is now available in toric and multi ocal designs and a range o replacement modalities, including daily disposable lenses. 

Myo p ia Co nt ro l Le nse s (2010) In 2010, CooperVision released into some Asian markets a daily disposable so lens that is designed to arrest the rate o progression o myopia. A variety o optical designs can be employed to achieve this so-called ‘anti-myopia’ e ect. T e CooperVision MiSight lens has a ‘dual- ocus’ design that contains a large central correction area surrounded by concentric zones o alternating distant and near powers. T e near power is intended as a ‘treatment’ zone to prevent myopic progression (see Chapter 33 or a detailed account o myopia control lenses). 

Co nt act Le ns ‘Flat Pack’ (2011) Japanese manu acturer Menicon introduced an ultra-thin orm o packaging – known as the ‘ at pack’ – or their ‘Magic’ brand o daily disposable contact lenses. As well as being highly e cient or storage and convenient or the user, this orm o packaging reduces lens contamination because the lens back sur ace is always presented to the patient upon opening the pack, which means that the person can pick up and insert the lens into the eye without touching and contaminating the posterior lens surace, which comes into contact with the eye (Nomachi et al., 2013). T e contact lens is essentially sandwiched within a 1 mm thick aluminium oil sleeve that is resistant to evaporation, thus preserving the small amount o uid trapped within the pack that moisturizes the lens. Fig. 1.11 presents a historical timeline o key developments in the contact lens eld rom the time contact (scleral) lenses were rst tted to human eyes in the late 1880s up to the present. 

The Fut ure So lenses are likely to dominate the uture contact lens market. Although rigid lenses are seldom tted today or purely cosmetic reasons, there are many clinical indications or rigid lenses, such as keratoconus, distorted corneas, irregular and / or high astigmatism, certain anterior eye pathologies and participation in extreme sports. Accordingly, specialized rigid lens ttings will continue to be an important aspect o contact lens practice, albeit at relatively low levels. T e recent renewed interest in scleral or mini-scleral lenses is unlikely to have a signi cant impact on the overall proportion o lenses prescribed owing to the specialist nature o tting such lenses. T e convenience and ocular health bene ts o daily disposable lenses are likely to see this modality o lens wear continue to increase in popularity. T is trend will be accelerated with improvements in methods and e ciency o lens mass production, which in turn will drive prices down and make these lenses more a ordable. O course, any increase in daily disposable lens usage will be matched by a commensurate decrease in the demand or, and use o , contact lens care solutions. Silicone hydrogels are set to continue as the main material type rom which lenses are abricated in view o their ability to obviate hypoxic complications o lens wear; however, the possibility o the arrival in the uture o an entirely new

8

Fig . 1.11

PART 1

Int ro d uct io n

Historical time line o contact le ns d e ve lop me nt. PMMA = p o lyme thyl me thacrylate ; HEMA = hyd roxye thyl me thacrylate .

category o lens material with even greater bene ts should not be discounted. Contact lenses are likely to be used increasingly or the correction o presbyopia; this trend may be uelled by the development o superior multi ocal lens designs and the increasing availability o such products as daily disposable lenses. Looking urther into the uture, contact lenses that switch power

electronically or through some other means may acilitate enhanced presbyopic correction. Extended wear is the ultimate modality in terms o patient convenience, but it is unlikely that this modality o lens wear will break through the ‘glass ceiling’ o a prescribing rate o around 10% o lenses tted in the oreseeable uture, in view o the ve times greater risk o microbial keratitis when sleeping in

1

all orms o contact lenses (Schein et al., 1989). Again, development or invention o an entirely new category o lens material with superior ocular biocompatibility or an ability to minimize microbial colonization would need to be developed be ore extended wear can capture an appreciably greater slice o the contact lens market. As better toric lens designs become available, especially in daily disposable modality, toric lenses tting is likely to increase steadily to represent approximately 45% o all so lenses prescribed, which is the level at which all astigmatism ≥ 0.75 D is being corrected. We may see a resurgence in tinted lens tting as the newly developed coloured silicone hydrogel lenses gain in popularity and similar products enter the market. Finally, current developments in innovative contact lens applications – such as lens sur ace modi cations to include channels and patterns or improving post-lens tear exchange (Weidemann

Hist o ry

9

and Lakkis, 2005; Lin et al., 2006), alternative anti-myopia designs (Sankaridurg et al., 2011), anti-in ective and anti-in ammatory lenses (Weisbarth et al., 2007; Zhu et al., 2008), drug delivery (Mohammadi et al., 2014), glucose monitoring and other orms o metabolic sensing (Farandos et al., 2015), intraocular pressure measurement (Chen et al., 2014), digital in ormation acquisition and display (e.g. a contact lens version o Google Glass [Google Inc., Mountain View, CA]) and liquid crystal diode optical switching (Milton et al., 2014) – may open up whole new markets or contact lenses and move at least part o the industry in new and interesting directions. Contact lens practitioners may need to acquire new knowledge and tting skills so that they can embrace any such innovative developments. Acce ss t he co mp le t e re fe re nce s list o nline at ht t p :/ / www.e xp e rt co nsult .co m.

REFERENCES Benjamin, W. J., Bergmanson, J. P. G., & Estrada, P. J. (1985). Disposable ‘eight-packs’. Int. Eyecare, 1, 494–499. Bergmanson, J. P. G., Soderberg, P. G., & Estrada, P. (1987). A comparison between the measured and the desirable quality o hydrogel extended wear contact-lenses. Acta Ophthalmol., 65, 417–423. Bra , S. M. (1983). T e Max Schapero Lecture: contact lens horizons. Am. J. Optom. Physiol. Opt., 60, 851–858. Chen, G. Z., Chan, I. S., Leung, L. K., et al. (2014). So wearable contact lens sensor or continuous intraocular pressure monitoring. Med. Eng. Phys., 36, 1134–1139. Dallos, J. (1936). Contact lenses, the ‘invisible spectacles’. Arch. Ophthalmol., 15, 617–623. E ron, N., & Pearson, R. M. (1988). Centenary celebration o Fick’s Eine Contactbrille. Arch. Ophthalmol., 106, 1370–1377. Enoch, J. M. (1956). Descartes’ contact lens. Am. J. Optom. Arch. Am. Acad. Optom., 33, 77–85. Farandos, N. M., Yetisen, A. K., Monteiro, M. J., et al. (2015). Contact lens sensors in ocular diagnostics. Adv. Healthc. Mater., 4(6), 792–810. http://dx.doi.org/10.1002/adhm.201400504. Feinbloom, W. (1936). A plastic contact lens. Trans. Am. Acad. Optom., 10, 37–44. Gaylord, N. G. (1974). Oxygen permeable contact lens composition methods and article o manu acture (to Polycon Lab Inc.). US Patent 3 808 178. Heitz, R. F., & Enoch, J. M. (1987). Leonardo da Vinci: an assessment on his discourses on image ormation in the eye. In A. Fiorentini, D. L. Guyton, & I. M. Siegel (Eds.), Advances in Diagnostic Visual Optics (pp. 19–26). New York: Springer-Verlag. Herschel, J. F. W. (1845). O the structure o the eye, and o vision. Vol. 4, Section XII, Light. In E. Smedley, H. J. Rose, & the late H. J. Rose (Eds.), Encyclopedia Metropolitana (pp. 341–586). London: B. Fellowes.

Holden, B. A., Sweeney, D. F., Vannas, A., et al. (1985). E ects o long-term extended contact lens wear on the human cornea. Invest. Ophthalmol. Vis. Sci., 26, 1489–1501. Lin, M. C., Soliman, G. N., Lim, V. A., et al. (2006). Scalloped channels enhance tear mixing under hydrogel contact lenses. Optom. Vis. Sci., 83, 874–878. Mandell, R. B. (1988). Historical development. Chapter 1, Section 1, Basic Principles. In R. B. Mandell (Ed.), Contact Lens Practice. (4th ed.). (p. 19). Spring eld, IL: Charles C. T omas. McMahon, . ., & Zadnik, K. (2000). wenty- ve years o contact lenses – the impact on the cornea and ophthalmic practice. Cornea, 19, 730–740. Mertz, G. W. (1997). Development o contact lenses. Ch. 5, Section II, Contact Lenses. In H. Hamano, & H. Kau man (Eds.), Corneal Physiology and Disposable Contact Lenses (pp. 65–99). Boston: Butterworth-Heinemann. Meyler, J., & Ruston, D. (2006). T e world’s rst daily disposables. Optician, 231(6053), 12. Milton, H. E., Morgan, P. B., Clamp, J. H., et al. (2014). Electronic liquid crystal contact lenses or the correction o presbyopia. Opt. Express, 22, 8035–8040. Mohammadi, S., Jones, L., & Gorbet, M. (2014). Extended latanoprost release rom commercial contact lenses: in vitro studies using corneal models. PLoS One, 9, e106653. http://dx.doi.org/10.1371/journal.pone.0106653. Morgan, P. B.Woods, C. A., ranoudis, I. G., (2015). International contact lens prescribing 2015. CL Spectrum, 31(1), 28-33. Müller, F. A., & Müller, A. C. (1910). Das kunstliche Auge. Wiesbaden: J. F. Bergmann, 68–75. Nomachi, M., Sakanishi, K., Ichijima, H., et al. (2013). Evaluation o diminished microbial contamination in handling o a novel daily disposable at pack contact lens. Eye Contact Lens, 39, 234–238. Pearson, R. M. (1989). Kalt, keratoconus and the contact lens. Optom. Vis. Sci., 66, 643–646.

Pearson, R. M. (2007). Karl Otto Himmler, manuacturer o the rst contact lens. Cont. Lens Anterior Eye, 30, 11–16. Pearson, R. M. (2009). T e Sämisch case and the Müllers o Wiesbaden. Optom. Vis. Sci., 86, 157– 164. Pearson, R. M. (2015). Comments on ‘Modern scleral contact lenses: a review’ [van der Worp et al. (2014)]. Cont. Lens Anterior Eye, 38, 73–74. Pearson, R. M., & E ron, N. (1989). Hundredth anniversary o August Müller’s inaugural dissertation on contact lenses. Surv. Ophthalmol., 34, 133–141. Sankaridurg, P., Holden, B. A., Smith, E., 3rd, et al. (2011). Decrease in rate o myopia progression with a contact lens designed to reduce relative peripheral hyperopia: one-year results. Invest. Ophthalmol. Vis. Sci., 52, 9362–9367. Schein, O. D., Glynn, R. J., Poggio, E. C., et al. (1989). T e relative risk o ulcerative keratitis among users o daily-wear and extended-wear so contact lenses. A case–control study. Microbial Keratitis Study Group. N. Engl. J. Med., 321, 773–778. Weidemann, K. E., & Lakkis, C. (2005). Clinical perormance o microchannel contact lenses. Optom. Vis. Sci., 82, 498–504. Weisbarth, R. E., Gabriel, M. M., George, M., et al. (2007). Creating antimicrobial sur aces and materials or contact lenses and lens cases. Eye Contact Lens, 33, 426–429. Wichterle, O. (1978). T e beginning o the so lens. Ch. 1, Section 1, Historical development. In M. Ruben (Ed.), Sof Contact Lenses. Clinical and Applied Technology (pp. 3–5). Eastbourne: Baillière indall. Wichterle, O., & Lim, D. (1960). Hydrophilic gels or biological use. Nature, 185(4706), 117–118. Young, . (1801). On the mechanisms o the eye. Phil. Trans. R. Soc. Lon. [Biol. Sci.], 91, 23–88. Zhu, H., Kumar, A., Ozkan, J., et al. (2008). Fimbrolidecoated antimicrobial lenses: their in vitro and in vivo e ects. Optom. Vis. Sci., 85, 292–300. Erratum in Optom. Vis. Sci., 85, 609.

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2

Ant e rio r Eye JO HN G LAWRENSO N

Int ro d uct io n A critical aspect o contact lens practice is monitoring the ocular response to lens wear, which ranges rom acceptable physiological changes to adverse pathology. In order to do this, practitioners must possess a thorough understanding o the normal structure and unction o the anterior eye, which is the subject o this chapter. In the course o reading other chapters in this book, the reader may need to re er back to this chapter on the unctional anatomy and physiology o the anterior eye in order to develop a uller understanding o the phenomena being described. 

The Co rne a T e cornea ul ls two important unctions: together with the sclera it orms a tough brous outer coat that encloses the ocular tissues and protects the internal components o the eye rom injury. Signi cantly, the cornea also provides twothirds o the re ractive power o the eye. It is particularly well suited to its role: the cornea is curved and transparent, and the air–tear inter ace provides a re ractive sur ace o good optical quality.

cornea is conventionally divided into our zones (central, paracentral, peripheral and limbal). T e central zone, which covers the entrance pupil o the eye, is spherical, approximately 4 mm wide, and principally determines high-resolution image ormation on the ovea. T e paracentral zone, which lies outside the central zone, is atter and becomes optically important in dim illumination when the pupil dilates. T e peripheral zone is where the cornea is attest and most aspheric (Klyce et al., 1998). Due to a di erence in curvature between its posterior and anterior sur aces, the cornea shows a regional variation in thickness. Centrally the thickness is approximately 0.54 mm (Doughty and Zaman, 2000), with a peripheral thickness between 11% and 19% higher than in the centre (Khoramnia et al., 2007).  Microscop ic Anatomy When the cornea is viewed in transverse section, ve distinct layers can be resolved: epithelium, Bowman’s layer, stroma, Descemet’s membrane and endothelium (Fig. 2.1). Epithelium. T e epithelium represents approximately 10% o the thickness o the cornea (55 µm) (Feng and Simpson, 2008). It is a strati ed squamous non-keratinized epithelium, consisting o 5–6 layers o cells (Fig. 2.2), which undergo a constant process o cyclic

CO RNEAL ANATO MY Gross Anatomy T e cornea is elliptical when viewed rom in ront, with its long axis in the horizontal meridian ( able 2.1). T is asymmetry is produced by a greater degree o overlap o the peripheral cornea by opaque limbal tissue in the vertical meridian. T e sur ace area o the cornea is 1.1 cm 2, which represents about 7% o the sur ace area o the globe (Maurice, 1984). opographically, the

TABLE

2.1

Co rne al Dime nsio ns and Re lat e d Me asure me nt s

Parame t e r

Value

Are a Diame te r Horizontal Ve rtical Rad ius of curvature Ante rior ce ntral Poste rior ce ntral Thickne ss Ce ntral Pe rip he ral Re fractive ind e x Powe r

1.1 cm 2

(Data ad ap te d rom Bron e t al., 1997.)

10

11.8 mm 10.6 mm 7.8 mm 6.5 mm 0.54 mm 0.67 mm 1.376 42 D

Fig . 2.1 Transve rse se ction throug h the corne a. The stroma, which re p re se nts 90% o the thickne ss o the corne a, is b ound e d b y the e p ithe lium (aste risk) and e nd othe lium (arrow).

2

shedding and replacement to maintain corneal integrity. T ree distinct epithelial cell types are recognized: a single row o basal cells, 2–3 rows o wing cells and 2–3 layers o super cial (squamous) cells. In addition, several non-epithelial cells are present (e.g. lymphocytes, macrophages and Langerhans cells). T e epithelium orms a permeability barrier to water, ions and hydrophilic molecules above a certain size, as well as orming an e ective barrier to the entry o pathogens. Further epithelial specialization enhances adhesion between cells, to withstand shearing and abrasive orces. Furthermore, throughout the thickness o the epithelium, adjacent cells are connected to one another by water channels (aquaporins) that are engaged in transcellular water transport and gap junctions to allow the trans er o ions and small molecules between cells (Bron et al., 2015). Super cial cells are structurally modi ed or their barrier unction and interaction with the tear lm. Scanning electron microscopy o sur ace cells shows extensive nger-like and ridge-like projections (microvilli and microplicae), which increase the epithelial sur ace area. Light, medium and dark cells can be distinguished depending on the number and pattern o sur ace projections (P ster, 1973). It has been suggested that dark cells, which are relatively ree o these sur ace eatures, are close to being desquamated into the tear lm. By contrast, the newly arrived light cells possess a more extensive array o sur ace projections. In high-power transmission electron micrographs, microvilli and microplicae show an extensive lamentous covering known as the glycocalyx. T e glycocalyx is ormed rom membrane-bound mucin glycoproteins and is important or spreading and attachment o the precorneal tear lm. In accordance with their barrier unction, a complex network o tight junctions links super cial cells that exclude watersoluble dyes such as uorescein (Bron et al., 2015). Wing cells are so named because o their characteristic shape, with lateral extensions and a concave in erior sur ace to accommodate the apices o the basal cells. T eir nuclei tend to be spherical or elongated in the plane o the cornea. T e cell borders o the polygonal wing cells show prominent in oldings that interdigitate with adjacent cells, and numerous desmosomes. T is arrangement results in a strong intercellular adhesion. T e cytoplasm contains prominent cytoskeletal elements (predominantly actin and cytokeratin intermediate laments), and although the usual complement o organelles is present they are ew in number.

Fig . 2.2 Corne al e p ithe lium (d e tail). Thre e ce ll typ e s are p re se nt: b asal ce lls (aste risk), wing ce lls (arrowhe ad ) and sq uamous ce lls (arrow). BL= Bowman’s laye r.

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Basal cells consist o single-layer columnar cells with a vertically oriented oval nucleus. Ultrastructurally, they are similar in appearance to wing cells. T e plasma membrane similarly shows pronounced in olding and the cytoplasm contains prominent intermediate laments. A variety o cell junctions are present including: desmosomes, which mediate adhesion between cells; hemidesmosomes, which are involved in the attachment o basal cells to the underlying stroma; and gap junctions, which allow or intercellular metabolic coupling. Basal cells orm the germative layer o the cornea, and mitotic cells are o en seen at this level.  Basal Lamina and Bowman’s Layer. T e basal lamina (basement membrane) is synthesized by basal cells. It varies in thickness between 0.5 and 1 µm, and under the electron microscope can be di erentiated into an anterior clear zone (lamina lucida) and a posterior darker zone (lamina densa). T e basal lamina is part o a complex adhesion system, which mediates the attachment o the epithelium to the underlying stroma (Fig. 2.3). Hemidesmosomes link the cytoskeleton via a series o anchoring brils to anchoring plaques in the anterior stroma. T e molecular components o this adhesion complex have been identi ed and include type VII collagen, integrins, laminin and bullous pemphigoid antigen (Gipson et al., 1987). Bowman’s layer (anterior limiting membrane) varies in thickness between 8 and 14 µm. With the light microscope it appears as an acellular homogeneous zone. Ultrastructurally, it is composed o a randomly oriented array o ne collagen brils, which merge with the brils o the anterior stroma (Hogan et al., 1971). Fibrils are composed primarily o collagen types I, III and V. Collagen VII, associated with anchoring brils, is also present. T ere is evidence that Bowman’s layer is ormed and maintained primarily by the epithelium, although its unction is unclear. T e absence o Bowman’s layer rom the cornea o most mammals, and the act that corneas devoid o this layer over the central cornea ollowing photore ractive keratectomy (PRK) apparently unction normally, suggest that it is not critical to corneal integrity (Wilson and Hong, 2000).  Stroma. T e stroma is approximately 500 µm thick, and accounts or 90% o the thickness o the cornea. It is composed predominantly o collagen brils (70% dry weight) embedded in a highly hydrated matrix o proteoglycans. A variety o collagen

Fig . 2.3 Sche matic re p re se ntation o the ad he sion syste m o the corne al e p ithe lium. Inte rme d iate lame nts in the cytoske le ton (CS) are linke d throug h he mid e smosome s (HD) via anchoring b rils (AF) to anchoring p laq ue s (AP) in the ante rior stroma. BL= b asal lamina; D = d e smosome .

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Fig . 2.4 Se ction throug h the stroma. Ke ratocyte s (arrowe d ) are locate d b e twe e n lame llae . Fig . 2.6 Flat se ction throug h the stroma staine d with g old chlorid e . Ke ratocyte s (arrowe d ) d isp lay a ste llate ap p e arance .

Fig . 2.5 Ele ctron microg rap h o stromal lame llae that cross e ach othe r ap p roximate ly at rig ht ang le s. Note the re g ular arrang e me nt o collag e n b rils within lame llae .

types have been identi ed. ype I is the major bril- orming collagen, with lesser amounts o types III and V. Non- brilorming collagens, including types VI and XII, are ound in the inter brillar matrix (Meek and Boote, 2009). A section taken perpendicular to the corneal sur ace reveals that the collagen brils are arranged in 200–250 layers (lamellae) running parallel to the sur ace (Fig. 2.4). Lamellae are approximately 2 µm thick and 9–260 µm wide, and extend rom limbus to limbus. Fibrils o adjacent lamellae make large angles with each other. In the super cial stroma the angles are less than 90°, but brils become orthogonal in the deeper stroma (Hogan et al., 1971; Meek and Boote, 2009). T is pre erred orthogonal orientation gradually changes in avour o circum erentially aligned collagen at the limbus. T is particular arrangement o collagen imparts a high tensile strength or corneal protection, which is important given its exposed position. Within lamellae, all collagen brils are parallel with uni orm size and separation (Fig. 2.5). Accurate

physiological measurements o collagen bre diameter and spacing can be obtained or the hydrated cornea with the aid o X-ray di raction. Using this technique, the mean bril diameter in the human cornea is 31 nm, with an inter brillar spacing o 55 nm (Meek and Leonard, 1993). T is narrow bril diameter and constant separation, which is a characteristic o corneal collagen, are necessary prerequisites or transparency. T e inter brillar space contains a matrix o proteoglycans (approximately 10% o dry weight). T ese molecules are highly sulphated, and along with bound chloride ions create a polyanionic stromal inter brillar matrix that induces osmotic swelling. As well as playing a major role in corneal hydration, collagen– proteoglycan interactions are also thought to be important in determining the size and spatial arrangement o stromal collagen brils (Scott, 1991; Quantock and Young, 2008). Collagen and proteoglycans are maintained by keratocytes. T ese cells occupy 3–5% o stromal volume and lie between collagen lamellae, attened in the plane o the cornea (Fig. 2.6). Keratocyte density examined by con ocal microscopy and biochemical methods (Møller-Pederson and Ehlers, 1995; Prydal et al., 1998) is non-uni orm. Density decreases rom super cial to deep stroma (Hollingsworth et al., 2001) and increases rom centre to periphery. Keratocytes display a large central nucleus and long slender processes extend rom the cell body. Processes rom adjacent cells sometimes make tight junctions with each other. Cell organelles are not numerous but the usual complement o organelles, including endoplasmic reticulum, Golgi apparatus and mitochondria, can be observed (Hogan et al., 1971). Newer lamellar corneal transplantation techniques have been developed that allow selective replacement o the diseased corneal layers. Deep anterior lamellar keratoplasty (DALK), which is increasingly being used to treat keratoconus and corneal scarring, involves replacement o the a ected stroma while retaining the host’s healthy Descemet’s membrane and endothelium. Separation between the posterior stroma and Descemet’s / endothelium can be achieved by intrastromal injection o air, viscoelastic or saline. Dua and co-workers per ormed a histological examination o donor corneas using air bubble separation and claimed to have identi ed a novel ‘pre-Descemet’s posterior stromal layer’, which was widely publicized (Dua et al.,

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Fig . 2.7 Hig h-p owe re d microg rap h o the p oste rior stroma. De sce me t’s me mb rane (DM) is locate d b e twe e n the stroma (S) and the e nd othe lium (arro w). Fig . 2.9 Tang e ntial (f at) se ction throug h the corne al e nd othe lium: a sing le laye r o p olyg onal ce lls with irre g ular b ord e rs can b e ob se rve d .

Fig . 2.8 Thre e -d ime nsional re p re se ntation o the p oste rior corne a showing the e nd othe lium (e ), De sce me t’s me mb rane (d ) and stroma (s). A stromal lame lla has b e e n re f e cte d to re ve al an intralame llar ke ratocyte (k).

2013). However, the current consensus amongst corneal experts is that this layer is not suf ciently unique to constitute a new corneal layer (Jester et al., 2013).  Descemet’s Membrane. Descemet’s membrane is the basement membrane o the corneal endothelium. It lies between the endothelium and the overlying stroma (Fig. 2.7). At birth it is 3–4 µm thick, and increases to a thickness o 10–12 µm in the adult. In the periphery o aged corneas, Descemet’s membrane displays periodic sections o thickening, which are known as Hassall–Henle warts. T e anterior one-third o Descemet’s membrane represents that part produced in etal li e and, under the electron microscope, is characterized by a periodic banded pattern (Fig. 2.8). T e posterior two-thirds, which is ormed postnatally, has a more homogeneous granular appearance. Descemet’s membrane has a unique biochemical composition in contrast with other basement membranes (Lawrenson et al., 1998). T e major basement membrane collagen type is type IV, whereas in Descemet’s membrane type VIII collagen predominates.  Endothelium. T e endothelium is a monolayer o squamous cells that lines the posterior sur ace o the cornea (Fig. 2.9) and plays a critical role in maintaining corneal transparency (Bonanno, 2012). As it has a limited capacity or mitosis to

replace damaged or e ete cells, there is a progressive reduction in endothelial cell number with age. At birth the cornea contains a total o approximately 500 000 cells, which represents a mean density o 4500 cells / mm 2. During in ancy, cell loss is particularly marked and a 26% reduction occurs in the rst year o li e (Sherrard et al., 1987). T erea er the rate o loss progressively declines into old age. Since gra ed corneas appear to maintain transparency and unctional normality with an endothelial cell density o less than 1000 cells / mm 2, it seems that normal cell density represents a considerable ‘physiological reserve’ (Klyce and Beuerman, 1998). When viewed en ace, or example using a specular microscope, the endothelium appears as a mosaic o polygonal (typically hexagonal) cells (E ron et al., 2001). In response to pathology, trauma, age and prolonged contact lens wear, the endothelial mosaic becomes less regular, and shows a greater variation in cell size (polymegethism) and shape (pleomorphism) as cells spread to ll gaps caused by cell loss. Under the electron microscope, the lateral borders o the cells are markedly convoluted and adjacent cells are linked by tight junctions (with less- requent gap junctions) (Hogan et al., 1971). T e complement o organelles seen in endothelial cells re ects their high metabolic activity, with numerous mitochondria and a prominent rough endoplasmic reticulum.  CO RNEAL INNERVATIO N Source and Distrib ution of Corne al Ne rve s T e cornea is the most richly innervated sur ace tissue in the body. Corneal nerves are responsible or the detection o somatosensory stimuli and play an important role in initiating the blink re ex, wound healing and tear secretion (see Shaheen et al., 2014, or a recent review). T e majority o corneal nerves are sensory and derive rom the nasociliary branch o the trigeminal nerve (Ruskell and Lawrenson, 1994). T ere is also evidence or the existence o a modest sympathetic innervation rom the superior cervical ganglion (Mar urt and Ellis, 1993). Branches rom the nasociliary nerve either pass directly to the eye as long ciliary nerves or traverse the ciliary ganglion, leaving it as short ciliary nerves that enter the eye close to the

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Fig . 2.10 Whole -mount g old chlorid e -staine d p re p aration o corne al ne rve s (arrows) locate d at b asal le ve l.

optic nerve. Nerves destined or the cornea travel initially in the suprachoroidal space, be ore crossing the sclera to advance radially towards the cornea. Most o the 50–80 precorneal nerve trunks, which contain a mixture o myelinated and unmyelinated bre bundles, enter the cornea at mid-stromal level. Myelin is soon lost and the unmyelinated nerve bre bundles divide repeatedly and move anteriorly to orm a rich plexi orm network in the anterior onethird o the stroma. Axons are particularly dense immediately beneath Bowman’s layer, orming an extensive subepithelial plexus (Oliveira-Soto and E ron, 2001). From this plexus, axons pass vertically through Bowman’s layer, losing their Schwann cell sheath in the process. Upon entering the epithelium, axons turn through 90° and divide into a series o ne branches that course between basal cells (Fig. 2.10). Some branches pass into the more super cial layers be ore terminating. T e density o nerve terminals is greatest centrally, corresponding to approximately 600 terminals / mm 2, which results in large overlapping receptive elds (Shaheen et al., 2014). Corneal nerves display a complex neurochemistry. A variety o neurotransmitters and neuromodulators have been identied, including acetylcholine, substance P, and calcitonin generelated peptide. However, it is unclear how these particular neurochemicals correlate with unction (Belmonte et al., 1997).  Functional Consid e rations Corneal nerves serve important sensory, re ex and trophic unctions. Interest in the sensitivity o the cornea dates back to the 19th century (Lawrenson, 1997), when the pioneering German physiologist von Frey concluded that pain was the only sensation perceived by the cornea. T is was consistent with his theory o the speci city o sensory receptors, which maintained that each sensory modality was subserved by a separate anatomically distinct nerve terminal. In his experiments on the cornea, von Frey could elicit only a sensation o pain and, as the cornea contained exclusively ree (unspecialized) nerve endings, he

concluded that ree nerve endings were the exclusive receptors or pain. Although the speci city theory has subsequently been challenged, particularly with respect to its exclusivity, the question as to whether pain is the only sensory modality perceived by the cornea remains. Modern experiments using care ully controlled corneal stimulation, with a variety o mechanical, chemical and thermal stimuli, have evoked only sensations o irritation or pain. By contrast, electrophysiological studies o corneal a erent neurones have identi ed neurones that respond to mechanical, thermal and chemical stimulation. However, since the conscious perception o these sensations has not been demonstrated, it is likely that such speci city o modality is lost during central nervous system processing. Electrophysiological recording also allows or the mapping o receptive elds. T ese are o en large and overlapping, which explains the inability o the cornea to localize a stimulus accurately (Belmonte et al., 1997). T e sensitivity o the cornea to mechanical stimulation is particularly acute, and acts as a trigger or the protective blink and lacrimal re exes. Cold receptors may be important in signalling evaporative cooling, which is a major determinant o spontaneous eye blink requency ( subota, 1998). Corneal a erent bres also exert important trophic in uences. Damage to corneal sensory nerves by surgery, trauma or in ection produces neuroparalytic keratitis – a condition that is characterized by progressive epithelial cell loss and oedema. T e mechanism o this trophic role is not ully understood, although the release o neuropeptides (e.g. substance P and calcitonin gene-related peptide) may be a actor. Sympathetic nerves also play a role in epithelial maintenance by regulating ion transport processes, and cell proli eration and migration during wound healing.  CO RNEAL METABO LISM Source of O xyg e n and Nutrie nts In order to per orm its vital unctions, the cornea requires a constant supply o oxygen and other essential metabolites (e.g. glucose, vitamins and amino acids). However, its avascularity dictates that alternative routes must exist or the provision o its metabolic needs. T ere are three possibilities: rom the perilimbal vasculature, rom the tear lm or rom the aqueous humour. In open-eye conditions the bulk o the oxygen required or the cornea is obtained rom the atmosphere via di usion across the precorneal tear lm. Under steady-state conditions it can be assumed that the tears are saturated with oxygen, and thereore at an oxygen tension corresponding to the atmosphere (155 mmHg at sea level). It has been estimated that the oxygen tension o the aqueous humour in the human eye lies between 30 and 40 mmHg (Klyce and Beuerman, 1998). Experiments using nitrogen- lled goggles or sealed scleral lenses have shown the corneal dependence on tear-side oxygen to avoid oedema and maintain normal unction. T e reason why the cornea swells during contact lens wear is explained in Fig. 2.11. During eye closure the oxygen level in the tears is in equilibrium with the palpebral vasculature (55 mmHg) (E ron and Carney, 1979). Signi cantly, corneal thickness increases by approximately 5% during sleep, and returns to baseline levels within 1 hour o eye opening. It has been suggested that overnight swelling is related to tear lm tonicity rather than reduced oxygen

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Fig . 2.11 (A) Cross-se ction o an e ye we aring a contact le ns, which imp e d e s ing re ss o oxyg e n into, and the e g re ss o carb on d ioxid e rom, the corne a. (B) The contact le ns b locks oxyg e n sup p ly to the corne a (1), causing lactic acid to accumulate in the stroma (2). This d raws in wate r (3), le ad ing to stromal oe d e ma (4). (Ad ap te d rom E ron, N. (1997). Contact le nse s and corne al p hysiolog y. Biol. Sci. Re v., 9, 29–31.)

availability (Klyce and Beuerman, 1998). T e oxygen ux into the cornea can be measured using polarographic oxygen sensors. It is in the region o 6 µl / cm 2 / h or the cornea as a whole, although the consumption rate or its composite layers is not equal. Consumption rates have been estimated as 40 : 39 : 21 or the epithelium, stroma and endothelium, respectively (Freeman, 1972). Several lines o evidence indicate that the aqueous humour is the primary source o glucose and essential amino acids or the cornea (Maurice, 1984). T e glucose concentration o tears is low compared with that in the aqueous humour, and the insertion o nutrient-impermeable implants into the stroma results in degeneration o the tissue lying anterior to the implant. Although exogenous glucose is primarily utilized, glycogen stores are present in all corneal cells to provide glucose in conditions o metabolic stress. T e role o the perilimbal vasculature in the provision o oxygen and nutrients appears limited and it is likely that it is signi cant only or the corneal periphery (Maurice, 1984).  O xid ative Me tab olism T e cornea derives its energy principally rom the oxidative breakdown o carbohydrates (Riley, 1969). Glucose, which is the primary substrate or the generation o adenosine triphosphate (A P), is catabolized by three metabolic pathways: glycolysis, the tricarboxylic acid (Krebs) cycle and the hexose monophosphate shunt (Fig. 2.12). Anaerobic glycolysis accounts or the majority (85%) o glucose metabolism. In this pathway, glucose is rst oxidized to pyruvate and then subsequently reduced to lactate, with a net yield o two molecules o A P per mole o glucose. T e CA cycle results in a greater energy yield (36 A P). T is pathway is most active in the corneal endothelium, which has the greatest energy requirement. Metabolic waste products can be potentially damaging i allowed to accumulate. Although carbon dioxide can readily di use out o the cornea across its limiting layers, lactate is less easily eliminated. Under normoxic conditions, lactate is able to

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Fig . 2.12 Me tab olic p athways p re se nt in the co rne a. HMP = he xose monop hosp hate shunt; TCA cycle = tricarb oxylic acid (Kre b s) cycle ; ATP = ad e nosine trip hosp hate ; NADPH = nicotinamid e ad e nine d inucle otid e p hosp hate (re d uce d orm).

di use slowly across the endothelium into the anterior chamber. However, during periods o hypoxia the proportion o glucose that is metabolized anaerobically increases. T e resulting accumulation o lactate causes stromal oedema via an increased osmotic load (Klyce, 1981) and localized tissue acidosis (Klyce and Beuerman, 1998). T e hexose monophosphate shunt (also known as the pentose phosphate shunt) plays an important role in the corneal epithelium (Berman, 1981), where it ul ls several important unctions, including the generation o intermediates or biosynthetic reactions and the prevention o oxidative damage by ree radicals.  CO RNEAL TRANSPARENCY Under normal conditions the cornea is highly transparent, transmitting more than 90% o incident light. Structurally, the cornea is a typical connective tissue consisting principally o a matrix o collagen and proteoglycans. Under normal circumstances such an arrangement would avour light scatter with consequent loss o transparency. T is raises two undamental questions: how is transparency achieved, and how is it maintained? o begin to answer these questions it is necessary to understand the spatial organization o the stromal matrix and the importance o corneal hydration control. Stromal O rg anization Maurice (1957) explained the transparency o the cornea on the basis o the small diameter and regular separation o the stromal collagen. He suggested that the collagen brils o the stroma were disposed in a regular crystalline lattice, and that light scattered by the brils is eliminated by destructive inter erence in all directions other than the orward direction. T is situation will hold as long as the axes o the collagen brils are arranged in a regular lattice with a separation less than the wavelength o light. It has been suggested, however, that the brillar arrangement need not be in a per ect crystal lattice to maintain transparency (Maurice, 1984), although disruption o short-range order between brils will lead to increased scatter and a loss o transparency.

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T e actors involved in the maintenance o collagen bril size and spatial order are not ully understood. It has been proposed that collagen bril diameters may be controlled by the incorporation o minor collagens (e.g. type V) into the predominantly type I brils (Meek and Leonard, 1993) and that their spatial separation is a unction o proteoglycan–collagen interactions (Scott, 1991). Proteoglycans are a amily o glycoproteins that consist o a protein core to which are attached sugar chains o repeating disaccharide units termed glycosaminoglycans (GAGs). T ese molecules are increasingly being recognized as important prerequisites or transparency (Quantock and Young, 2008; Hassell and Birk, 2010). Proteoglycans were originally classi ed according to their glycosaminoglycan composition; however, current nomenclature groups them into amilies based on homologous sequences o amino acids in their protein core. Corneal stromal proteoglycans are members o the small leucine-rich amily, which are small enough to t in the space between collagen brils. T e most prevalent glycosaminoglycan in the cornea is keratan sulphate, which is ound in three types o proteoglycan: lumican, keratocan and mimecan (Funderburgh et al., 1991; Funderburgh 2000). T e other type o corneal proteoglycan is decorin, which contains a hybrid chondroitin sulphate / dermatan sulphate side chain. Evidence rom several sources has shown that lumican and decoran play important roles in regulating collagen bril diameter and maintaining the spacing between brils once ormed, which are essential or transparency.  Hyd ration Control T e state o corneal hydration is another important determinant o corneal transparency (Bonanno 2012). T e hydrophilic properties o the stroma are to a large part determined by proteoglycans, which contribute to the xed negative charge o the stroma and produce a passive gel swelling pressure through electrostatic repulsion (Hodson, 1997). Physiologically, corneal hydration is maintained at approximately 78%. I the cornea is allowed to swell ±5% o this value, it begins to scatter signi cant quantities o light (Hodson, 1997). Maintenance o physiological corneal hydration is to a large part dependent on the corneal endothelium, which possesses both a barrier property and a metabolically driven pump. T e endothelial barrier to the ree passage o molecules rom the aqueous humour is ormed principally by ocal tight junctions between adjacent endothelial cells. However, in contrast to other barrier epithelia, these junctions are o low electrical resistance and allow the passage o ions and small molecules. T is leak is o set by the metabolically driven pumping o ions out o the stroma by the endothelium, which maintains a transcellular potential di erence (aqueous side negative) to balance stromal swelling pressure (Maurice, 1984). Disruption o this osmotic gradient will result in stromal uid imbibition. T e speci c endothelial ion transport mechanisms responsible or the maintenance o physiological hydration are not ully understood. A simpli ed model representing our current level o knowledge is represented in Fig. 2.13. T ere is compelling evidence that a ux o bicarbonate ions is the predominant component o the endothelial ion transport system (Hodson and Miller, 1976). Subsequent studies have identi ed that Cl− transporters may also be important in maintaining the pump (Bonanno 2012). T e bicarbonate is generated either by a Na+ / HCO3− co-transporter located on the basolateral plasma membrane or via the intercellular conversion o carbon

Fig . 2.13 Sche matic re p re se ntation o the corne al e nd othe lial p ump . CA = carb onic anhyd rase ; TJ = tig ht junctio n.

dioxide by the enzyme carbonic anhydrase. Bicarbonate leaves the cell via an apical bicarbonate ion channel. T e driving orce or the bicarbonate ux is generated by a sodium–potassium A Pase that resides on the basolateral endothelial membrane. T e energy associated with subsequent sodium re-entry (via Na+ / H + and Na+ / HCO3− transporters) is coupled to active HCO3− ux (Hodson et al., 1991). T e epithelium also contributes to corneal hydration control (Klyce and Beuerman, 1998). T e tight junctions between super cial epithelial cells orm an e ective permeability barrier to ions and polar solutes. For example, the anionic molecule sodium uorescein does not penetrate an intact epithelium. However, damage to the super cial cells allows uorescein to enter the epithelium, with resulting corneal staining. In addition to its barrier properties, the epithelium also possesses active ion transport systems or Na+ and Cl−. As these pumps contribute to the tonicity o the tear lm, it is likely that they are involved in the maintenance o stromal hydration.  Re sp onse to O e d e ma When corneas swell, light scattering increases, with an ensued transparency loss due to the disruption o the regular collagen matrix. T e collagen brils themselves swell very little and most o the additional water goes into the inter brillar spaces. ransmission electron micrographs o oedematous corneas show bril aggregation, with the result that large areas are devoid o collagen brils (Stiemke et al., 1995). T ere is evidence that collagen aggregation occurs as a result o loss o GAGs, which previously had maintained bre separation (Stiemke et al., 1995).  CO RNEAL EPITHELIAL WO UND HEALING A smooth and intact corneal epithelium is necessary in order or the cornea to maintain clear vision. However, due to its exposed position the cornea is potentially vulnerable to a variety o external insults. It possesses several protective mechanisms to avoid injury, but i tissue damage occurs it is capable o an e ective wound-healing response (Gipson and Inatomi, 1995; Nishida and anaka, 1996). Corneal epithelial repair is a complex process involving an orchestrated interaction between cells and extracellular matrix, which is coordinated by a variety

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o growth actors. T e process can be divided into three phases: (1) initial covering o the denuded area by cell migration, (2) cell proli eration to replace lost cells and (3) epithelial di erentiation to re- orm the normal strati ed epithelial architecture. Following a ull-thickness epithelial de ect, bronectin, an adhesive glycoprotein, is synthesized and covers the sur ace o the bared stroma where it serves as a temporary matrix or cell migration. T e adhesion between bronectin and the epithelium is mediated by integrin–matrix interactions (integrins are a amily o cell sur ace receptors that bind to certain extracellular matrix proteins). Several growth actors have been implicated in the control o the wound-healing response, including epidermal growth actor, trans orming growth actor beta, platelet-derived growth actor and broblast growth actor (Gipson and Inatomi, 1995). Growth actors, which are produced by a variety o sources (e.g. ocular sur ace epithelia and the lacrimal gland), are able to regulate the process o epithelial migration, proli eration and di erentiation. T ere is evidence that epithelial–stromal interactions play an important role in corneal wound healing (Wilson, 2000). Epithelial injury triggers keratocyte apoptosis (programmed cell death) in the anterior stroma, via the release o apoptosis-inducing cytokines rom epithelial cells. Keratocyte apoptosis subsequently triggers a wound-healing cascade, which in uences epithelial repair. Regeneration o the corneal epithelium is highly dependent on the integrity o the limbus (Lavker et al., 2004). Cumulative evidence indicates that a proportion o limbal basal epithelial cells possess the properties o stem cells, which are ultimately responsible or corneal epithelial replacement. Stem cells have several unique characteristics: they are poorly di erentiated, long lived and have a high capacity or sel -renewal. When these cells divide, one o the daughter cells replenishes the stem cell pool, whilst the other is destined to undergo urther cell divisions be ore di erentiating. Such a cell is re erred to as a transient ampli ying cell. ransient ampli ying cells undergo several rounds o cell division be ore ully di erentiating. T ese cells play an important role in epithelial wound healing, where their proli erative capacity is increased by shortening cycle times and increasing the number o times that the transient ampli ying cells can divide be ore maturation. 

The O cular Ad ne xa T e ocular adnexa are those structures that support and protect the eye, and include the eyelids, conjunctiva and lacrimal system. T ey play an important role in the ormation o the preocular tear lm and collectively de end the eye against antigenic challenge. EYELIDS T e eyelids are two mobile olds o skin that per orm several important unctions: they act as occluders that shield the eyes rom excessive light, and through their re ex closure they a ord protection against injury. T e lids also orm a precorneal tear lm o uni orm thickness during the upturn phase o each blink. T e action o blinking is also important or tear drainage. Gross Anatomy T e eyelids are joined at their extremities, termed ‘the canthi’, and when the eye is open, an elliptical space, the palpebral ssure, is ormed between the lid margins. In the adult, the length

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Fig . 2.14 Sur ace anatomy o the e ye lid s. (Ad ap te d rom Bron, A. J., Trip athi, R. C. & Trip athi, B. (1997). Wol ’s Anatomy o the Eye and O rb it (8th e d .). Lond on: Chap man and Hall.)

o the ssure is approximately 30–31 mm, with a vertical height o 10–11 mm. In the primary position, the upper lid, which is the larger and more mobile o the two, typically covers approximately the upper third o the cornea, whilst the lower lid is level with the in erior corneal limbus (Fig. 2.14). Important di erences in eyelid anatomy exist between Asian and Caucasian eyes (Saonanon, 2014). T e most obvious eature o the Asian eye is the absent or very low lid old and uller upper eyelid. T e eyelid margins are about 2 mm thick rom ront to back. T e posterior quarter consists o conjunctival mucosa and the anterior three-quarters is skin. T e junction between the two is re erred to as the mucocutaneous junction. T ere has recently been a renewed interest in the marginal zone o the human eyelid, with the identi cation o the role o the inner lid border, termed the ‘lid-wiper’ owing to the analogy to a windscreen wiper, in the distribution o the tear lm (Knop et al., 2011a, 2012). wo or three rows o eyelashes (cilia) arise rom the anterior border o the lid margins. T ese are longer and more numerous in the upper lid. T e lashes receive a rich sensory nerve supply, and their sensitivity provides an e ective alerting mechanism. T e meibomian (tarsal) gland ori ces emerge just anterior to the mucocutaneous junction (Fig. 2.15). About 30–40 glands open onto the upper margin, and slightly ewer (20–40) onto the lower. On eversion o the lids the yellowish meibomian acini are visible as yellow clusters through the tarsal conjunctiva (Bron et al., 1991; Knop et al., 2011b). Meibomian glands can be more clearly visualized using in rared meibography, and instruments that use this method are now commercially available (Srinivasan et al., 2012) (Fig. 2.16). At the medial angle, the eyelid margins enclose a triangular space – the lacus lacrimalis – which contains the plica semilunaris and the caruncle. Lacrimal papillae are small elevations located 5–6 mm rom the medial canthal angle, which contains a small aperture (punctum) that is the opening to the lacrimal drainage system.  Muscle s of the Eye lid s Movements o the eyelids are governed by the coordinated action o several muscles. Orbicularis Oculi. T e orbicularis oculi is the sphincter muscle o the eyelids, and can anatomically be divided into two main divisions: the palpebral and the orbital (Fig. 2.17). Fibres o the palpebral division arise rom the medial palpebral ligament and arc across the eyelids in a series o hal -ellipses, meeting at the lateral canthus to orm a lateral raphe. T e lateral palpebral

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Fig . 2.17 Sche matic re p re se ntation o the d ivisions o the orb icularis oculi and the rontalis. a = p re tarsal; b = p re se p tal; c = orb ital; d = rontalis. (Ad ap te d rom Bron, A. J., Trip athi, R. C. & Trip athi, B. (1997). Wol ’s Anatomy o the Eye and O rb it (8th e d .). Lond on: Chap man and Hall.)

Fig . 2.15 (A) Sche matic re p re se ntation o the e ye lid marg in. mcj = mucocutane ous junction. (B) Gross ap p e arance o the e ye lid marg in. O p e ning s o the me ib omian g land s are cle arly visib le (arrow). (Ad ap te d rom Bron, A. J., Trip athi, R. C. & Trip athi, B. (1997). Wol ’s Anatomy o the Eye and O rb it (8th e d .). Lond on: Chap man and Hall.)

ligament also acts as an anchor point. T e palpebral division can be urther subdivided into marginal, pretarsal and preseptal parts. T e marginal part (pars ciliaris), which is also known as Riolans muscle, is responsible or maintaining the apposition o the lid to the cornea during lid closure. A third part o the muscle (pars lacrimalis) is closely associated with the lacrimal out ow pathway. T e pars lacrimalis (also known as Horner’s muscle) encloses the canaliculi and provides attachments to the lacrimal sac and its associated ascia. T e orbital part o the orbicularis oculi lies outside the palpebral division and extends or some distance beyond the orbital margins. Muscle bres arise predominantly rom bone at the medial orbital rim and appear to sweep around the lids without interruption as a series o complete ellipses. However, studies have shown that the muscle bres o the orbital and palpebral division o the orbicularis are relatively short (0.4–2.1 mm) and overlapping (Lander et al., 1996). T e regional divisions o the orbicularis also show a unctional distinction. T e action o the palpebral part o the muscle is to produce the re ex or voluntary closure o the lids during blinking. Contraction o the orbital division produces the orcible closure o the lids that occurs in sneezing or in response to a pain ul stimulus.  Levator Palpebrae Superioris. T e levator palpebrae superioris is primarily responsible or elevating the upper lid during blinking and or maintaining an open palpebral aperture. T e levator palpebrae arises rom the lesser wing o the sphenoid, above and anterior to the optic canal, and runs orward along the roo o the orbit above the superior rectus be ore terminating anteriorly in a an-shaped tendon (aponeurosis). Some bres are attached to the anterior sur ace o the tarsal plate, whilst the remainder pass between ascicles o the orbicularis (Fig. 2.18). T e superior palpebral sulcus orms at the upper border o the attachment to the orbicularis. 

Fig . 2.16 Normal me ib omian g land s o the up p e r tarsus (top ) and lowe r tarsus (b ottom) o a 38-ye ar-old woman, imag e d using no n-invasive in rare d me ib og rap hy. (Imag e courte sy o Re iko Arita.)

Superior and Inferior Tarsal Muscles (of Müller). T ese smooth muscles arise rom the lower border o the levator in the upper lid and the in erior rectus in the lower lid, and insert into the orbital margins o the tarsal plates. T e role o the superior tarsal muscle is to assist the levator in maintaining the width o the palpebral aperture. A mild degree o ptosis results rom damage to its sympathetic nerve supply (Horner’s syndrome). 

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Fig . 2.18 Diag ram showing the re lations o the le vator p alp e b rae sup e rioris. a = le vator ap one urosis; tm = sup e rior tarsal muscle (o Mülle r); t = tarsal p late ; s = orb ital se p tum. (Ad ap te d rom Gray, H., Banniste r, L. H., Be rry, M. M. & Williams, P. L. (1995) Gray’s Anatomy: The Anatomical Basis o Me d icine and Surg e ry (38th e d .). Ed inb urg h: Churchill Living stone .)

Control of Eye lid Move me nts Movements o the eyelids occur through the coordinated action o several muscles – the levator palpebrae, tarsal muscles, the orbicularis oculi and the rontalis. T e elevation o the upper lid and the control o its vertical position are mediated principally by the levator. In vertical gaze, lid position and eye movements are closely linked. During elevation the state o contraction o the levator is varied to maximize visibility. In extreme upgaze, lid retraction is augmented by the action o the rontalis, which elevates the eyebrows. In downgaze, coordinated lid movements similarly occur through levator relaxation. In periodic and re ex blinks, the levator is spontaneously inhibited prior to orbicularis contraction in lid closure. Similarly, in lid opening the orbicularis relaxes, ollowed by contraction o the levator. Spontaneous eye-blink activity is in uenced by both central and peripheral actors ( subota, 1998). Compared with the upper lid, the lower lid is relatively immobile and has no counterpart to the levator palpebrae. T e depression o the lower lid that occurs in downgaze is due to the attachment o the sheaths o the in erior oblique and in erior rectus muscles to the tarsal plate via a brous extension.  Microscop ic Anatomy T e histological appearance o the upper and lower lids is similar, and in sagittal section the ollowing six tissue layers can be resolved: skin, subcutaneous connective tissue, muscle layer, submuscular connective tissue, tarsal plate and palpebral conjunctiva (Fig. 2.19). T e eyelid skin is thin and very elastic. It is continuous with the palpebral conjunctiva at the lid margin, and keratinization is maintained up to this mucocutaneous junction. T e subcutaneous connective tissue is composed o a loose areolar tissue and contains hair ollicles and associated glands. T e muscle layer consists o striated muscle bres o the orbicularis oculi, which are arranged in bundles ( ascicles) separated by connective tissue. T e orbicularis extends throughout the lid. T e marginal part o the muscle (Riolan’s muscle) is separated rom the pretarsal portion by connective tissue that contains the eyelash ollicles. T e loose submuscular connective tissue layer lies between the orbicularis and the tarsal plate and contains the major nerves and vessels o the lid. T e tarsal plates (tarsi) are composed o dense brous connective tissue and provide support and determine lid shape.

Fig . 2.19 Sag ittal se ction throug h the up p e r lid . TP = tarsal p late ; O O c = orb icularis oculi; R = Riolan’s muscle ; EF = e ye lash ollicle s; PC = p alp e b ral conjunctiva; ES = e ye lid sur ace .

T ey are anchored to the orbital margins by the medial and lateral palpebral ligaments. Each tarsus is approximately 25 mm long and 1–2 mm thick. T e upper tarsus is semioval with a height o 11 mm at its midpoint, whereas the in erior tarsus is narrower (4 mm in height). T e posterior sur ace o the eyelid is lined by the palpebral conjunctiva, which is rmly adherent to the underlying tarsal plate.  Gland s of the Eye lid s Meibomian Glands. T e tarsal plates contain the acini and ducts o the meibomian (tarsal) glands. Ducts are vertically oriented with respect to the lid margins, with multiple secretory acini that open laterally onto each duct. T e glands occupy nearly the ull length and width o each tarsus, and are ewer and shorter in the lower lid. Histologically, the acini are lined by a layer o undi erentiated basal cells that divide, and cells are displaced rom the basement membrane. As they progress towards the duct they gradually enlarge and develop lipid droplets in their cytoplasm (Fig. 2.20). Ultimately, cell membranes rupture and cellular debris, together with the lipid product, is discharged into the duct. T e stimulus or meibomian gland secretion is unclear. Although a modest autonomic innervation o the meibomian glands has been demonstrated, there is still some doubt regarding a neuromodulation o glandular secretion; it is likely that the principal control o the glands is hormonal, and both androgens and oestrogens have been shown to regulate meibomian secretion (Sullivan et al., 2000; Knop et al., 2011b). A long ductal system carries the secretion to the lid margin, and the compressive action o the palpebral division o the orbicularis oculi on the meibomian ducts acilitates the ow o lipid and its eventual delivery onto the lid margins. 

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Fig . 2.20 Histolog ical se ction showing me ib omian g land acini. Se cre tory ce lls d e g e ne rate (aste risk) as the y ap p roach the d uct (D).

Fig . 2.22 Sche matic re p re se ntation o a mid -sag ittal se ction throug h the e ye lid and conjunctival sac showing the d i e re nt conjunctival re g ions. M = marg inal; T = tarsal; O = orb ital; B = b ulb ar; L= limb al; F = ornical.

Fig . 2.23 Static d ime nsions o the conjunctival sac in millime tre s. M = me d ial canthus. (Ad ap te d ro m Ehle rs, N. (1965). O n the size o the co njunctival sac. Acta O p hthalmol., 43, 205–210.) Fig . 2.21 Histolog ical se ction throug h the ciliary zone o the e ye lid . Gland s o Ze is (Z) d ischarg e the ir conte nts into an e ye lash ollicle (EF), which contains the re mnants o an e ye lash. M = g land o Moll.

Glands of Zeis and Moll. Ciliary glands o Zeis and Moll are ound in association with eyelash ollicles ( akahashi et al., 2013) (Fig. 2.21). Zeis glands are unilobular sebaceous glands that open directly into the ollicle. T e unction o their oily secretion is to lubricate the lashes to prevent them rom drying out and becoming brittle. Glands o Moll are modi ed sweat glands (apocrine) consisting o an unbranched spiral tubule. T e exact unction o these glands is unclear, although their secretion is rich in IgA, which suggest a role in the immune de ence o the ocular sur ace (Stoeckelhuber et al., 2003).  Blood and Ne rve Sup p ly Nerves of the Eyelids. T e levator palpebrae and orbicularis oculi muscles are innervated by the oculomotor and acial nerves, respectively. T e sensory supply o the upper lid derives rom branches o the ophthalmic nerve (supraorbital, supratrochlear and lacrimal). T e supply to the lower lid comes rom branches o the maxillary nerve (zygomatic, in raorbital).  Blood and Lymphatic Supply to the Eyelids. T e arterial supply derives rom branches o the ophthalmic, lacrimal and in raorbital arteries, which contribute to two palpebral arcades in the upper lid and one in the lower. Branches rom these arcades supply the skin, orbicularis, tarsal glands and

palpebral conjunctiva. Veins o the eyelids empty into veins o the orehead and temple, and some empty into the ophthalmic vein. Lymphatics drain to the preauricular and submandibular lymph nodes.  THE CO NJ UNCTIVA Gross Anatomy T e conjunctiva is a thin transparent mucous membrane that extends rom the eyelid margins anteriorly, providing a lining to the lids, be ore turning sharply upon itsel to orm the ornices, rom where it is re ected onto the globe, covering the sclera up to its junction with the cornea. It thus orms a sac that opens anteriorly through the palpebral ssure. T e conjunctiva is conventionally divided into the ollowing regions: marginal, tarsal, orbital (these three collectively orm the palpebral conjunctiva), bulbar and limbal (Fig. 2.22). T e static dimensions o the conjunctival sac in the primary position are illustrated in Fig. 2.23 (Ehlers, 1965). T e marginal zone extends rom a line immediately posterior to the openings o the tarsal glands and passes around the eyelid margin, rom where it continues on the inner sur ace o the lid as ar as the subtarsal old (a shallow groove that marks the marginal edge o the tarsal plate). T e tarsal conjunctiva is highly vascular and is rmly attached to the underlying brous connective tissue. From the convex border o the tarsal plate, the orbital zone

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Fig . 2.24 Hig h-p owe r slit-lamp vie w o the conjunctival p alisad e s o Vog t (aste risks) at the lowe r limb us.

extends as ar as the ornices. Over this region the conjunctiva is more loosely attached to underlying tissues, and so readily olds. Elevations o the conjunctival sur ace in the orm o papillae and lymphoid ollicles are commonly observed in this region. T e transparency o the bulbar conjunctiva readily permits the visualization o conjunctival and episcleral blood vessels. Here, the conjunctiva is reely movable owing to its loose attachment to enon’s capsule (the ascial sheath o the globe). As the bulbar conjunctiva approaches the cornea, its sur ace becomes smoother and its attachment to the sclera increases. T e limbal conjunctiva extends approximately 1–1.5 mm around the cornea. Its junction with the cornea is ill de ned, particularly in the vertical meridian, owing to a variable degree o conjunctival / scleral overlap. T e limbus has a rich blood supply, and in the majority o individuals a radial array o connective tissue elevations – the palisades o Vogt – can be seen adjacent to the corneal margin (Fig. 2.24). T e palisades are most prominent in the vertical meridian, and their visibility is enhanced in pigmented eyes.  Microscop ic Anatomy In histological section, two distinct layers can be resolved: an epithelium containing a variable number o goblet cells, and a vascular stroma that consists o a super cial lymphoid layer and a deep brous layer (Fig. 2.25). T e appearance o the conjunctiva shows a marked regional variability. Epithelium. In the marginal zone, the epithelium is strati ed and squamous with relatively ew goblet cells, although this has recently been disputed ollowing the description o intracellular crypts lined with goblet cells lying deep within the epithelium in the region o the so-called ‘lid wiper’ region (Knop et al., 2012). It has been suggested that a subpopulation o epithelial cells that lie close to the mucocutaneous junction may be acting as stem cells or the palpebral conjunctiva (Wirtscha er et al., 1999). Approaching the tarsus, the epithelium thins to 2–3 layers o cuboidal cells with scattered goblet cells. T e epithelium o the orbital zone is slightly thicker (2–4 cells) with more numerous goblet cells. T e number o goblet cells declines over the bulbar conjunctiva and at the limbus the epithelium is again strati ed squamous, and goblet cells are absent. T e limbus contains a

Fig . 2.25 Histolog ical se ction throug h the b ulb ar conjunctiva. E = e p ithe lium; S = stro ma. Gob le t ce lls can b e se e n in the e p ithe lium (arrows). The stroma can b e re solve d into an ad e noid laye r (arrowhe ad ) and a d e e p b rous laye r (aste risk).

Fig . 2.26 Histolog ical se ction throug h the p alisad e re g ion. Conne ctive tissue rid g e s can b e se e n p roje cting into the ove rlying e p ithe lium (arrows).

unique array o connective tissue ridges (the palisades o Vogt), which project into the overlying epithelium (Fig. 2.26). Clinical and experimental evidence suggests that the palisades are the repositories o stem cells and there ore act as the regenerative organ o the corneal epithelium (Dua and Azuara-Blanco, 2000). T e conjunctival epithelium additionally contains several non-native cell types, including dendritic cells, melanocytes and lymphocytes.  Goblet and Other Secretory Cells. Goblet cells provide the mucous component o the tear lm. T ey arise in the basal cell layers and migrate to the sur ace, there becoming ully di erentiated. Mature goblet cells are larger than the surrounding epithelial cells and contain a peripherally placed nucleus. T e cytoplasm is packed with membrane-bound secretory granules that discharge rom the apical sur ace in an apocrine manner. T e number o goblet cells shows a marked regional variation in density (Kessing, 1968) (Fig. 2.27), and these cells are occasionally seen lining intraepithelial crypts (o Henle).

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Fig . 2.27 Diag ram showing the re g ional variation in g ob le t ce ll d e nsity. Gob le t ce ll d e nsity is g re ate st ove r the caruncle , p lica se milunaris and in e rior nasal p alp e b ral conjunctiva. (Ad ap te d rom Bron, A. J., Trip athi, R. C. & Trip athi, B. (1997). Wol ’s Anatomy o the Eye and O rb it (8th e d .). Lond on: Chap man and Hall. Re p rod uce d ro m Bron, 1997.)

Fig . 2.28 Histolog ical se ction throug h a lymp hoid ollicle (F). Note the mod i cation o the ove rlying e p ithe lium (aste risk).

T e apices o many sur ace epithelial cells o the conjunctiva contain numerous carbohydrate-containing secretory vesicles, which are seen to migrate to the cell sur ace where they use with the plasma membrane (Dilly, 1985). It is likely that this represents a mechanism or recycling the cell surace glycocalyx rather than a secondary source o secretory mucin.  Conjunctival Stroma. T e conjunctival stroma (substantia propria) is variable in thickness. It can be resolved into two distinct layers: a super cial adenoid layer and a deeper brous layer (see Fig. 2.25). T e adenoid layer contains numerous lymphocytes with local accumulations in the orm o lymphoid ollicles (Fig. 2.28). Follicles represent aggregates o predominantly B cells, which orm part o the so-called conjunctiva-associated lymphoid tissue (Knop and Knop, 2005). T e adenoid layer also contains a large number o mast cells, which play a major role in ocular allergy (McGill et al., 1998). T e deep brous layer is generally thicker than the adenoid layer and contains the majority o conjunctival blood vessels and nerves.  Inne rvation and Blood Sup p ly Nerves. he conjunctiva receives nerves rom sensory, sympathetic and parasympathetic sources. Sensory nerves, which are trigeminal in origin, reach the conjunctiva via branches o the ophthalmic nerve. he principal unction o these ibres is to equip the conjunctiva with the ability to detect a variety o sensations – or example, touch, pain, warmth and cold. Sensory nerve terminals include both ree (unspecialized) nerve endings and the more complex corpuscular endings (classically re erred to as Krause end bulbs) (Lawrenson and Ruskell, 1991). Conjunctival blood vessels receive a dual autonomic innervation. Parasympathetic ibres issuing rom the pterygopalatine ganglion and sympathetic ibres rom the superior cervical ganglion are responsible or vasodilation and vasoconstriction, respectively. 

Fig . 2.29 Hig h-p owe r slit-lamp p hotog rap h showing the limb al vascular arcad e s. (Courte sy o Eric Pap as.)

Blood Vessels and Lymphatics. T e arterial supply derives rom two sources: palpebral branches o the nasal and lacrimal arteries, and anterior ciliary arteries. Palpebral vessels serve two vascular arcades within the eyelid. T e in erior (marginal) arcade sends branches through the tarsal plate to the eyelid margin and tarsal conjunctiva. T e superior (palpebral) arcade supplies the tarsal, orbital, ornix and bulbar conjunctiva. T e limbal zone, in contrast, is served by anterior ciliary arteries. T e anterior ciliary arteries travel along the tendons o the rectus muscles and give o branches at episcleral level prior to dipping down into the sclera to link with the major iridic circle. Episcleral branches pass orward and loop back a ew millimetres short o the cornea to become conjunctival vessels. Forward extensions o these vessels orm the limbal arcades (limbal loops), which are a complex network o ne capillaries (Fig. 2.29). Conjunctival veins are more numerous than arteries. T ey can be readily di erentiated rom arteries owing to their larger calibre, darker colour and more tortuous path. 

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Fig . 2.31 Low-p owe r lig ht microg rap h o the lacrimal g land . Acini are arrowe d . Ad ip ose conne ctive tissue (aste risks) e xte nd s across the g land . Fig . 2.30 Late ral vie w o the orb it showing the position o the lacrimal g land. The levator ap oneurosis (LA) p artially divid e s the g land into an orb ital (OD) and palpe bral (PD) d ivision. (Adapted rom Kron eld, P. C., McHug h, S. L. & Polyak, S. L. (1943). The Human Eye in Anatomical Transp are ncie s. Roche ste r, NY: Bausch & Lomb .)

Functional Consid e rations T e conjunctiva contributes the mucin component o the preocular tear lm and plays an important role in the de ence o the ocular sur ace against microbial in ection. Mucins are a amily o high-molecular-weight glycoproteins that include membrane-bound and secretory varieties (Cor eld et al., 1997; Gipson and Inatomi, 1997; Hodges and Dartt, 2013). Goblet cells are the primary source o secretory mucin, whilst surace epithelial cells o both the conjunctiva and cornea possess mucin-like molecules within their glycocalyx. T e conjunctiva also orms part o a common mucosal de ence system, which is an important component o the de ence o the human body against microorganisms (McClellan, 1997; Knop and Knop, 2005). T e conjunctiva possesses the immunological capacity or antigen processing, and cell-mediated and humoral immunity. Humoral immunity is provided by speci c antibodies (particularly immunoglobulin A [IgA]) produced by trans ormed B cells (plasma cells) in the stroma. lymphocytes orm the basis o cell-mediated immunity.  LACRIMAL SYSTEM T e lacrimal apparatus provides or the production and maintenance o the preocular tear lm. T e normal unction o this system is essential or the integrity o the ocular sur ace and the provision o a smooth re ractive sur ace. T e lacrimal apparatus comprises a secretory system that includes the main and accessory lacrimal glands, and a drainage system that consists o the paired puncta and canaliculi, the lacrimal sac and the nasolacrimal duct. Lacrimal Gland Gross Anatomy. T e main lacrimal gland is the key provider o the aqueous component o the tears. T e gland is located in a shallow depression o the rontal bone behind the superolateral orbital rim (Fig. 2.30). It is partially split by the aponeurosis o the levator palpebrae into an upper larger orbital lobe and

Fig . 2.32 Ele ctron microg rap h o p art o a lacrimal acinus showing lig ht and d ark se cre tory ce lls.

a lower palpebral lobe, which can o en be visualized through the conjunctiva upon lid eversion (Bron, 1986). T e gland is pinkish in colour, with a lobulated sur ace. Between 6 and 12 ducts leave the gland through the palpebral lobe and discharge into the conjunctival sac at the upper lateral ornix.  Microscopic Anatomy. T e lacrimal gland is tubuloacinar in orm (Fig. 2.31). Its secretory units (acini) contain secretory cells surrounded by myoepithelial cells (Ruskell, 1975). Acinar secretory cells show extensive olding o their plasma membrane and apical microvilli. Adjacent cells are linked by tight junctions that restrict di usion between cells. T e most prominent eature o these cells is the presence o abundant secretory granules. wo principal secretory cell subtypes have been identi ed on the basis o their granule content (Fig. 2.32). T e majority o cells contain dark granules (dark cells), with a smaller number o cells containing light granules (light cells). T e unctional signi cance o this heterogeneity is uncertain at present. Ducts consist o a single layer o cuboidal cells that lack secretory granules. Myoepithelial cells are dendritic cells that are closely associated with the perimeter o acini and ducts. It is likely that these contractile cells play a role in the expulsion o tears rom the gland. T e interstices o the gland contain numerous blood vessels and nerves. A large population o immune cells (particularly IgAsecreting plasma cells) are also ound between acini. 

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Fig . 2.33 Diag ram showing the role o the g astrointe stinal tract g e ne rating sp e ci c immunog lo b ulin A (Ig A) in the lacrimal g land . Antig e ns which challe ng e the ocular sur ace ultimate ly d rain to the g astrointe stinal (GI) tract whe re the y stimulate B ce lls in Pe ye r’s p atche s (g ut-associate d lymp hoid tissue ). Se nsitize d B ce lls the n p ass to the lacrimal g land via the circulation. SC = se cre tory comp one nt. (Ad ap te d rom Allansmith, M. R. (1992). The Eye and Immunolog y. Maryland He ig hts, MO : Mosb y. Cop yrig ht Else vie r 2002.)

Blood and Nerve Supply. T e arterial supply to the lacrimal gland is provided by the lacrimal artery, which enters the posterior border o the gland. Venous drainage occurs via the lacrimal vein. A rich autonomic innervation includes secretomotor (parasympathetic) bres that issue rom the pterygopalatine ganglion and sympathetic (vasomotor) bres rom the carotid plexus. T e lacrimal nerve traverses the gland to provide a sensory innervation to the conjunctiva and lateral aspect o the eyelid.  Accessory Lacrimal Glands. Numerous small accessory lacrimal glands, which include the eponymous glands o Wol ring and Krause, are ound within the conjunctival stroma. T ey have a particular predilection or the upper ornix and above the tarsal plate, and, on the basis o proportion o total lacrimal tissue, it has been estimated that they contribute 5–10% o aqueous tear volume. Structurally, they have a similar appearance to the lacrimal gland proper. However, true acini are absent and glands consist o elongated tubules that connect with ducts opening onto the conjunctival sur ace (Sei ert et al., 1993).  Functional Considerations. In addition to its role as the principal provider o the aqueous phase o the tear lm, the lacrimal gland is also a major component o the ocular sensory immune system, which acts as the rst line o de ence against microbial in ection (Sullivan and Sato, 1994). T e secretory immune system is mediated through secretory IgA. T e lacrimal gland is the main source o tear IgA and the gland contains a large number o IgAproducing plasma cells. T e mechanism by which an antigenic challenge o the ocular sur ace induces a lacrimal antibody response is not ully understood. However, as the administration o an antigen by a gastrointestinal route raises speci c IgA levels in tears, one suggested mechanism is that ocular antigens – a er drainage through the nasolacrimal duct – stimulate B cells in gut Peyer’s patches. T ese sensitized B cells then populate the lacrimal gland where they trans orm into plasma cells (Fig. 2.33).

Fig . 2.34 Illustration o the lacrimal d rainag e syste m. C = canaliculi; LS = lacrimal sac; P = p unctum; NLD = nasolacrimal d uct. (Ad ap te d rom Kron e ld , P. C., McHug h, S. L. & Polyak, S. L. (1943). The Human Eye in Anatomical Transp are ncie s. Roche ste r, NY: Bausch & Lomb .)

It has been demonstrated that the lacrimal gland also secretes into the tears growth actors that are important or the maintenance o the ocular sur ace and epithelial wound healing (P ug elder, 1998). Prominent amongst these growth actors are epidermal growth actor and trans orming growth actor beta.  Lacrimal Drainag e Syste m ears collect at the medial canthal angle, where they drain into the puncta o the upper and lower lids. Each punctum is a small oval opening approximately 0.3 mm in diameter that is located at the summit o an elevated papilla. From each punctum the canaliculus passes rst vertically or about 2 mm and then turns sharply to run medially or about 8 mm (Fig. 2.34). At the angle, a slight dilation, the ampulla, can be seen. T e canaliculi converge towards the lacrimal sac, usually orming a common canaliculus be ore entry. T e lacrimal sac occupies a ossa ormed by the maxillary and lacrimal bones. It measures 1.5–2.5 mm in diameter and approximately 12–15 mm in vertical length. From the lacrimal sac tears drain into the nasolacrimal duct, which extends or about 15 mm, passing through a bony canal in the maxillary bone, to an opening in the nose beneath the in erior nasal turbinate. A old o mucosa is o en observed at the termination o the duct: this has been termed ‘the valve o Hasner’, although there is no strong evidence that it unctions as a valve. T e process o tear drainage is an active process mediated by the contraction o the orbicularis during blinking (Doane, 1981). ears enter the canaliculi principally by capillary action. During the early part o the blink the puncta are occluded as the orbicularis urther contracts. T e canaliculi and lacrimal sac are also compressed, orcing uid into the nose. An alternative hypothesis has been proposed whereby orbicularis contraction dilates the sac, creating a negative pressure, which draws in the tears rom the canaliculi (Jones, 1961). An investigation by Paulsen et al. (2000) described a vascular plexus embedded in the wall o the lacrimal sac and duct that may in uence tear out ow. It is postulated that opening and closing o the lumen o

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Physical Pro p e rt ie s o f t he Pre o cular Te ar Film

Parame t e r

Value

O smolarity pH Thickne ss Volume Rate of p rod uction Unstimulate d Stimulate d Re fractive ind e x

302 (± 6.3) mO sm/l 7.45 3 µl 7.0 (± 0.2) µl 1–2 µl / min >100 µl / min 1.336

the lacrimal passages can be achieved by regulating blood ow within this plexus. 

The Pre o cular Te ar Film FUNCTIO N AND PRO PERTIES O F THE PREO CULAR TEAR FILM T e tear lm is a complex uid that covers the exposed parts o the ocular sur ace ramed by the eyelid margins. T e physical characteristics o this uid are summarized in able 2.2. Classically, the tear lm has been regarded as a trilaminar structure with a supercial lipid layer secreted by the meibomian glands, which overlies an aqueous phase derived rom the main and accessory lacrimal glands, and an inner mucinous layer consisting o membranespanning mucins o the ocular sur ace epithelium and secretory mucins produced mainly by conjunctival goblet cells. T e tear lm per orms several important unctions, which can be broadly classi ed as optical, metabolic support, protective and lubrication. By smoothing out irregularities o the corneal epithelium, the tear lm creates an even sur ace o good optical quality that is reormed with each blink. T e air–tear inter ace orms the principal re ractive sur ace o the optical system o the eye and provides twothirds (43 D) o its total re ractive power. As the cornea is avascular it is dependent on the tear lm or its oxygen provision. When the eye is open the tear lm is in a state o equilibrium with the oxygen in the atmosphere, and gaseous exchange takes place across the tear inter ace. T e constant turnover o the tear lm also provides a mechanism or the removal o metabolic waste products. ears play a major role in the de ence o the eye against microbial colonization. T e washing action o the tear uid reduces the likelihood o microbial adhesion to the ocular surace. Moreover, the tears contain a host o protective antimicrobial proteins. T e tear lm acts as a lubricant, smoothing the passage o the lids over the corneal sur ace and preventing the transmission o damaging shearing orces. o acilitate this, tear uid displays non-Newtonian behaviour with respect to shear ( i any, 1991). Newtonian uids maintain a constant viscosity with increasing shear rates. By contrast, tear uid has a relatively high viscosity between blinks to aid stability, and with increasing shear rates during the blink process the viscosity alls dramatically, thereby easing the movement o the lids over the ocular sur ace. Te ar Prod uction Jones (1966) rst used the terms ‘basic (basal)’ and ‘re ex’ to describe tear ow. He proposed that the accessory lacrimal glands were the basic (minimal ow) secretors, and that re ex secretion

Fig . 2.35 Schematic rep rese ntation o the orbital g lands, which contrib ute the various components o the preocular tear lm. (Adapted rom Dartt, D. A. (1992). Physiology o tear production. In M. A. Lemp & R. Marquardt (ed s) The Dry Eye: A Comprehensive Guide. Berlin: Springer-Verlag.)

(i.e. in response to strong physical or emotional stimulation) is mediated by the main lacrimal gland. However, Jordan and Baum (1980) questioned the concept o basic and re ex secretion, and suggested that it is more accurate to think o tear output as a continuum, whereby the rate o production is proportional to the degree o sensory or emotive stimulation (Dartt, 2009). T is concept would also mean that a unctional distinction between main and accessory lacrimal glands, in terms o basal and re ex tear production, is unnecessary. Rather, it is more likely that tear ow is the combination o contributions rom both glands, although the output rom the accessory glands alone is suf cient to maintain a stable tear layer (Maitchouk et al., 2000).  SO URCES AND CO MPO SITIO N ears are composed o a complex secretion that combines the products o several glands (Fig. 2.35). Although the precise composition o tear uid varies with collection method, ow rate and time o day, it can be considered as a watery secretion containing electrolytes and proteins, with lesser amounts o lipid and mucin.

26

TABLE

2.3

PART 1

Int ro d uct io n

Bio che mical Co mp o sit io n o f t he Pre o cular Te ar Film

Co mp o ne nt

Co nce nt rat io n

ELECTRO LYTES* Na + Cl− K+ HCO 3 − Ca 2+ Mg 2+

135 mEq / l 131 mEq / l 36 mEq / l 26 mEq / l 0.46 mEq / l 0.36 mEq / l

MAJ O R PRO TEINS* Lysozyme Se cre tory Ig A Lactofe rrin Lip ocalin Alb umin Ig G

2.07 g / l 3.69 g / l 1.65 g / l 1.55 g / l 0.04 g / l 0.004 g / l

LIPIDS† Wax e ste rs Chole ste ryl e ste rs Polar lip id s Hyd rocarb ons Die ste rs Triacylg lyce rid e s Fatty acid s Fre e ste rols

41% 27.3% 14.8% 7.5% 7.7% 3.7% 2.0% 1.6%

MUCIN ‡ MUC1 MUC5AC MUC4 MUC16

nd nd nd nd

(Data ad ap te d rom Ti any, 1997.) Source s: *Main and acce ssory lacrimal g land s. †Me ib omian g land s. ‡Ep ithe lial ce lls / g ob le t ce lls. nd = not d e te rmine d .

Ele ctrolyte s Human tears contain approximately the same range o electrolytes as ound in plasma ( i any, 1997). able 2.3 gives typical values or the ionic composition o human tears. However, as the electrolyte content o tears varies with ow rate, there is signi cant variation in measured values. During the process o secretion by the lacrimal gland, there is a process o active electrolyte transport that is coupled to the passive movement o water by an osmotic process. Acinar-derived uid is essentially an isotonic ultra ltrate o plasma. Its composition is altered as it passes along the ductal system, where urther chloride and potassium ions are secreted. A variety o ion transport proteins have been identi ed in acinar cells, including sodium–potassium A Pase and potassium and chloride channels.  Prote ins ear proteins are thought to originate rom three main sources: the lacrimal gland, ocular sur ace epithelia and conjunctival blood vessels. T e major lacrimal proteins include secretory IgA, lysozyme, lacto errin and lipocalin ( ormerly known as tear-speci c prealbumin) (see able 2.3). IgA, which is the major immunoglobulin in tears, is secreted as a dimer by plasma cells in the interstices between lacrimal acini. It then binds to a receptor on the basolateral aspect o acinar cells, and is transcytosed across the cell and secreted into tear uid. IgA

is a constitutively secreted lacrimal protein whose rate o secretion is independent o ow rate. During sleep, the levels o IgA increase as secretory IgA production continues and as acinar secretion declines (Sack et al., 1992). IgA plays an important role in the de ence o the ocular sur ace against microbial in ection by preventing bacterial and viral adhesion, and inactivating bacterial toxins. Other immunoglobulins (e.g. IgG and IgM) are present in tears at much lower levels. Lysozyme, lacto errin and lipocalin, in contrast, originate rom acinar cells and their rate o secretion roughly matches ow rate. Lysozyme is a well-known bacteriolytic protein that has the ability to lyse the cell wall o several Gram-positive bacteria. Lacto errin serves an important bacteriostatic unction by binding iron and making it unavailable or bacterial metabolism. It also acts as a ree radical scavenger, thereby reducing ree-radical-mediated cell damage ( i any, 1997). Lipocalins are a amily o lipid-binding proteins with an af nity or a broad array o lipids, including atty acids, phospholipids and cholesterol. It has been suggested that tear lipocalins act as scavengers or a wide range o meibomian lipids, which could spill onto the corneal sur ace and perturb its wettability (Glasgow et al., 2000). Furthermore, lipocalin may promote lipid solubility at the aqueous–lipid inter ace to acilitate the ormation o a thin layer o lipid on the sur ace o the tear lm.  Mucins Mucins are a amily o high-molecular-weight glycoproteins, o which sugars contribute up to 85% o their dry weight. Structurally, they consist o a polypeptide backbone to which chains o sugar molecules attach via O-linkages to the amino acids serine and threonine. Mucins are a heterogeneous group o molecules that can be subdivided into secretory and integrated-membrane varieties (Cor eld et al., 1997; Hodges and Dartt, 2013). So ar, modern molecular biology techniques have identi ed up to 20 mucin (MUC) genes, although only our o these (MUC1, MUC5AC, MUC4 and MUC16) are expressed on the human ocular sur ace (Gipson and Inatomi, 1997; McKenzie et al., 2000; P ug elder et al., 2000; Mantelli and Argüesco, 2008). T e epithelia o the cornea and conjunctiva express the transmembrane mucins MUC1, and to a lesser extent MUC4 and MUC16, which attach to apical microvilli where they orm a hydrophilic base to acilitate the spreading o the goblet-cell-derived mucin MUC5AC. Mucins play a major role in stabilizing and spreading the tear lm and provide protection against desiccation and microbial invasion (Gipson and Inatomi, 1997; Hodges and Dartt, 2013).  Lip id s T e source o lipids in the tear lm is the meibomian glands embedded within the tarsal plates o each lid. T e blinking process is an important mechanism in the expulsion o the secretion rom the glands ( i any, 1995). Meibomian lipid (also known as meibum) is delivered directly as a clear oil onto the lid margins and is spread over the tear lm rom the inner edge o the lid margins with each blink. T e thickness o the lipid layer is variable (mean thickness 42 nm, range 15–157 nm; KingSmith et al., 2000, 2010), and depending on thickness gives rise to characteristic inter erence patterns when viewed in specular re ection (Fig. 2.36) (Guillon, 1998). Meibomian secretion consists o a complex mixture o lipids ( able 2.3), including wax and cholesteryl esters (which together constitute approximately 70% o meibum), atty acids and atty alcohols ( i any, 1995; Butovich, 2013). T e primary unctions o this secretion are to

2

Fig . 2.36 Lip id laye r o the p re ocular te ar lm vie we d in sp e cular re f e ction. A ‘wave ’ ap p e arance can b e se e n, which re p re se nts the most commonly ob se rve d lip id p atte rn in the p op ulation.

provide a hydrophobic barrier at the lid margin to prevent overspill o tears, and to cover the sur ace o the tear lm to retard evaporation (Craig and omlinson, 1997).  MO DELS O F TEAR FILM STRUCTURE T e classical trilaminar model o tear lm structure in terms o a super cial lipid layer, a middle aqueous layer and deep mucin layer, rst proposed by Wol and subsequently modi ed by Holly and Lemp (1977), has received broad acceptance. However, the results o recent studies have led to a re-evaluation o the nature o the aqueous and mucinous layers. Several pieces o evidence have suggested that the mucin contribution to the tear lm is much greater than was previously thought (Prydal et al., 1992), and an alternative tear lm model, which possesses a substantial mucinous phase, has been proposed (Fig. 2.37). T e nature o the mucinous phase has not been ully established, but

Ant e rio r Eye

27

Fig . 2.37 Diag ram showing the comp osition o the p re ocular te ar lm. Inse ts sho w d e tails o the g lycocalyx and lip id –aq ue ous inte r ace . (Ad ap te d rom Corf e ld , A. P., Carring ton, S. D., Hicks, S. J. e t al. (1997). O cular mucins: p urif cation, me tab olism and unctions. Prog . Re tin. Eye Re s., 16, 627–656.)

is thought to consist o a mixture o soluble and gel- orming mucins (Hodges and Dartt, 2013). 

Co nclusio n It is clear rom the above account that our understanding o the structure and unction o the anterior eye is ar rom complete, which places certain limits on our understanding o clinical, contact-lens-related phenomena. It is essential, there ore, that uture research continues to ocus on undamental aspects o ocular anatomy and physiology, as well as on the more applied clinical applications that are described in the remainder o this book. Acce ss t he co mp le t e re fe re nce s list o nline at ht t p :/ / www.e xp e rt co nsult .co m.

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3

Visual O p t ics W NEIL CHARMAN

Int ro d uct io n T e human eye is a remarkable optical instrument (Navarro, 2009; Artal, 2014). Its per ormance has been honed by millennia o evolution to meet admirably the needs o the neural system that it serves. At its best, ew human-engineered photographic lens systems can match its semi eld o more than 90°, its range o -numbers rom about / 11 to better than / 3, and its near di raction-limited axial per ormance when stopped down under photopic light levels. Moreover, the ocus o the eye o the young adult can be adjusted with reasonable accuracy or distances between about 0.1 m and in nity. Nevertheless, all eyes su er rom a variety o regular and irregular aberrations, while a substantial subset displays clinically signi cant spherical and astigmatic re ractive errors. In addition, the ability to change the power o the crystalline lens to view near objects is an asset that declines with age, to disappear entirely by the mid 50s, when presbyopia is reached. T e invention o spectacles in the 13th century, and their subsequent relatively slow re nement, ollowed by the more rapid development o contact lenses in the 20th century, has done much to provide solutions to the problems o both re ractive error and presbyopia: improvements in the design o both types o lens continue to be made. Re ractive surgical techniques, including both laser-based methods that modi y the corneal contour and intraocular lenses, are beginning to compete with spectacle and contact lens corrections, although unanswered questions still remain concerning the long-term e cacy and sa ety o some o the procedures used. In this chapter the basic optics o the eye and its components will rst be reviewed. T is will be ollowed by a discussion o the modi cations that the correction o re ractive error – particularly by contact lenses – produces in actors such as spectacle magni cation, accommodation and convergence (Douthwaite, 2005). 

The Basic O p t ics o f t he Eye and Ame t ro p ia

T e distribution across the population o parameters such as sur ace radii, component spacing and re ractive indices has been studied by a variety o authors (McBrien and Barnes, 1984; Charman, 2010). Re ractive indices o the media vary little between eyes, apart rom the non-uni orm re ractive index distribution within the lens, which changes with age as the lens grows throughout li e (Pierscionek et al., 1988; Pierscionek, 1995; Jones et al., 2005; Kasthurirangan et al., 2008). Each dimensional parameter appears to be approximately normally distributed amongst di erent individuals (Stenstrom, 1946; Sorsby et al., 1957). T e values o the di erent parameters in the individual eye are, however, correlated so that the resultant distribution o re ractive error is strongly peaked near emmetropia, rather than being normal (Fig. 3.2). T is correlation is thought to be due to a combination o genetic and environmental actors; visual experience helping to ‘emmetropize’ the eyes actively ( roilo, 1992; Saunders, 1995; Wildsoet, 1997; Weale, 2003; Mutti, 2010; Flitcro 2014). T e apparently greater incidence o myopia in recent times has been attributed to the greater prevalence o near tasks and other changes in environment and li estyle (Rosen eld and Gilmartin, 1998; Pan et al., 2011).  MO DEL EYES AND AMETRO PIA Many authors have produced paraxial models o the emmetropic eye, based on typical measured values o the ocular parameters (T ibos and Bradley, 1999; Atchison and Smith, 2000; Rabbetts, 2007; Atchison, 2009). T ese simpli y the optical complexities o the real eye while having approximately the same basic imaging characteristics. Some examples are given in able 3.1; uller details o these and other more elaborate eye models (e.g. Goncharov and Dainty, 2007) are given in the cited re erences. Using the parameters o the model eyes it is straight orward to calculate the positions o the cardinal points, which, in thick-lens theory, can be used to summarize paraxial imagery (Fig. 3.3).

GENERAL O PTICAL CHARACTERISTICS T e amiliar, and deceptively simple, optical layout o the eye is shown in Fig. 3.1. About three-quarters o the optical power comes rom the anterior cornea, with the crystalline lens providing supplementary power that, in the pre-presbyope, can be varied to ocus sharply on objects at di erent distances. T e actual optical design is, however, subtle, in that all the optical sur aces are aspheric, while the lens, and probably also the cornea, displays a complex gradient o re ractive index. T ere is little doubt that such re nements play an important role in controlling aberration. 28

Fig . 3.1

Sche matic horizontal se ction o the human e ye .

3

It is, however, important to stress that these eye models are only representative. In practice, an eye o shorter or longer axial length may still be emmetropic. T is behaviour and the various possible origins o re ractive error are easy to understand in terms o these basic models. Consider, or simplicity, the generic reduced eye shown in Fig. 3.4, with a single re ractive sur ace o radius r, re ractive index n′ and axial length k′. T e power o the eye, Fe, is given by: Fe = (n' − 1) /r

Visual O p t ics

29

For a distant object (zero object vergence) the image vergence n′ / l′ equals Fe. For emmetropia we require that the image o the distant object lies on the retina, i.e. l′ = k′, implying that Fe = n′ / k′ = K′, where K′ = n′ / k′ is the dioptric length o the eye. T ere are, then, in principle an in nite number o matching pairs o values o Fe and K′ that lead to emmetropia, so that eyes that are relatively larger or smaller than the ‘standard’ models may still be emmetropic. In the case o ametropia Fe and K′ are no longer equal. I the power o the eye is too high (Fe > K′) we get myopia; i too low (Fe < K′) we get hypermetropia. T e ocular re raction K is given by: K= K' − Fe

T us, or example, myopia (K negative) can occur i K′ is too low, corresponding to an axial length k′ that is relatively too great (axial ametropia), or i Fe is relatively too large (re ractive ametropia). A high Fe may arise as a result o either too small a corneal radius r or because n′ is too large (note, however, that changes in n′ a ect both Fe and K′). Although more sophisticated eye models are characterized by more parameters, the possible origins o ametropia are essentially the same. Astigmatism can arise either because one or more o the optical sur aces is toroidal or because o tilts o sur aces with respect to the axis, particularly o the lens. How accurate do our models and associated calculations have to be? Although in the laboratory it may theoretically be possible to measure all the parameters o an individual eye, in general all that will be known in the consulting room is that the eye is ametropic. T us, in clinical contact lens practice, precise calculation o the optical e ects in the uncorrected or corrected eye is rarely possible; it is more important that the general magnitude o the e ects be borne in mind and that the approximate changes brought about by correction be ully understood.  ACCO MMO DATIO N AND THE PRECISIO N O F O CULAR FO CUS Fig . 3.2 Distrib ution o some ocular p arame te rs and o re ractive e rror. (A) Rad ius o curvature o the ante rior corne a. (B) Ante rior chamb e r (A.C.) d e p th. (C) Le ns p o we r. (D) Axial le ng th. (E) Sp he rical e q uivale nt re ractive e rror. In (A)–(D) the d ashe d curve re p re se nts the corre sp ond ing normal d istrib ution. Note that, whe re as ind ivid ual p arame te rs are d istrib ute d ap p roximate ly normally, re ractive e rrors are strong ly p e ake d ne ar e mme trop ia. (Afte r Ste nstrom, S. (1946). Unte rsuchung e n ub e r d e r Variation und Kovaration d e r op tische Ele me nte d e s me nschliche n Aug e s. Acta O p hthalmol., 15(Sup p l. 26). [Translate d b y Woo lf, D.]) TABLE

3.1

T e decline with age in the subjective amplitude o accommodation (i.e. the reciprocal o the distance, measured in metres, o the nearest point at which vision remains subjectively clear to the distance-corrected patient) is illustrated in Fig. 3.5A. Few everyday tasks require accommodation in excess o about 4 D, so that it is normally only as individuals approach 40 years o age that marked problems with near vision start to appear. It is, however, important to recognize that, even or objects lying

Parame t e rs o f So me Paraxial Mo d e ls o f t he Human Eye Sche mat ic Eye (mm)

Simp lifie d Sche mat ic Eye (mm)

Re d uce d Eye (mm)

Surface rad ii (mm)

Ante rior corne a Poste rior corne a Ante rior le ns Poste rior le ns

7.80 6.50 10.20 −6.00

7.80 — 10.00 −6.00

5.55 — — —

Distance s from Ante rior Corne a (mm)

Poste rior corne a Ante rior le ns Poste rior le ns Re tina

0.55 3.60 7.60 24.20

— 3.60 7.20 23.90

— — — 22.22

Re fract ive Ind ice s

Corne a Aq ue ous humour Le ns Vitre ous humour

1.3771 1.3374 1.4200 1.3360

— 1.333 1.416 1.333

— 1.333 — —

(Data from Charman, W. N. (1991) O p tics of the human e ye . In W. N. Charman (e d .) Vision and Visual Dys unction. Vol. 1: Visual O p tics and Instrume ntation (p p . 1–26). Lond on: Macmillan.)

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Fig . 3.3 Examp le s o p araxial mod e ls o the human e ye . In e ach case F and F ′ re p re se nt the f rst and se cond ocal p oints, re sp e ctive ly, P and P ′ the f rst and se cond p rincip al p oints, and N and N ′ the f rst and se cond nod al p oints. (A) Unaccommod ate d sche matic e ye with our re racting sur ace s. (B) Simp lif e d , unaccommod ate d e ye with thre e re racting surace s. (C) Re d uce d e ye with a sing le re racting sur ace . (Ad ap te d from Charman, W. N. (1991). O p tics of the human e ye . In W. N. Charman (e d .) Vision and Visual Dys unctio n. Vol. 1: Visual O p tics and Instrume ntation (p p . 1–26). Lond on: Macmillan.)

Fig . 3.5 (A) The d e cline in monocular sub je ctive amp litud e o acco mmod ation, re e re nce d to the sp e ctacle p lane , with ag e . (B) Typ ical ste ad y-state accommod ation re sp onse / stimulus curve , showing lag s o accommod ation or ne ar stimuli. (Data in (A) from Duane , A. (1922). Stud ie s in monocular and b inocular accommod ation with the ir clinical imp licatio ns. Am. J. O p hthalmol., 5, 865–877.) Fig . 3.4 A g e ne ric re d uce d e ye mod e l, with p arame te rs as ind icate d . r is the rad ius o curvature o the re racting sur ace , k′ the axial le ng th and n ′ the re ractive ind e x. The e ye shown is hyp e rme trop ic.

within the available range o accommodation, accommodation is rarely precise. ‘Lags’ o accommodation usually occur in near vision and ‘leads’ or distance vision (Fig. 3.5B). As the accommodation system is driven via the retinal cones, these lags increase i the environmental illumination is reduced to mesopic levels and the accommodation system is e ectively inoperative at scotopic light levels, when the system reverts to its slightly myopic (around −1 D) tonic level (Ciu reda, 1991, 1998). 

Co rne al To p o g rap hy It has already been stated that the optical sur aces o the eye are not necessarily spherical. T e topography o the anterior cornea is o particular interest since, as the dominant re ractive surace, its orm has a major inf uence on overall re ractive error and ocular aberration. In contact lens work, it is o enormous importance to the tting geometry.

We have already seen (see Fig. 3.2A) that the radius o curvature over the central region, as measured by conventional keratometers, shows considerable individual variation, and it has been recognized or more than a century that many corneas display marked astigmatism. Corneal astigmatism is not, o course, necessarily equal to the total ocular astigmatism, as additional astigmatism (residual astigmatism) may be contributed by the crystalline lens. Earlier work on corneal topography using modi cations o traditional keratometers concentrated on approximating the orm o the corneal sur ace by a conicoid, in which each meridian is a conic section. In this approach the anterior corneal sur ace can be described by the ollowing equation (Bennett, 1966, 1988): x2 + y2 + pz2 = 2r 0 z

where the coordinate system has its origin at the corneal apex, z is the axial coordinate, r0 is the radius o curvature at the cornea apex and the shape actor p is a constant parameter characterizing the orm o the conic section or the individual eye. Values o p < 0 represent hyperboloids, p = 0 paraboloids, 0 < p < 1 f attening (prolate) ellipsoids, p = 1 spheres and p > 1 steepening (oblate)

3

Visual O p t ics

31

Fig . 3.6 (A) Histog ram showing the d istrib ution o the shap e actor, p , in 176 e ye s. (B) Typ ical re sult rom a top og rap hic instrume nt, showing the local variation in nominal sp he rical p owe r across our astig matic corne as. (Ad ap te d from Kie ly, P. M., Smith, G. & Carne y, L. G. (1982). The me an shap e of the human corne a. O p tica Acta, 29, 1027–1040.)

ellipsoids. T e same equation is sometimes written in terms o the Q- actor or the eccentricity e o the conic section, where: p = 1 + Q = 1 − e2

Kiely et al. (1982) ound mean r0 and p values o 7.72 ± 0.27 mm and 0.74 ± 0.18, respectively, these values being supported by the results o Guillon et al. (1986), that is, 7.85 ± 0.25 mm and 0.85 ± 0.15; broadly similar p values are ound in di erent racial groups: 0.70 ± 0.12 in Chinese eyes (Zhang et al., 2011) and 0.74 ± 0.19 in A ro-Americans (Fuller and Alperin, 2013). T us the typical general orm o the cornea is that o a f attening ellipsoid, with the curvature reducing in the periphery (Fig. 3.6A). Recent years have seen the introduction o a range o topographic instruments, marrying optical with electronic and computer technology, that can routinely give a much uller picture o the corneal contour (see Chapters 36, 41). T ese videokeratographic and scanning-slit results show that the conicoidal model is only a rst approximation to corneal shape and that individual eyes show a wide range o individual asymmetries. In particular, the rate o corneal f attening is o en di erent in di erent meridians (Fig. 3.6B), while the corneal cap o steepest curvature may be displaced with respect to the visual axis, on average lying about 0.8 mm below (Mandell et al., 1995). More elaborate models have been devised to describe these asymmetries in corneal shape (Navarro et al., 2006) Currently the most popular orm o output or the topographic data is probably a colour-coded map o the cornea, showing regions o di erent axial (sagittal) power (see Chapter 36). T is may be slightly misleading, since each local area o the cornea is toroidal rather than spherical. For this reason both sagittal and tangential power maps are o en used (Mount ord et al., 2004). It is possible that other orms o representation, such as those that plot local departures in height rom a best- tting sphere, will ultimately prove more use ul, particularly in relation to the tting o rigid contact lenses (Salmon and Horner, 1995; Horner et al., 1998). T e contribution o the cornea to the overall ocular wave aberration can be deduced rom the videokeratogram (Hemenger et al., 1995; Guirao and Artal, 2000). Scanning slit instruments, such as the Orbscan and Pentacam, allow the orm o the posterior sur ace o the cornea to be deduced, as well as that o the anterior sur ace (see Chapters 36, 41). 

Fig . 3.7 Formation o the re tinal b lur circle or a myop ic e ye . D is the p up il d iame te r and d is the b lur circle d iame te r.

Pup il Diame t e r and Re t inal Blur Circle s As will be discussed below, although the retinal image is always blurred by both aberration and di raction, in ametropia and presbyopia it is o en de ocus blur that is the major source o degradation. De ocus will occur whenever the object point lies outside the range o object distances embraced by the ar and near points o the individual. As noted earlier, even within this range, small errors o ocus will normally occur owing to the steady-state errors that are characteristic o the accommodation system. Using a reduced eye model and simple geometric optical approximations (Smith, 1982, 1996; Atchison and Smith, 2000; Rabbetts, 2007) – which are normally valid or all errors o ocus over about 1 D – such blur depends on the dioptric error o ocus and the pupil diameter. From Fig. 3.7 it can be seen that, or any object point and assuming that the eye pupil is circular, spherical de ocus produces a ‘blur circle’ on the retina. Using similar triangles, it is easy to show that the diameter (d, in mm) o this blur circle is: d = ΔFD/K'

where ΔF is the dioptric error o ocus with respect to the object point, D is the pupil diameter in millimetres and K′ is the dioptric length o the eye. I astigmatism is present, the blur patch

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is an ellipse, with major and minor axes corresponding to the ocus errors in the two principal meridians. We can express the blur circle diameter in angular terms as: α = ΔFD 10 − 3 rads = 3.44ΔFD min arc

Eq. 3.1

T us, or a 3 mm diameter pupil, the blur circle diameter increases by roughly 10 min arc per dioptre o de ocus. Chan et al. (1985) measured blur circle diameters experimentally and ound that results or pupil diameters between 2 and 6 mm and de ocus between 1 and 12 D were quite accurately predicted by Eq. 3.1. T e impact o blur on visual acuity depends somewhat on the acuity target chosen and the criteria and observation conditions used. We would expect the minimum angle o resolution (MAR) to be somewhat smaller than the blur circle diameter. Smith (1996) suggests that, or errors o ocus above about 1 D, letter targets, a 50% recognition rate, and normal chart luminances o about 150 cd / m 2 (giving pupil diameters o about 4 mm): MAR = 0.65ΔFD min arc

Eq. 3.2

With errors o ocus smaller than about 1 D, di raction, aberration and the neural capabilities o the visual system are more important than de ocus blur and the MAR exceeds that predicted by Eq. 3.2. T e natural pupil diameter is chief y dependent on the ambient light level. Fig. 3.8 shows typical results or this relationship in young adults. Pupil diameters at any light level tend to decrease with age (senile miosis: Winn et al., 1994) and with accommodation, as well as varying with a variety o emotional and other actors (Loewen eld, 1998). Some typical values or older eyes under di erent lighting conditions are given in able 3.2. Clearly, reducing the pupil size results in smaller amounts o blur in the retinal image or any given level o de ocus, and thus the depth o ocus is increased. For example, an uncorrected low myope may experience minimal levels o distance blur under good photopic levels o illumination but may notice considerable blur when driving at night, when the pupil is large. Pupil diameter strongly in luences the design and per ormance o bi ocal and other types o contact lens or the presbyope (Koch et al., 1991; see Chapter 23). 

Effe ct s o f Diffract io n and Ab e rrat io n As noted above, these are chie ly important when the eye is close to its optimal ocus. he point image or a spherical error o ocus then no longer approximates to a blur circle (or a point in the absence o de ocus) but has more complex orm. DIFFRACTIO N I the optical per ormance o the eye were limited only by di raction, the in- ocus retinal image o a point object would be an Airy di raction pattern. T e angular radius o the rst dark ring in this pattern would be: θmin = 1.22λ/D radians = 4194λ/D min arc Fig . 3.8 De p e nd e nce o p up il d iame te r on f e ld luminance in young ad ults. (Ad ap te d from Farre ll, R. J. & Booth, J. M. (1984). De sig n Hand b ook or Imag e ry Inte rp re tation Eq uip me nt (Se c. 3.2, p . 8). Se attle , WA: Boe ing Ae rosp ace Co.)

TABLE

3.2

where the wavelength λ and the pupil diameter D are expressed in the same units. It is usually assumed that it will be possible to resolve the images o two identical point objects i their angular separation equals this value (the Rayleigh limit). 

Me ans, St and ard De viat io ns and (Bracke t e d ) Rang e s o f Pup il Diame t e r in Vario us Visual Tasks and Illuminance s fo r Pre sb yo p ic Pat ie nt s o f Diffe re nt Ag e s

Co nd it io n Nig ht d riving Re ad ing (low illumination, 215 lux) Re ad ing (hig h illumination, 860 lux) O utd oors (ind ire ct sunlig ht, 3400 lux) O utd oors (d ire ct sunlig ht, 11 000 lux)

Pup il Diame t e r Ag e s 40–49 (mm)

Pup il Diame t e r Ag e s 50–59 (mm)

5.2 ± 0.8 (3.8–6.2) 3.5 ± 0.6 (2.l6–4.6) 2.9 ± 0.5 (2.2–3.9) 2.7 ± 0.5 (1.9–3.4) 2.3 ± 3.4 (1.8–3.1)

4.6 ± 0.8 (3.1–5.8) 3.0 ± 0.5 (2.3–4.4) 2.6 ± 0.3 (2.1–3.6) 2.5 ± 0.4 (1.9–3.4) 2.2 ± 0.3 (1.8–2.9)

(Data from Koch D. D., Samue lson S. W., Haft E. A. & Me rin L. M. (1991). Pup illary size and re sp onsive ne ss. Imp lications for sele ction of a b ifocal intrao cular le ns. O p hthalmolog y, 98, 1030–1035.)

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MO NO CHRO MATIC ABERRATIO NS Aberration obviously acts to introduce additional blur into both in- ocus and out-o - ocus images. Monochromatic aberration can arise rom a variety o causes. T e eye would be expected to display the classical Seidel aberrations (spherical aberration, coma, oblique astigmatism, eld curvature and distortion) inherent in any system o spherical centred sur aces but, due to the various asphericities, tilts, decentrations and irregularities that may occur in its optical sur aces (see Fig. 3.6B), its aberrational behaviour is much more complex than that which would be expected on the basis o simple schematic eye models o the type illustrated in Fig. 3.3 and able 3.1. Early authors attempted to analyse ocular aberration in terms o the individual Seidel aberrations. However, these attempts were o limited value because o the lack o rotational symmetry in the system. Monochromatic aberration is now most commonly expressed in terms o the wave ront aberration (Atchison, 2004; Charman, 2005). T e behaviour o a ‘per ect’ optical system, according to geometrical optics, can be visualized either as involving rays radiating rom an object point to be converged to a unique image point, or as spherical wave ronts diverging rom the object point to converge at the image point, so that the object point is the centre o curvature o the object wave ronts and the image point is that o the image wave ronts (Fig. 3.9A). T e rays and wave ronts are everywhere perpendicular to one another. I we have aberration, the image rays ail to intersect at a single image point. Similarly, the wave ronts, which are still everywhere perpendicular to the rays, are no longer spherical (Fig. 3.9B). It is usual to express the wave ront aberration at any point in the pupil as the distance between the ideal spherical wave ront, centred on the gaussian image point, and the actual wave ront, where both are selected to coincide at the centre o the exit pupil (Fig. 3.9C). Recent years have seen an explosion o interest in ocular aberrations, largely uelled by the realization that the earlier excimer-laser re ractive surgical techniques o en resulted in poor vision because these procedures introduced unacceptably high levels o aberration. As a result, a variety o commercial aberrometers have become available or measuring the waveront aberration o the eye (Krueger et al., 2004; Atchison, 2005). One o the more elegant designs involves the use o a Hartmann–Shack wave ront sensor (Liang et al., 1994, 1997; Liang and Williams, 1997). A regular array o identical microlenses allows the slope o the wave ront across a lattice o points in the pupil to be determined. T e principle can be understood with re erence to Fig. 3.10. Suppose we have a point source on the retina o a per ect emmetropic eye. T e light leaving the eye can be envisaged either as a bundle o parallel rays or as a series o plane waveronts (Fig. 3.10A). We now place our array o microlenses in the path o the emerging light. Evidently each lens will converge the parallel rays to its second ocal point, so that in the common ocal plane we shall see an absolutely regular array o image points. I now the eye su ers rom aberration, the emergent rays are no longer parallel and the associated wave ronts are no longer f at (Fig. 3.10B). T us the rays no longer come to a ocus on the axes o the lenses; the lateral displacement rom the ocal point o each lens is directly proportional to the local inclination o the ray or the slope o the wave ront. It is, then, easy to calculate the orm o the emergent wave ronts and the waveront aberration rom the distorted pattern o image points.

Visual O p t ics

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Examples o some typical axial results or normal eyes corrected or any spherocylindrical re ractive error are shown in Fig. 3.11. T e wave ront error is usually expressed in microns (micrometres). Departures rom the re erence sphere (in this case o in nite radius) o more than a quarter o a wavelength (i.e. around 0.14 µm or the green region o the spectrum) would be expected to degrade image quality. What is striking is the wide variation between the aberrations shown by di erent eyes. T e aberration in the central 2–3 mm o the pupil is usually modest, but much larger amounts may be ound in the periphery o dilated pupils. On the basis o wave ront aberration results, it is possible to calculate monochromatic point and line spread unctions and also the ocular modulation and phase trans er unctions or any pupil diameter (Hopkins, 1962). Note that the wave ront maps shown in Fig. 3.11 were obtained on axis with the eyes under cycloplegia. In each case, ocular aberrations get worse nearer to the peripheral pupil, as with most optical systems. In practice, the aberrations on the visual axis o each individual eye vary slightly with time owing

Fig . 3.9 (A) With a p e r e ct le ns, rays rom the ob je ct (O ) conve rg e to a sing le imag e p oint. Alte rnative ly we can visualize d ive rg e nt sp he rical wave ronts (shown as d ashe d line s) rom the ob je ct p oint co nve rg ing as sp he rical wave ronts to the imag e p oint. (B) I the le ns su e rs rom ab e rration, the imag ing rays ail to conve rg e to a sing le p oint and the corre sp ond ing wave ronts are not sp he rical. (C) The wave ront ab e rration, W ′, is sp e cif e d as the d istance b e twe e n the id e al wave ront, or re e re nce sp he re , ce ntre d on the g aussian imag e p oint, O ′, and the actual wave ront in the e xit p up il. It is usually ad juste d to b e ze ro at the ce ntre o this p up il.

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Fig . 3.10 Principle o the Hartmann–Shack techniq ue . (A) E ects with a pe r ect e mmetropic eye, where the imag es are ormed on the axis o each microle ns and hence are regularly sp aced . (B) E ects with an ab errated eye, where the image array is irre g ular as the imag es are no long er ormed on the axe s o the le nse s (se e te xt). f′ is the ocal le ngth o the microle nse s.

(A–C) the signal-to-noise o the Hartmann–Shack point images is poor in some cases; this may lead to errors in the estimates o the corresponding local slope and orm o the wave ront. Although the basic wave ront map gives a use ul impression o the orm and extent o the wave ront errors, it is help ul to be able to quanti y this in some way. Various methods are available, but those that are the most popular at the present time are the total root mean square (RMS) wave ront error and the values o the Zernike coe cients or the wave ront error. T e basic method or obtaining RMS wave ront error or any diameter o pupil is easily understood. We divide the pupil into equal small areas, and sum the squared values o the wave ront error or each small area. T is sum is then divided by the number o areas and the square root o this result gives the RMS wave ront error. It can be shown that, i the RMS aberration is less than a 14th o a wavelength (i.e. about 0.04 µm), there is negligible loss in retinal image quality in comparison with an eye whose per ormance is limited only by di raction. Obviously or any eye the RMS error will vary with pupil diameter: in general, as the wave ront aberration tends to increase in the outer zones o the dilated pupil, the RMS aberration increases with pupil diameter. Applegate et al. (2007) investigated axial RMS wave ront errors as a unction o pupil diameter and age in a large sample o normal eyes that were corrected or spherical and cylindrical re ractive error. able 3.3 gives the means and standard deviations o their data or subjects aged 30–39 years. It is interesting to note that the typical axial RMS wave ront error or a 3 mm pupil (see able 3.3) is close to the limit at which the image di ers negligibly rom that rom an aberrationree system (about 0.04 µm). T e luminance at which this pupil diameter is ound, a ew hundred cd / m 2, corresponds to that ound on cloudy days in the UK. T us, in most eyes, wave ront aberration can play only a minor role in vision under daylight conditions. o give some clinical insight into the image degradation caused by these levels o RMS wave ront aberration, we can roughly evaluate the blurring e ect o the RMS aberration by equating it with those o an ‘equivalent de ocus’ – that is, the spherical error in ocus that produces the same magnitude o RMS aberration or the same pupil size. T e equivalent de ocus is given by: Equivalent defocus (D) = 4.31/2 [RMS error] /R2

Fig . 3.11 (A–C) Wave ront se nsor imag e s on the visual axis or thre e e ye s with a p up il d iame te r o 7.3 mm. An ab e rration- re e e ye wo uld g ive a re g ular he xag onal lattice o p oints. (D–F) Corre sp ond ing d e rive d wave ront ab e rration. Contours are at 0.15 µm inte rvals or sub je ct O P and at 0.3 µm inte rvals or sub je cts JL and ML. The p e ak-to-valle y wave ront e rror or a 7.3 mm p up il is ab out 7, 4 and 5 µm or JL, O P and ML, re sp e ctive ly. Note that or an ab e rration- re e e ye the re would b e a comp le te ab se nce o contours. (Re p rod uce d with p e rmission from Liang , J. & Williams, D. R. (1997). Ab e rrations and re tinal imag e q uality of the human e ye . J. O p t. So c. Am. A, 14, 2873–2883.)

to actors such as accommodation f uctuations and tear-layer changes a er a blink (Ho er et al., 2001; Cheng et al., 2004; Montés-Micó et al., 2004). T ere will also be variation in the measured wave ront errors owing to the limited reliability o any aberrometer. It can be seen, or example, that in Fig. 3.11

where the RMS aberration is measured in microns and the pupil diameter, R, in millimetres. able 3.3 includes values or the equivalent de ocus at each pupil diameter; except at the largest pupil diameter, the equivalent de ocus is always less than 0.25 D. Although the assumption that equal RMS error produces equal degradation o vision is not completely justi ed (Applegate et al., 2003), it is evident that, in normal eyes, the impact o optical blur due to monochromatic aberration is modest under most photopic conditions. For comparison, the reliability o clinical re ractive techniques is only around ±0.3 D (O’Leary, 1988; Bullimore et al., 1998). T e second common way o speci ying aberrations is in terms o Zernike coe cients (Atchison, 2004; Charman, 2005). T e idea here is that, as very di erent orms o wave ront can have the same total RMS error yet still produce somewhat di erent e ects on vision, it is better to break the complex waveront patterns o the type shown in Fig. 3.11 into a set o simpler ‘building blocks’. Each ‘block’, mathematically described by a

3

TABLE

3.3

35

Variat io n in t he Me an Axial Hig he r-o rd e r Mo no chro mat ic RMS Wave fro nt Erro r and it s St and ard De viat io n in t he Eye s o f Sub je ct s Ag e d 30–39 Ye ars*

Pup il Diame t e r (mm) 3 4 5 6 7

Visual O p t ics

Typ ical Luminance Le ve l (cd / m 2 )

RMS Wave fro nt Erro r (µm)

Eq uivale nt De fo cus (D)

400 70 7 0.1 0.0005

0.052 ± 0.022 0.102 ± 0.041 0.174 ± 0.062 0.289 ± 0.091 0.513 ± 0.138

0.16 0.18 0.19 0.22 0.29

*Also g ive n is the typ ical amb ie nt luminance le ve l at which the natural p up il d iame te rs occur (take n rom Fig . 3.8) and the e q uivale nt d e ocus (se e te xt). (Data from Ap p le g ate , R. A., Donne lly, W. J., Marsack, J. D. e t al. (2007). Thre e -d ime nsional re lationship b e twe e n hig he r-ord e r root-me an-sq uare wave front e rro r, p up il d iame te r, and ag ing . J. O p t. Soc. Am. A, 24, 578–587.)

Zernike polynomial, corresponds to a speci c type o wave ront de ormation: some o these are closely related to the traditional Seidel aberrations. T e set o polynomials, named a er their originator Fritz Zernike (1888–1966), has the advantage that the individual polynomials are mathematically independent o one another. T e overall complex wave ront can then be specied in terms o the size o the contributions made by each o these constituent wave ront de ormations: the size o the contribution that each makes is given by the value o the coe cient o the corresponding polynomial. In the recommended ormulation in current use, each coe cient gives the RMS wave ront error (in microns) contributed by the particular Zernike mode (Atchison, 2004; Charman, 2005): the overall RMS wave ront error is given by the square root o the sum o the squares o the individual coe cients. T e relative sizes o the di erent Zernike coe cients thus give detailed in ormation on the relative importance o the di erent aberrational de ects o any particular eye. T e Zernike polynomials can be expressed in terms o polar coordinates (ρ, θ) in the pupil, where ρ = R / Rmax is the relative radial coordinate, Rmax being the maximum pupil radius, and θ is the azimuthal angle, de ned in the same way as in the optometric notation, except that it can rise to 360°. Each polynomial, or wave ront building block, is de ned by the highest power (n) to which ρ is raised (the radial order) and the multiple (m) or the angle θ (the angular requency): m = −2, or example, means that θ appears as sin2θ, while m = +3 means that it appears as cos3θ. T e polynomials and coe cients are, then, conveniently m described as Zm n and Cn respectively. Fig. 3.12 shows the rst ew levels o the ‘Zernike tree’ ormed by the di erent polynomials, the levels corresponding to successively greater powers o n. T e top two rows o the tree (n = 0 and n = 1) are o no signi cance or image quality: piston (n = 0) just corresponds to a longitudinal shi o the wave ront and tilts (n = 1) to small prismatic shi s in the image point. T e second-order terms (n = 2) all depend upon the square o the radius in the pupil. T is is, o course, a amiliar eature o the sag ormula and in act Z02 represents spherical de ocus and the other terms astigmatism in crossed-cylinder orm, with the principal meridians either at 45 / 135 ( Z−− 22 ) or 90 / 180 ( Z− 22 ). T us, collectively, the secondorder terms correspond to our amiliar spherocylindrical de ocus and can be compensated or by an appropriate contact lens or other type o correction. T e higher-order (third and greater) polynomials represent the residual aberrations, which, in the past, it has not normally been possible to correct. Clinically these higher-order aberrations have o en been described rather loosely by terms such as ‘irregular astigmatism’ and ‘spherical

aberration’. T e third order includes vertical and horizontal primary coma and the ourth order primary spherical aberration. What levels o Zernike aberrations are ound on the visual axis in normal eyes? It must be remembered that, like the total RMS aberration, the values will tend to increase with pupil diameter, but a variety o studies involving large numbers o subjects give very similar results (Salmon and van de Pol, 2006). T e study by Applegate et al. (2007) generated mean values or the magnitudes o di erent types o third- and ourth-order Zernike aberration or di erent pupil sizes and age (coe cients or still higher-order Zernike modes are usually much smaller). able 3.4 gives their values or 30–39-year-old eyes. Note that, where appropriate, the coe cients or similar, but di erently oriented, Zernike polynomials have been combined. It is evident that, at the smaller 3 mm pupil size, third-order coma and tre oil aberrations tend to dominate over ourth-order aberrations, including spherical aberration, although spherical aberration becomes comparable to coma or the larger 6 mm pupil. A somewhat di erent picture emerges i we average the signed coe cients, rather than considering the RMS values. Fig. 3.13 gives some typical data, in this case or a large sample (109) o normal eyes with a pupil diameter o 5.7 mm (Porter et al., 2001). What is striking is that almost all the modes have a mean close to zero, although individual eyes may have substantial aberration, as shown by the relatively large standard deviations. A notable exception is the j = 12, Z04 spherical aberration mode, where the mean is positive and di ers signi cantly rom zero. T us, the picture that emerges is that most eyes have a central tendency to be ree o all higher-order aberration, except or spherical aberration, which shows a signi cant bias towards slight positive (undercorrected) values. T e Zernike coe cients o normal individual eyes vary randomly about these mean values in a way that presumably depends upon the idiosyncratic sur ace tilts, decentrations and other asymmetries o the individual eye. T e aberrations o eyes where pathology, such as keratoconus, is present may, however, be much larger. Away rom the visual axis, the major contribution to retinal image blur in the axially corrected eye is usually oblique astigmatism (Atchison, 2012a). T e magnitude o the dioptric di erence between the sagittal and tangential oci is similar in most eyes. Atchison and Smith (2000) suggest that this di erence between the two power errors can be described by: A (θ) = 2.66 × 10 − 3 θ2 − 2.09 × 10 − 7 θ4

where θ degrees is the eld angle with respect to the visual axis and the oblique astigmatism, A(θ), is in dioptres. Although the amount o astigmatism shows little variation, the relationship

36

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Int ro d uct io n

Fig . 3.12 The f rst f ve le ve ls o the Ze rnike ‘p yramid ’ or ‘tre e ’ showing the contour map s corre sp ond ing to the f rst 15 Ze rnike p olynomia ls (up to the ourth ord e r). The contour scale is arb itrary and , in the ind ivid ual e ye , will vary with the coe f cie nt o e ach p olynomial. Rows re p re se nt succe ssive ord e rs, n (i.e . the maximal p owe r to which the normalize d p up il rad ius is raise d ) and columns d i e re nt azimuthal re q ue ncie s, m. Also shown (in b racke ts) are the ind e x numb e rs, j, o the p olynomials and some o the name s use d to d e scrib e the m: p olynomials (11) and (13) are o te n calle d se cond ary astig matism. H / V astig matism = horizontal / ve rtical astig matism.

TABLE

3.4

Me an Ab so lut e Le ve ls RMS Wave fro nt Erro rs (WFE) o f Diffe re nt Typ e s o f Hig he r-o rd e r Ze rnike Ab e rrat io n, and t he ir St and ard De viat io ns, fo r 30–39-ye ar-o ld Sub je ct s and Tw o Pup il Diame t e rs

Ab e rrat io n Tre foil (j = 6 and 9) Coma (j = 7 and 8) Te trafoil (j = 10 and 14) Se cond ary astig matism (j = 11 and 13) Sp he rical ab e rration Total hig he r-ord e r RMS (j = 12)

Co mb inat io n o f Co e fficie nt s

RMS WFE (µm) fo r 3 mm Pup il Diame t e r

RMS WFE (µm) fo r 6 mm Pup il Diame t e r

0.027 ± 0.017 0.031 ± 0.022 0.010 ± 0.004 0.015 ± 0.008

0.139 ± 0.089 0.136 ± 0.087 0.056 ± 0.030 0.055 ± 0.027

0.014 ± 0.010 0.052 ± 0.022

0.130 ± 0.090 0.289 ± 0.091

(Data from Ap p le g ate , R. A., Donne lly, W. J., Marsack, J. D., e t al. (2007) Thre e -d ime nsional re lationship b e twe e n hig he r-ord e r root-me an-sq uare wave front e rror, p up il d iame te r, and ag ing . J. O p t. Soc. Am. A, 24, 578–587.)

3

Fig . 3.13 Typ ical d ata or the me ans o the sig ne d value s o the Ze rnike coe f cie nts o e ye s at a p up il d iame te r o 5.7 mm: among the hig he ro rd e r coe f cie nts only j = 12 ( ), sp he rical ab e rration, has a value that d i e rs sig nif cantly rom ze ro. ANSI = Ame rican National Stand ard s Institute . (Ad ap te d from Porte r J., Guirao, A., Cox, I. G. & Williams, D. R. (2001). The human e ye ’s monochromatic ab e rrations in a larg e p op ulation. J. O p t. Soc. Am. A, 18, 1793–1803.)

Visual O p t ics

37

Fig . 3.14 The long itud inal chromatic ab e rration o the e ye as ound b y d i e re nt inve stig ators. (Ad ap te d from Charman, W. N. (1991). O p tics o f the human e ye . In W. N. Charman (e d .) Vision and Visual Dys unction. Vol. 1: Visual O p tics and Instrume ntation (p p . 1–26). Lond on: Macmillan.)

between the two image sur aces and the retina varies across eyes and re ractive groups. It has been speculated that those eyes where the mean sphere shows relative hyperopia in the periphery may be more susceptible to the development o myopia (Charman and Radhakrishnan, 2010; Smith, 2011; Flitcro , 2012). For this reason there is ongoing interest in exploring the extent to which modi ying the pattern o peripheral re raction, in particular by reducing relative peripheral hyperopia, by the wearing o suitably designed spectacles or contact lenses (Shen et al., 2010; Sankaridurg et al., 2011; Aller and Wildsoet, 2013), or by orthokeratology (Cho et al., 2005; Walline et al., 2009; Si et al., 2015), may reduce myopia progression in children. Results to date appear to be promising.  CHRO MATIC ABERRATIO N As the re ractive indices o all the ocular media vary with wavelength, the eye su ers rom both longitudinal and transverse chromatic aberration. At the ovea, the ormer is more important – the amount o aberration approximating to that which would occur i the eye media were all water. Unlike the monochromatic aberrations, longitudinal chromatic aberration varies very little between individuals and equals about 2.5 D across the visible spectrum (Fig. 3.14). As the visual axis is usually displaced rom the nominal optical axis o the eye by about 5°, some individually varying, transverse chromatic aberration is ound at the ovea, typically amounting to about 0.8 min arc (Rynders et al., 1995); this urther degrades oveal image quality.  O VERALL O PTICAL PERFO RMANCE O F THE EYE IN WHITE LIGHT Both monochromatic and chromatic aberration will degrade the in- ocus retinal image in comparison with that which would be expected or an aberration- ree eye with the same pupil size. Fig. 3.15 illustrates this or the case o the image o a ne line – that is, the line spread unction. T e experimental results are compared with the calculated pro les or the aberration- ree case (Campbell and Gubisch, 1966). With small pupils, aberration has only minor e ects, but

Fig . 3.15 White -lig ht op tical line sp re ad unctions or e ye s with d i e re nt p up il d iame te rs (mm) at op timal ocus. The solid line curve s g ive the e xp e rime ntal me asure me nts, and the d ashe d curve s the the ore tical re sult or a d i raction-limite d syste m. (Ad ap te d from Camp b e ll, F. W. & Gub isch, R. W. (1966). O p tical q uality of the e ye . J. Physiol. (Lond on), 186, 558–578.)

the per ormance de cit due to aberration steadily increases as the pupil diameter increases. It should, however, be borne in mind that under natural conditions large pupils are ound only when eld luminances are low and neural per ormance is poor. T us di raction-limited optical per ormance with large pupils would be o little value as the neural retina could not utilize the available in ormation. Although monochromatic aberrations at constant pupil diameter tend to increase in later li e, under natural conditions the pupil diameter is smaller, so that the quality o the retinal image changes very little (Applegate et al., 2007). 

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Int ro d uct io n

O CULAR DEPTH O F FO CUS I the retinal image is gradually de ocused, its quality will deteriorate owing to de ocus blur. Nevertheless, there is a nite range o ocus over which this blur causes no appreciable deterioration in visual per ormance. T e precise value o the total depth o ocus depends on how it is assessed (e.g. Atchison, 2012b), but Fig. 3.16 gives some representative photopic values rom di erent studies. It can be seen that, or typical photopic pupils o about 4 mm diameter, visual per ormance will remain relatively una ected provided that the spherical error o ocus does not exceed about ±0.25 D.  CO RRECTIO N O F HIGHER-O RDER O CULAR ABERRATIO N Conventional corrections are designed to compensate or the spherocylindrical errors o the eye. As noted earlier, in waveront terms these correspond to second-order wave ront aberrations. Would it be possible to improve visual per ormance urther by also correcting the higher-order aberrations o the eye, as we can now easily measure these under clinical conditions? Until recently, the irregular and individual nature o the monochromatic wave ront aberrations o the eye made it impossible to correct them ully, although some reduction in the average spherical aberration could be achieved with appropriately aspheric contact lenses (see Chapters 6 and 13). Longitudinal chromatic aberration can be corrected by a suitable achromatizing doublet lens, but the improvement in retinal image quality in white light is small and occurs mainly at intermediate spatial requencies (Campbell and Gubisch, 1967); no improvement in conventional high-contrast, white-light visual acuity is normally detectable (Hartridge, 1947). More recently, however, real progress has been made in correcting monochromatic aberration using either adaptive optics

or liquid crystal phase plates (Liang et al., 1997; Vargas-Martin et al., 1998). Although all these corrections are, at present, easible only in the laboratory, they do show that marked improvements in spatial vision can be achieved over the uncorrected eye, particularly i both monochromatic and chromatic aberrations are corrected; i only monochromatic aberrations are corrected, per ormance in white light improves only modestly (Yoon and Williams, 2001). Will it prove possible to correct axial ocular aberrations in everyday li e? In theory, having measured the wave aberrations o the individual eye, the orm o the cornea could be appropriately shaped, or example by a computer-controlled scanning spot excimer laser, to compensate or the aberrations. T is has been the inspiration behind the development o many commercial aberrometers that, when coupled to suitably controlled excimer lasers, are used in wave ront-guided re ractive surgery (Krueger et al., 2004). In practice, rather than eliminating monochromatic aberrations, this approach has so ar only been able to ensure that postsurgery aberrations are comparable with normal levels, partly because o our limited knowledge o regression e ects associated with corneal healing. Alternatively a tight- tting, customized, contact lens with minimal transverse and rotational movement might be engineered to play the same role (Klein and Barsky, 1995; Klein, 1998; Schweigerling and Snyder, 1998). T e lenses would lack rotational symmetry and would be customized so that their local optical thickness varied in such a way as to compensate or the wave ront aberration o the individual eye. o improve optical per ormance in eyes with normal levels o aberration, any lens decentration should be less than about 0.5 mm and any rotation less than 10° (Bara et al., 2000; Guirao et al., 2001). However, such approaches would reduce only the monochromatic aberrations, which, in any case, change with the level o accommodation (Ivano , 1956; Lopez-Gil et al., 1998) and other actors (Charman and Chateau, 2003). T e blur e ects due to chromatic aberrations would remain uncorrected. Moreover, the worst monochromatic aberration occurs in the periphery o the dilated pupil, and pupil dilation occurs only when light levels are low and visual per ormance is largely limited by neural, rather than optical, actors. For these reasons, customized correction o aberration seems likely to be pro table only in the case o individuals whose monochromatic aberration is particularly high, as in keratoconus (Jinabhai et al., 2012). T is problem is discussed urther in Chapters 6 and 13. 

Effe ct ivit y, Sp e ct acle Mag nificat io n, Acco mmo d at io n and Co nve rg e nce Effe ct s w it h Co nt act Le ns and Sp e ct acle Co rre ct io ns

Fig . 3.16 Examp le s o e xp e rime ntal me asure me nts o p hotop ic, total monocular d e p th o ocus / f e ld as a unction o p up il d iame te r (op timal ocus lie s mid way throug h the total d e p th o ocus). (Ad ap te d from Charman, W. N. (1991). O p tics of the human e ye . In W. N. Charman (e d .) Vision and Visual Dys unction. Vol. 1: Visual O p tics and Instrume ntation (p p . 1–26). Lond on: Macmillan.)

Many patients may wish to change rom a spectacle to a contact lens correction, and vice versa. Although the corrections may be equally e ective in producing in- ocus retinal images in both eyes, they do have a number o slightly di erent secondary e ects, most o which are associated with the act that, whereas contact lenses are placed directly on the cornea, spectacle lenses are placed at a signi cant distance, typically 10–20 mm, in ront o this sur ace. Corrections achieved by corneal ablation using excimer lasers, such as photore ractive keratectomy (PRK), laser in situ keratomileusis (LASIK) or other corneal surgical procedures, such as radial keratotomy or intrastromal rings, produce

3

broadly similar e ects to contact lenses, although their e ective optical zones are usually smaller. EFFECTIVITY T e role o the distance correction is to produce an intermediate image at the ar point o the particular eye. Due to the non-zero vertex distance o any spectacle correction, this ar point will lie at slightly di erent distances rom the two types o correcting lens. T us the spectacle and contact lens powers required to correct a particular eye will di er. From Fig. 3.17A we can see that, using a reduced eye model, i the vertex distance is a (taken as positive) and the ocular re raction is K, giving a ar point distance rom the cornea k = 1 / K, the second ocal point o the correcting lens lies at a distance a + k. T us the power o the correcting lens (Fc) is: Fc = 1/ (a + k) = 1 (a + 1/K) = K/ (1 + aK)

For a contact lens, a will be zero so that the required value o Fc equals the ocular re raction in this simple model. T is does not apply with a spectacle lens. T e result is that a hypermetrope will require a higher-powered contact lens than a spectacle lens, the reverse occurring or a myope. T e di erence between the two correcting powers is plotted as a unction o the spectacle correction or a vertex distance o 14 mm in Fig. 3.17B, rom which it can be seen that the di erence between the required powers o correction becomes signi cant (i.e. greater than 0.25 D) only

Visual O p t ics

39

when the magnitude o the ocular re raction exceeds about ±4 D. Appendix C provides a tabulation o ocular re raction values based on spectacle lens re ractions or various vertex distances.  SPECTACLE MAGNIFICATIO N Spectacle magni cation, as its name implies, describes the ratio o the image size in the corrected ametropic eye to that in the uncorrected eye. It is particularly signi cant in cases o anisometropia, where a er correction the di erential magni cation o the two retinal images may give rise to symptoms o aniseikonia, and with cylindrical errors, where the di erent magni cations in the two principal meridians caused by the correction may lead the patient to complain o distorted images. T e retinal images o any object in the eyes o an uncorrected ametrope have a scale that is governed by the chie rays passing rom the extremities o the object through the centres o the entrance and exit pupils o the eye. Each image point will, o course, be blurred (Fig. 3.18A). Although placing a contact lens on the cornea does not a ect the course o the chie ray, and hence does not alter the size o the retinal image, this is not the case with a spectacle lens. A positive correction increases the angle that the chie ray makes with respect to the axis, whereas a negative correction reduces it. Fig. 3.18B illustrates this e ect or a positive, thin lens correction and a reduced eye with both entrance and exit pupils at the cornea. We de ne the spectacle magni cation, SM, as the retinal image height in the corrected eye, h′, divided by that in the uncorrected eye, h0′. From the diagram it can be seen that, i all angles are assumed to be small: SM = h'/h0 ' = w'k'/w0 'k' = w'/w0 ' = (w/n') / (w0 /n') = w/w0

Fig . 3.17 (A) Ge ome try re lating the ar p oint o an ame trop ic e ye (hyp e rme trop ic in the case shown) and the corre cting le ns. (B) Di e re nce b e twe e n the re q uire d p owe rs o contact le ns and sp e ctacle corre ctions, as a unction o the sp e ctacle corre ction, assuming that the ve rte x d istance o the sp e ctacle le ns is 14 mm.

Fig . 3.18 (A) Ray g e ome try in the case o an uncorre cte d hyp e rme trop e . (B) E e ct o a corre cting sp e ctacle le ns. Note that the ang le that the incid e nt chie ray make s with the axis is incre ase d rom w0 without corre ction to w with corre ction. Corre sp ond ing ly, the ang le o the chie ray with the axis a te r re raction is incre ase d rom w0′ to w′ a te r co rre ction.

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Int ro d uct io n

Fig . 3.19 Typ ical value s or sp e ctacle mag nif cation ob taine d with sp e ctacle le ns and contact le ns corre ctions. The ratio o the two sp e ctacle mag nif cations is also shown. E e ctivity has b e e n allowe d or, so that p oints on any ve rtical line re e r to the same ame trop ia. (Ad ap te d from We sthe ime r, G. (1962). The visual world of the ne w contact le ns we are r. J. Am. O p tom. Assoc., 34, 135–138.)

As the role o the correcting lens is to orm an image o height h1′ at the ar point, we thus have (Fig. 3.18B): SM = [h1 '/ (fc' − a)] / [h1 '/fc'] = 1/ [1 − aFc]

In this simple model, the spectacle magni cation will be unity or contact lenses (vertex distance a = 0), less than unity or negative, myopic spectacle corrections and greater than 1 or positive corrections. Somewhat perversely, spectacle correction is o en expressed as the percentage by which it di ers rom unity, so that a spectacle magni cation o 1.05× would be described as ‘5% magni cation’. In practice we cannot strictly treat corrections as thin lenses and the entrance and exit pupils do not lie at the cornea. For practical purposes the pupils may be taken as being situated about 3 mm behind the cornea. Using a thick-lens extension o the arguments already used, it can then be shown that: SM = [(1 − bF ′v) (1 − (t/n) F1 )]

−1

≈ (1 + bF ′v) (1 + (t/n) F1 )

where b is the vertex distance measured rom the back sur ace lens to the entrance pupil, and t, n, F1 and F′ v are the lens thickness, re ractive index, anterior sur ace power and back vertex power, respectively. It can be seen that the magni cation is a unction o both lens design and vertex distance. Fig. 3.19 shows typical values o spectacle magni cation or both contact lens and spectacle corrections. Note particularly that spectacle magni cation is always close to unity or contact lenses, so that there are likely to be ew magni cation-related complaints rom patients when moving directly rom no correction to a contact lens correction or rom one contact lens correction to another. Casual contact lens wearers who normally wear spectacle corrections may theoretically notice spatial distortion, although or myopes this is counterbalanced by the bene t o relatively larger retinal images, which may improve acuity. Spectacle magni cation e ects a er corneal re ractive surgery are similar to those with contact lenses (Applegate and Howland, 1993).

In addition to spectacle magni cation as de ned above, relative spectacle magni cation (RSM) is sometimes discussed. T is is the ratio o the retinal image size in the corrected ametropic eye to that in a speci ed emmetropic schematic eye. T eoretically it has the advantage o putting retinal image size on an absolute basis. However, in most clinical work it is the changes described by spectacle magni cation that are o interest and RSM is o limited practical use. As noted earlier, when anisometropes are corrected by spectacle lenses, marked di erences in spectacle magni cation may occur between the two eyes, which may result in symptoms o aniseikonia. It is obvious that these are much reduced in the case o contact lenses, which there ore minimize the possibility o aniseikonic symptoms (Winn et al., 1986). A closely related e ect occurs when the anisometrope looks in di erent directions with the head in a xed position. When ordinary spectacles are worn and the visual axes do not pass through the optical centres, prismatic e ects are introduced, o magnitude given by Prentice’s rule P = cFc, where P is the induced prism power in prism dioptres, c the decentration in centimetres and Fc the lens power in dioptres. I the corrections are the same or both eyes, these prismatic e ects cause no problems or the spectacle wearer. In anisometropia, however, the prismatic e ects will be di erent or each eye. For example, in reading, the visual axes o a young anisometrope would normally intercept the lenses o the distance correction at some distance below the optical centres. Assuming this distance to be 8 mm and the corrections to be right eye (RE) −3.00 D, le eye (LE) −6.00 D, the prismatic e ects would be RE 2.4Δ and LE 4.8Δ, respectively, both base-down. T e di erence in vertical prism power exceeds normal usional abilities, so that, to avoid the problem, the spectacle-corrected anisometrope would have to execute head turns during reading rather than simply depress the visual axes. T is problem is absent with well-centred contact lenses.  ACCO MMO DATIO N DEMAND Just as the position o the correcting lens a ects the correcting power required and the spectacle magni cation, so too does it inf uence the accommodation required to view a near object. T e accommodation necessary with any particular correction can easily be calculated or any given object distance, lens position and correcting power by determining the di erence between the vergence o the light striking the cornea when viewing a near object and that or a distant object. However, an adequate approximation or most purposes is that the accommodation demand (A, in dioptres) is given by: A ≈ −L(1 + 2aK)

where L is the object vergence (negative or real objects), a is the vertex distance and K is the ocular re raction. In this approximation, a is zero or a contact lens, so that we can see that or a myope (negative K) the accommodation demand is higher with contact lenses than with spectacle lenses, whereas the reverse is true or hypermetropes. I we consider an object at 33 cm (L = −3 D) and a spectacle vertex distance a = 14 mm, we nd that the di erence in demand with the two types o correction becomes signi cant (>0.25 D) when the magnitude o the re ractive error, K, is larger than about 3 D. T us, higher myopes approaching presbyopia might slightly delay the need or a reading addition by wearing spectacles, whereas

3

Visual O p t ics

41

CO NVERGENCE DEMAND

Fig . 3.20 Ocular accommodation required when a patient with the spectacle ametropia given on the abscissa views targets at either 0.50 or 0.33 m (vergence, L= −2 or −3 D) when corrected with either spectacles (a = 14 mm) or contact lenses. (Adapted from Westheimer, G. (1962). The visual world of the new contact lens wearer. J. Am. Optom. Assoc., 34, 135–138.)

Contact lenses move with the eyes, hence convergence demands when viewing near objects are identical to those applying in the uncorrected state. In contrast, myopes with a negative spectacle correction or distance observe near objects through base-in prisms, as they are no longer looking through the optical centres o their lenses (Fig. 3.21). T e base-in prismatic e ects reduce the convergence requirement, compared with the naked eye or contact lens situation. Spectacle-corrected hypermetropes, however, experience a base-out e ect at near, which increases the convergence demand. Allowing or a typical interpupillary distance o 65 mm and the centre o rotation o each eye being about 12 mm behind the cornea, application o Prentice’s rule shows that, or an object distance o 33 cm, the convergence demand or each eye is reduced by about 0.25Fc prism dioptres or a negative spectacle correction and similarly increased or a positive correction. In most cases, then, the change in convergence demand is small compared with the usion reserves. Since both accommodation and convergence demands are higher or myopes with contact lenses, and lower or hypermetropes, the accommodation–convergence links are minimally disturbed. 

O t he r O p t ical Effe ct s T ere are certain additional phenomena related to prismatic e ects o ophthalmic lenses that are not encountered by contact lens wearers. T ese phenomena, which are experienced by spectacle lens wearers, relate to the e ective eld o view in static gaze, the extent o eye movements required to maintain xation and the appearance o the eyes as viewed by another person (or when looking in a mirror). FIELDS O F VIEW AND FIXATIO N

Fig . 3.21 Prismatic e e cts o d istance sp e ctacle corre ctions d uring ne ar vision.

hypermetropes would nd near vision easier with a contact lens correction. Fig. 3.20 shows results rom a slightly more re ned model or the accommodation demand at two object distances. 

With spectacle lenses, the prismatic e ects associated with the lens peripheries result, when the eyes are stationary, in an annular zone o the visual eld being invisible (a ring scotoma) with a positive correction, and being seen diplopically with a negative correction. Analogously, when the eye is rotated to view objects away rom the axis o the correction, a larger eye movement, in comparison with the uncorrected eye, is required with a negative spectacle lens and a smaller one with a positive correction. T is can be seen in Fig. 3.22. I C is the centre o rotation o the eye, the e ective eld o view as seen through the spectacle lens is governed by the position o its image, C′, as ormed by the correcting lens. T ese xation e ects are absent with contact lens corrections, as the lenses ollow the movements o the eyes rom xation to xation. T e periphery o the static eld o view may, however, be slightly a ected i the contact lens or its optical zone is small, and in the case o rigid lenses f are or glare may occur owing to discontinuities at the edge o the lens or optic zone a ecting ray pencils rom the periphery o the eld. A er laser re ractive surgery, the optic zone diameter may be smaller than the dilated pupil, leading to complaints o haloes at night.  APPARENT SIZE O F THE EYES A cosmetic disadvantage o spectacle lenses is that they alter the apparent size o the eyes o the wearer as seen by other people:

42

PART 1

Int ro d uct io n

Because l is small (50% EWC < 0.2% IO NIC CO NTENT) Biotrue O ne d ay Bausch & Lomb Dailie s Aq uaCom ort Plus Alcon Focus Dailie s All Day Com ort Alcon O mnif e x Coop e rvision Procle ar Coop e rvision Sauf on-55 Sauf on So Le ns d aily d isp osab le Bausch & Lomb UltraWave UltraVision CLPL

HEMA, VP PVA PVA MMA, VP HEMA, PC HEMA, VP HEMA, VP HEMA, GMA

78 69 69 70 62 55 59 57

Ne so lcon A Ne lcon A Ne lcon A Lid o lcon-A O ma lcon B N/A Hila lcon B Hioxi lcon A

FDA GRO UP III ( < 50% EWC 50% EWC >0.2% IO NIC CO NTENT) Acuvue 2 Johnson & Johnson 1-Day Acuvue Moist Johnson & Johnson Biome d ics 55UV Coop e rVision Fre shlook ColorBle nd s Alcon Fre q ue ncy 55 Coop e rVision Pe rmale ns* CIBA Vision

HEMA, MAA HEMA, MAA HEMA, MAA HEMA, MAA HEMA, MAA HEMA, VP, MAA

58 58 55 55 55 71

Eta lcon A Eta lcon A O cu lcon D Phe m lcon A Me tha lcon A Pe r lcon-A

*No long e r availab le . FDA = Food and Drug Ad ministration; EWC = e q uilib rium wate r conte nt; HEMA = 2-hyd roxye thyl me thacrylate ; VP = N-vinyl p yrrolid one ; MMA = me thyl me thacrylate ; PC = p hosp horylcholine ; GMA = g lyce ryl me thacrylate ; MAA = me thacrylic acid ; VA = vinyl ace tate ; PMA = p olyme thyl acrylate ; PVA = p olyvinyl alcohol; PVP = p olyvinyl p yrrolid one (i.e . g ra t cop olyme r); BMA = b utyl (p rob ab ly isob utyl) me thacrylate ; DAA = d iace tone , acrylamid e ; USAN = Unite d State s ad op te d name .

o macromolecular entrapment and / or release o hydrophilic sur ace-active polymers at the lens sur ace in order to improve end-o -day com ort by stabilizing the pre-lens tear lm. T e Dailies AquaCom ort Plus lens is manu actured rom nel lcon A, which consists o a cross-linked unctionalized polyvinyl alcohol (PVA) macromer with the addition o non- unctionalized PVA (Winterton et al., 2007). T is un unctionalized PVA macromer is ree to elute rom the lens into the tear lm with each blink. T is PVA is thought to emerge rom the lens matrix as ‘strands’ at the lens sur ace, and it is this e ect together with the e ect o soluble PVA in the tear lm that is re erred to as the ‘sur ace modi cation’ o these lenses. T e released PVA may improve lens com ort by decreasing the sur ace tension o the tears, or by mimicking mucin, ound naturally in the tear lm (Mahomed et al., 2004). T e blister packaging also contains hydroxypropyl methylcellulose (HPMC), which is a lubricating agent used to improve com ort on lens applications, as well as polyethylene glycol (PEG), which is a hydrophilic wetting agent with a high af nity or PVA used or enhancing com ort throughout the day. PVA and polyvinyl pyrrolidone (PVP) are common soluble polymeric components in com ort drops and arti cial tears and have a viscous consistency at elevated concentrations and molecular weights, giving them good sur ace spreading characteristics. T e 1-Day Acuvue Moist lens is manu actured rom the eta lcon A polymer (HEMA / MAA) together with the incorporation o small concentrations o low-molecular-weight PVP into the ionic material network. Here the PVP is ‘locked’

into the lens matrix and is not released rom the lens during wear. T e PVP is adsorbed on to the pre ormed lens sur ace a er manu acture rom solution. T e lens packaging states that the lenses are supplied in ‘bu ered saline with povidone’. Povidone is another name or PVP. PVP is quite polar and it is likely to be relatively strongly attracted to the eta lcon material, potentially providing a mechanism or its retention on the lens sur ace. T e persistence o the PVP at the lens sur ace during wear has been veri ed by Ross et al. (2007), who have also described the PVP as being in a predominantly ‘looped structure’ across the lens sur ace. T e PVP is thought to reduce the coef cient o riction o the lens sur ace T e So Lens daily disposable lens is modi ed by the adsorption o etronic 1107 – a hydrophilic sur ace-active polymer composed o ethylene oxide / propylene oxide block copolymer – onto the lens sur ace. T e etronic at the sur ace lowers the coef cient o riction o the lens, but it has been shown to be progressively lost rom the sur ace during wear (Ross et al., 2007). It is likely, there ore, that the etronic is held by weak orces at the lens sur ace, which would explain the lowest in-eye persistence o three ‘enhanced’ lenses investigated by Ross et al. in 2007.  SILICO NE HYDRO GEL MATERIALS When Holden and Mertz (1984) de ned the critical oxygen levels in order to avoid corneal oedema or daily and extended wear they concluded that 24.1 Barrer / cm was the oxygen transmissibility required or daily wear and 87 Barrer / cm was that

56

PART 2

So ft Co nt act Le nse s

required or overnight wear. T ese values were re-evaluated by Harvitt and Bonanno (1999), who ound that the minimum oxygen transmissibility required to avoid anoxia throughout the entire cornea was 35 Barrer / cm or the open eye and 125 Barrer / cm or the closed eye. Fig. 4.12 shows the relationship between the EWC and the Dk o hydrogels and silicone hydrogels. From the graph, it is obvious that there is an upper limit to how much oxygen permeability can be attained simply by increasing the EWC o hydrogel materials. A hydrogel with a theoretical EWC o 90% and a central thickness o 0.1 mm would have an oxygen transmissibility in the region o 60 Barrer / cm, which still alls ar short o that required to avoid additional overnight corneal oedema. Such a lens would need to be in the region o 0.06 mm thick, which is unrealistic rom both a manu acturing and a clinical point o view (Holden et al., 1986). I reducing the thickness o lenses made rom hydrogels was not an option or achieving success in extended wear, then polymer scientists had to come up with an altogether new kind o material. T at material was silicone. T e element silicon (Si) is the most abundant element on earth a er oxygen (e.g. in the orm o silicates or oxides such as sand and clay). Silicones are organic compounds o silicon and oxygen. Incorporating silicone into contact lens materials was not a new concept when scientists began trying to produce silicone hydrogels. Indeed, the rst material to be used in contact lenses was silicone dioxide (glass). Additionally, silicone rubber (polydimethyl siloxane, PDMS) (Fig. 4.14) has been used with limited success as a contact lens material in the orm o silicone elastomer lenses. T ese lenses have not become popular mainly because o lens-tightening and sur ace wettability problems (PDMS is extremely hydrophobic) (Josephson and Ca ery, 1987). PDMS has an oxygen permeability in the region o 600 Barrer but is unwettable by tears, deposits high degrees o lipid

and needs to be sur ace treated. Sur ace treatments o silicone elastomer lenses have not been particularly success ul in the past because Si—O chains have a tendency to rotate very easily and any hydrophilic parts o a newly treated sur ace tend to disappear inside the polymer. Silicon, however, has been very success ully incorporated into rigid lens materials and it was this development that proved to be a key milestone in the subsequent development o silicone hydrogel materials. T e work o Norman Gaylord at Polycon Laboratories drove the development o the rst siloxane-based rigid lens material that merged the properties o MMA with the increased oxygen per ormance o silicone rubber (Gaylord, 1974, 1978). T e resultant siloxymethacrylate monomer was tris(trimethylsiloxy)-methacryloxy-propylsilane (see Fig. 4.14) and is more commonly re erred to as RIS. T e patent literature shows that combining silicone with hydrogel monomers has been a goal or polymer scientists since the late 1970s. T e biggest obstacle to this approach, however, is that silicone is hydrophobic and poorly miscible with hydrophilic monomers, resulting in opaque, phase-separated materials. In order to solve this problem, two main approaches have been utilized ( ighe, 2004). T e rst approach involves the insertion o polar groups into the section o the RIS molecule, arrowed in Fig. 4.14, in order to aid its miscibility with hydrophilic monomers ( anaka et al., 1979; Künzler and Ozark, 1994). T e second approach is that o utilizing macromers. Macromers are large monomers ormed by preassembly o structural units that are designed to bestow particular properties on the nal polymer ( ighe, 2004). T is is illustrated in Fig. 4.15 with an example rom an Alcon patent (Nicolson et al., 1996) that contains poly( uoroethylene oxide) segments and oxygen-permeable polysiloxane units. Fig. 4.12 demonstrates the relationship between Dk and EWC or silicone-containing

Fig . 4.14 Silicone -b ase d mate rials. PDMS = p olyd ime thyl siloxane ; TRIS = tris(trime thylsiloxy)-me thacryloxy-p rop ylsilane ; TPVC = carb amate sub stitute d TRIS.

4

hydrogels based on RIS, highlighting the bene ts o increased oxygen per ormance. T e rst two silicone hydrogels were launched in the late 1990s – the PureVision lens (Bausch & Lomb) and the Air Optix Night and Day lens (Alcon) and are now commonly re erred to as ‘ rst-generation’ silicone hydrogels. Both were licensed or 30 days o continuous wear ( able 4.2). T e exact compositions o these materials are proprietary, but the USAN-registered components o the bala lcon A material show that it is based on a carbamate-substituted RIS-based material known as PVC (see Fig. 4.14). T e PVC is then copolymerized with NVP to orm the bala lcon material. T e Air Optix Night and Day lens (lotra lcon A) ( able 4.2) is ‘biphasic’. ighe (2004) describes the lens as being a uoroether macromer copolymerized with RIS and N,N-dimethyl acrylamide (DMA) in the presence o a diluent. Its biphasic (two-channel) structure means that oxygen and water permeability channels are not reliant on each other. T e silicone-containing phase allows passage o oxygen whilst the water phase primarily allows the lens to move. Without urther treatment both o these rst-generation silicone hydrogel lenses would be unsuitable or wear owing to the act that the resultant material sur aces are very hydrophobic. In order to overcome this problem, both lenses are sur ace treated using gas plasma techniques. High-energy gases or gas mixtures (the plasma) are used to modi y the lens sur ace properties without changing the bulk properties. T e result or the bala lcon lens is that sur ace wettability is gained via plasma oxidation, which produces glassy silicate islands on the lens sur ace. T e lotra lcon lens is coated with a dense 25 nm thick coating. Both resultant sur aces have low molecular mobility, which minimizes the migration o hydrophobic silicone groups to the sur ace. However, despite these sur ace modi cations, wettability problems with these lenses were reported. It is generally accepted that silicone hydrogel lenses have in erior wettability compared with hydrogels, which occurs as a result o the hydrophobic interaction o silicone with the tear lm. Another important di erence between these rst-generation silicone hydrogel materials and hydrogels is that they have signi cantly greater elastic moduli (i.e. they are ‘sti er’). Such mechanical characteristics mean that the lenses are easy to handle, but have also been implicated in the aetiology o a number o clinical complications (Dumbleton, 2003). T ese include higher incidences o super cial epithelial arcuate lesions, mucin balls and CLPC (in particular, localized CLPC compared with generalized CLPC), especially with continuous wear o these lenses (Skotnitsky et al., 2002). T e sti ness o the material may contribute to the mechanical irritation o the lens rubbing against the conjunctiva o the upper eyelid producing a localized response.

Fig . 4.15

Typ ical siloxy-containing macrome r (macromonome r) structure .

So ft Le ns Mat e rials

57

T e design o a lens, and in particular the edges, may also have an impact on ocular compatibility. It has been suggested that the design o the lens edge in conjunction with the mechanical properties o silicone hydrogel lenses may be responsible or increased conjunctival staining and conjunctival epithelial aps observed with these lenses (Lo strom and Kruse, 2005). A kni e-point edge or chisel-shaped edge may cause more conjunctival staining and ap ormation than a round edge by ‘carving’ into the conjunctival tissue (Back, 2007). It has been proposed that certain edge designs incorporating localized increases in posterior edge li , reduced peripheral thickness or peripheral channels may reduce the pressure on the conjunctiva (Back, 2007). However, more recent work has suggested that lenses that produce more circumlimbal staining are not associated with reduced levels o com ort (Maissa et al., 2012). In an attempt to improve on the problems encountered with these rst-generation lenses, manu acturers have engaged in a programme o research aimed at manu acturing silicone hydrogel lenses with improved mechanical and sur ace characteristics. T is has resulted in the gradual emergence o ‘secondgeneration’ and ‘third-generation’ silicone hydrogel lenses such as Acuvue Oasys, 1-Day Acuvue ruEye, Avaira, Clariti, Dailies otal 1 and MyDay lenses (see able 4.2). T e main advantage o these newer silicone hydrogels compared with the early silicone hydrogels is that they have increased water contents, reduced moduli and do not need to be sur ace treated. T e mechanical and sur ace properties o the newest lenses are now similar to that o hydrogels (see able 4.2). Recent clinical work indicates that there may be a lower incidence o CLPC with these lenses (Maldonado-Codina et al., 2004). Some o the lenses in able 4.2 are based on materials containing RIS-like components. Acuvue Advance and Acuvue Oasys are based on anaka’s original patent ( ollowing its expiration a er 25 years) using a modi ed RIS molecule, a silicone macromer and hydrophilic monomers such as HEMA and DMA. Alcohol is used as a solvent to aid the miscibility o these ingredients and is then extracted ollowing polymerization. High-molecular-weight PVP is the internal wetting agent (the Hydraclear) used in these lenses, which is entangled and there ore ‘entrapped’ within the lens matrix and which allows them to be manu actured without requiring a sur ace treatment (Maiden et al., 2002; McCabe et al., 2004). T e PVP essentially works by shielding the silicone rom the tear lm at the lens inter ace. T e Bio nity (com lcon A) and Avaira lenses (en lconA) are not based on RIS chemistry. T ey are comprised solely o silicon-containing macromers and require no sur ace treatment or wetting agent. T e patents surrounding the materials re er to a mono unctional macromer (which contains only one double bond taking part in the polymerization process) being

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PART 2

T

58 So ft Co nt act Le nse s

4

Fig . 4.16

So ft Le ns Mat e rials

59

Exte nt o silicone hyd rog e l contact le ns f tting as a p e rce ntag e o all so t le nse s p re scrib e d in se ve n nations b e twe e n 2000 and 2015.

combined with another rubber-like siloxy macromer, resulting in a material with much longer chains (higher molecular weight) compared with the other silicone hydrogels (Iwata et al., 2005, 2006). T e patents also discuss other hydrophilic monomers, which are presumably the key to why these materials do not need to be sur ace treated. T e introduction o second- and third-generation lenses has seen a signi cant rise in the number o silicone hydrogel lenses being prescribed on a daily-wear basis in addition to the introduction o daily disposable silicone hydrogel lenses (Morgan et al., 2016) (Fig. 4.16). 

Classi icat io n o So t Le ns Mat e rials T ere are two main classi cation systems or so contact lens materials. T ese classi cation systems are expanded upon in Appendix 4.1, below. 

Co nclusio n A basic understanding o the materials rom which contact lenses are made as well as their behaviour is vitally important to any contact lens practitioner as it is likely to orm an important aspect o patient management. So contact lenses have come a long way since the pioneering e orts o Pro essor Otto Wichterle in the late 1950s in terms o material, design and the way

they are manu actured. T ick pHEMA lenses that were replaced every ew years are now a thing o the past. Whilst extended-wear hypoxia-related problems with hydrogels have been resolved with the introduction o silicone hydrogel materials, a number o mechanical and sur ace material-related complications still remain, despite the introduction o secondand third-generation polymers. For daily wear, there has been somewhat o a renaissance towards tting hydrogel materials in recent years. T is has come about because o the lack o evidence or increased com ort with silicone hydrogels, the lack o evidence to show that signi cant pathology results owing to oxygen levels reaching the anterior eye during daily wear and the conrmation that the incidence o microbial keratitis is no di erent between the two lens material groups. Future development o so contact lens materials is likely to concentrate on trying to resolve the issues o in ammation and in ection, improving lens com ort (particularly towards the end o the day), enhancing post-lens tear exchange and improving sur ace wettability. ACKNO WLEDGEMENTS T e author wishes to thank Andy Broad, revor Glasbey, David Ruston, Guy Whittaker and Inma Perez-Gomez or use ul comments and discussions. Acce ss t he co mp le t e re fe re nce s list o nline at ht t p :/ / www.e xp e rt co nsult .co m.

60

PART 2

APPENDIX

4.1

So ft Co nt act Le nse s

CLASSIFICATIO N O F SO FT LENS MATERIALS

Fo o d and Drug Ad minist rat io n (FDA) Classi icat io n Syst e m T e FDA classi cation system or so lens materials is shown in able 4A.1. T e classi cation system groups lens materials based on their water content and physical charge. For many years the classi cation system consisted o our hydrogel groups. However, since silicone hydrogels were introduced, this classi cation system has not been ideal because these lenses are undamentally di erent in their material chemistry. As a result, a h group or silicone hydrogels has been introduced. 

The ISO Classi icat io n Syst e m BS EN ISO 18369-1 / DAM1: 2009 sets out the new international standard method or the classi cation o a contact lens material given as a six-part code as ollows: (pre x) (stem) (series suf x) (group suf x) (Dk range) (surace modi cation code) For so lens materials, the classi cation denotes whether the material is ionic and the range in which the water content o the material lies. T e presence or absence o sur ace modi cations is also indicated. T e pre x is a term assigned to a material to designate a speci c chemical ormulation. Use o this pre x, which is administered by the United States Adopted Names (USAN) Council, is optional or all countries other than the USA. wo types o stem are used. T e lcon stem is af xed to the pre x and is applied or materials that contain ≥10% water by mass (hydrogel materials). Focon is applied to materials containing ≤10% water by mass (i.e. non-hydrogel materials). T e series suf x is also administered by the USAN council, and is used in cases in which the original ratio o the monomers o an existing contact lens polymeric material is changed to make a new material. In this case, the capital letter A is added a er the stem designation. Subsequent changes in monomer ratio are designated by the next letter o the alphabet. T ese letters are used to di erentiate copolymers o unchanged monomer units, but with di erent ratios. It can be omitted i there is only one ormulation. T e group suf x, represented by a Roman numeral, indicates the range o water content and ionic character o the material ( able 4A.2). able 4A.3 shows how the oxygen permeability o the materials is classi ed. T e modi cation code, designated by a letter m, denotes whether the lens has a sur ace modi cation that renders the sur ace characteristics di erent to the bulk material. Such treatments include plasma treatment, acid / base hydrolysis and incorporation o a material that migrates to the sur ace. Certain types o tinted lens may also be considered sur ace modi ed. In the case o an unmodi ed sur ace, this suf x is omitted. 

TABLE

4A.1

FDA Classif cat io n Syst e m o r So t Le ns Mat e rials

Gro up

Mat e rial

I II III IV V

Low-wate r-conte nt (50%), non-ionic p olyme rs Low-wate r-conte nt (50%), ionic p olyme rs Silicone hyd rog e l mate rials

TABLE

4A.2

BS EN ISO Hyd ro g e l Su f x Gro up s

Gro up Suffix

Mat e rial

I

Low-wate r-conte nt, non-ionic: mate rials that contain le ss than 50% wate r and contain 1% or le ss (e xp re sse d as mole raction) o monome rs that are ionic at p H 7.2 Medium- and high-water-content, non-ionic: materials that contain 50% water or more, and contain 1% or less (expressed as mole raction) o monomers that are ionic at pH 7.2 Low-wate r-conte nt, ionic: mate rials that contain le ss than 50% wate r and contain g re ate r than 1% (e xp re sse d as mole raction) o monome rs that are ionic at p H 7.2 Medium- and high-water-content, ionic: materials that contain 50% water or more, and contain greater than 1% (expressed as mole raction) o monomers that are ionic at pH 7.2 Enhance d oxyg e n p e rme ab le mate rials (e .g . silicone hyd rog e l)

II

III

IV

V

TABLE

4A.3

BS EN ISO Hyd ro g e l Dk Gro up s

Gro up

Dk Rang e (ISO Dk Unit s)

0 1 2 3 4 5 6 7, e tc.

10 D) may be present rom birth and is related to a number o ocular and systemic disorders (Jensen, 1997). High myopia is also associated with cranio acial anomalies, which can make the wearing o spectacles di cult (Fig. 28.4). T e myopic eye is larger than normal and tends to have a f atter than average corneal radius and larger corneal diameter. Adult-sized lenses can o en be used in young in ants and children. Myopia can also result rom buphthalmos where the corneal diameter is much larger than normal (>12.5 mm) and so requires a f atter and larger lens. Contact lenses in unilateral high myopia have been shown to be more satis actory than spectacle lenses in the management o amblyopia in regard to cosmesis, com ort and treatment compliance (Mets and Price, 1981).  O CULAR MO TILITY DISO RDERS Contact lenses can be use ul in the management o ocular motility disorders (Evans, 2006). Some uses include:

Fig . 28.4 An in ant with a hig h myop ia in association with a cranio acial anomaly.

Est imat e d Hyd ro g e l Le ns Sp e cif cat io ns Base d o n Ag e o r an Ap hakic Eye o No rmal Size *

Ag e (mo nt hs) 1 2 3 6 12

Bab ie s and Child re n

BO ZR (mm)

TD (mm)

Po we r (D)

7.00 7.20 7.50 7.80 8.10

12.00 12.50 13.00 13.50 13.50

+ 35.00 + 32.00 + 30.00 + 25.00 + 20.00

*The se le nse s would orm the b asis o a p ae d iatric ap hakic d iag nostic tting se t. BO ZR = b ack zone op tic rad ius; TD = total d iame te r.

270

PART 5

Sp e cial Le nse s and Fit t ing Co nsid e rat io ns

et al. (2008), where two cases o upper-eyelid entropion secondary to neonatal conjunctivitis resolved spontaneously ollowing the insertion o bandage contact lenses. Previously early surgical intervention was advocated to correct the eyelid abnormality and prevent any permanent corneal scarring and visual loss. T e tting o therapeutic lenses is described in more detail in Chapter 29. 

Ele ct ive Co nt act Le ns We ar in Child re n

Fig . 28.5 An ap hakic e ye , ollowing trauma and a ull-thickne ss corne al lace ration, tte d with a hyd rog e l toric contact le ns.

• aniseikonia induced by anisometropia exceeding 6 D • accommodative esotropia (older children) • nystagmus • occlusion.  IRREGULAR ASTIGMATISM Irregular astigmatism derived rom primary corneal ectasia is extremely rare in childhood. Most causes o corneal irregularity are secondary in nature – or example, ollowing corneal in ection or laceration (Fig. 28.5). Neutralization o irregular astigmatism is important during the visual development period so as to prevent deprivational amblyopia. T e optimum orm o contact lens correction in this situation is a rigid gas-permeable lens, although sometimes, i the irregularity is less severe, a toric so lens may su ce. Rigid gas-permeable lenses have been shown to o er a use ul re ractive treatment alternative in children with traumatized eyes (Pradhan et al., 2014)  TINTED AND PRO STHETIC LENSES T e aim o this type o contact lens in paediatric use is to enhance visual per ormance by reducing the e ect o photophobia or improving the cosmesis o the child by camouf aging an ocular de ect. T e most common reasons or tting these lenses in childhood are: • albinism • aniridia • achromatopsia • iris de ects, e.g. coloboma • nanophthalmos or microphthalmos • corneal anomalies, e.g. sclerocornea or Peter’s anomaly. he itting o these lenses is described in more detail in Chapter 22.  THERAPEUTIC LENSES Silicone hydrogel lenses have been shown to be sa e and e cacious or continuous-wear therapeutic use in children (Benoriene and Vogt, 2006). T erapeutic contact lens use in the paediatric population is similar to its use in adults – mainly or the relie o pain, promotion o corneal healing and protection o the cornea. An example o their use is reported by Maycock

Contact lenses can be considered as an additional option to ull-time spectacle wear or or use while participating in sporting activities or both myopic and hypermetropic children and teenagers. A study by Jones-Jordan et al. (2010) ound that both rigid gas-permeable and so contact lenses could be considered, although gas-permeable lenses took longer to adapt to and resulted in slightly less com ortable wearing times. Daily disposable contact lenses are particularly use ul whilst participating in sporting activities, especially i wear is intermittent. E ron et al. (2011) conducted an international survey to determine the types o contact lenses prescribed or in ants (aged 0 to 5 years), children (6 to 12 years), and teenagers (13 to 17 years). Up to 1000 survey orms were sent to contact lens tters in each o 38 countries between January and March every year or 5 consecutive years (2005–2009). Practitioners were asked to record data relating to the rst 10 contact lens ts or re ts per ormed a er receiving the survey orm. Data were received relating to 105 734 ts (137 in ants, 1672 children, 12 117 teenagers, and 91 808 adults [age ≥18 years]); the proportion o minors ( 0.6

± 0.75

± 0.09

± 0.025

± 0.05

Back p e rip he ral rad ius

± 0.10

± 0.10

Front p e rip he ral rad ius

± 0.10

± 0.10

Back p e rip he ral d iame te r

± 0.20

± 0.20

Back op tic zone rad ii o toroid al sur ace s whe re the d i e re nce in rad ii is:

Sag itta at sp e cif e d d iame te r

± 0.05

Back op tic zone d iame te r

± 0.20

Total d iame te r

± 0.20

± 0.10

± 0.10

Front op tic zone d iame te r

± 0.20

± 0.20

± 0.20

Bi ocal se g me nt he ig ht

−0.10 to + 0.20

−0.10 to + 0.20

Ce ntre thickne ss

± 0.02

± 0.02

Ce ntre thickne ss, whe re the nominal value is: ≤0.10

± 0.010 + 10%

> 0.10

± 0.015 + 5%

TABLE

B

O p t ical To le rance s fo r So ft , Po lyme t hyl Me t hacrylat e (PMMA) and Rig id Le nse s

Dime nsio n Back ve rte x p owe r ≤5 D ≤10 D ≤15 D ≤20 D >20 D

So ft Le nse s

PMMA Le nse s

Rig id Le nse s

± 0.50 D ± 1.00 D

± 0.12 D ± 0.18 D ± 0.25 D ± 0.37 D ± 0.50 D

± 0.12 D ± 0.18 D ± 0.25 D ± 0.37 D ± 0.50 D

Cylind e r p owe r ≤0.2 2–4 >4

± 0.25 D ± 0.37 D ± 0.50 D

± 0.25 D ± 0.37 D ± 0.50 D

± 0.25 D ± 0.37 D ± 0.50 D

Cylind e r axis

± 5°

± 5°

± 5°

± 0.25 D

TABLE

C

Mat e rial Pro p e rt y

To le rance

Re ractive ind e x

± 0.005

Wate r conte nt

± 2%

O xyg e n p e rme ab ility

± 20%

•T

Prismatic e rror (measure d at the g e ometric centre o the op tic zone) Back ve rte x p owe r ≤6 D >6 D Pre scrib e d p rism

440

± 0.25 cm / m

± 0.25 cm / m

± 0.50 cm / m

± 0.50 cm / m

± 0.25 cm / m

± 0.25 cm / m

Mat e rial Pro p e rt y To le rance s fo r So ft Le nse s

• •

e tolerances outlined in this appendix were obtained rom the ollowing standards: ISO 8321–1: 1991 Optics and optical instruments – contact lenses – part 1: speci cation or rigid corneal and scleral contact lenses. BS EN ISO 8321–2: 2000 (BS 7208–24:2000) Ophthalmic optics – speci cations or material, optical and dimensional properties o contact lenses – part 2: single-vision hydrogel contact lenses. PMMA tolerances are given here because trial lens tting sets are of en abricated rom this material due to its resilience. See also: Hough, . (2000) A Guide to Contact Lens Standards. British Contact Lens Association.

APPENDIX

C

Ve rt e x Dist ance Co rre ct io n

Effe ct ive Po w e r (D) o f Plus- and Minus-p re scrip t io n Sp e ct acle Le nse s at t he Co rne al Plane fo r Vario us Ve rt e x Dist ance s (mm)* PO WER (D) AT CO RNEAL PLANE FO R DIFFERENT VERTEX DISTANCES (mm) Sp e c Rx

8 mm

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Plus

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Plus

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4.13

3.88

4.17

3.85

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3.82

4.24

3.79

4.27

3.76

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4.40

4.11

4.44

4.08

4.48

4.04

4.52

4.01

4.56

3.98

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4.67

4.34

4.71

4.31

4.76

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4.80

4.23

4.85

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4.75

4.94

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4.99

4.53

5.04

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5.14

4.41

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5.21

4.81

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4.76

5.32

4.72

5.38

4.67

5.43

4.63

5.25

5.48

5.04

5.54

4.99

5.60

4.94

5.67

4.89

5.73

4.84

5.50

5.75

5.27

5.82

5.21

5.89

5.16

5.96

5.11

6.03

5.06

5.75

6.03

5.50

6.10

5.44

6.18

5.38

6.25

5.32

6.33

5.27

6.00

6.30

5.73

6.38

5.66

6.47

5.60

6.55

5.54

6.64

5.47

6.25

6.58

5.95

6.67

5.88

6.76

5.81

6.85

5.75

6.94

5.68

6.50

6.86

6.18

6.95

6.10

7.05

6.03

7.15

5.96

7.25

5.89

6.75

7.14

6.40

7.24

6.32

7.34

6.24

7.45

6.17

7.57

6.09

7.00

7.42

6.63

7.53

6.54

7.64

6.46

7.76

6.38

7.88

6.29

7.25

7.70

6.85

7.82

6.76

7.94

6.67

8.07

6.58

8.20

6.50

7.50

7.98

7.08

8.11

6.98

8.24

6.88

8.38

6.79

8.52

6.70

7.75

8.26

7.30

8.40

7.19

8.54

7.09

8.69

6.99

8.85

6.90

8.00

8.55

7.52

8.70

7.41

8.85

7.30

9.01

7.19

9.17

7.09

8.25

8.83

7.74

8.99

7.62

9.16

7.51

9.33

7.40

9.50

7.29

8.50

9.12

7.96

9.29

7.83

9.47

7.71

9.65

7.60

9.84

7.48

8.75

9.41

8.18

9.59

8.05

9.78

7.92

9.97

7.80

10.17

7.68

9.00

9.70

8.40

9.89

8.26

10.09

8.12

10.30

7.99

10.51

7.87

9.25

9.99

8.61

10.19

8.47

10.40

8.33

10.63

8.19

10.86

8.06

9.50

10.28

8.83

10.50

8.68

10.72

8.53

10.96

8.38

11.20

8.25

9.75

10.57

9.04

10.80

8.88

11.04

8.73

11.29

8.58

11.55

8.43

10.00

10.87

9.26

11.11

9.09

11.36

8.93

11.63

8.77

11.90

8.62

10.25

11.17

9.47

11.42

9.30

11.69

9.13

11.97

8.96

12.26

8.81

10.50

11.46

9.69

11.73

9.50

12.01

9.33

12.31

9.15

12.62

8.99

10.75

11.76

9.90

12.04

9.71

12.34

9.52

12.65

9.34

12.98

9.17

11.00

12.06

10.11

12.36

9.91

12.67

9.72

13.00

9.53

13.35

9.35

11.25

12.36

10.32

12.68

10.11

13.01

9.91

13.35

9.72

13.72

9.53

11.50

12.67

10.53

12.99

10.31

13.34

10.11

13.71

9.91

14.09

9.71

11.75

12.97

10.74

13.31

10.51

13.68

10.30

14.06

10.09

14.47

9.89

12.00

13.27

10.95

13.64

10.71

14.02

10.49

14.42

10.27

14.85

10.07

12.25

13.58

11.16

13.96

10.91

14.36

10.68

14.79

10.46

15.24

10.24

12.50

13.89

11.36

14.29

11.11

14.71

10.87

15.15

10.64

15.63

10.42

12.75

14.20

11.57

14.61

11.31

15.05

11.06

15.52

10.82

16.02

10.59

13.00

14.51

11.78

14.94

11.50

15.40

11.25

15.89

11.00

16.41

10.76 Continue d

441

442

APPENDIX C Ve rt e x Dist ance Co rre ct io n

Effe ct ive Po w e r (D) o f Plus- and Minus-p re scrip t io n Sp e ct acle Le nse s at t he Co rne al Plane fo r Vario us Ve rt e x Dist ance s (mm)* (Continue d ) PO WER (D) AT CO RNEAL PLANE FO R DIFFERENT VERTEX DISTANCES (mm) Sp e c Rx

8 mm

10 mm

12 mm

14 mm

16 mm

Plus

Minus

Plus

Minus

Plus

Minus

Plus

Minus

Plus

Minus

13.25

14.82

11.98

15.27

11.70

15.76

11.43

16.27

11.18

16.81

10.93

13.50

15.13

12.18

15.61

11.89

16.11

11.62

16.65

11.35

17.22

11.10

13.75

15.45

12.39

15.94

12.09

16.47

11.80

17.03

11.53

17.63

11.27

14.00

15.77

12.59

16.28

12.28

16.83

11.99

17.41

11.71

18.04

11.44

14.25

16.08

12.79

16.62

12.47

17.19

12.17

17.80

11.88

18.46

11.60

14.50

16.40

12.99

16.96

12.66

17.55

12.35

18.19

12.05

18.88

11.77

14.75

16.72

13.19

17.30

12.85

17.92

12.53

18.59

12.23

19.31

11.93

15.00

17.05

13.39

17.65

13.04

18.29

12.71

18.99

12.40

19.74

12.10

15.25

17.37

13.59

17.99

13.23

18.67

12.89

19.39

12.57

20.17

12.26

15.50

17.69

13.79

18.34

13.42

19.04

13.07

19.80

12.74

20.61

12.42

15.75

18.02

13.99

18.69

13.61

19.42

13.25

20.21

12.90

21.06

12.58

16.00

18.35

14.18

19.05

13.79

19.80

13.42

20.62

13.07

21.51

12.74

16.25

18.68

14.38

19.40

13.98

20.19

13.60

21.04

13.24

21.96

12.90

16.50

19.01

14.58

19.76

14.16

20.57

13.77

21.46

13.40

22.42

13.05

16.75

19.34

14.77

20.12

14.35

20.96

13.95

21.88

13.57

22.88

13.21

17.00

19.68

14.96

20.48

14.53

21.36

14.12

22.31

13.73

23.35

13.36

17.25

20.01

15.16

20.85

14.71

21.75

14.29

22.74

13.89

23.83

13.52

17.50

20.35

15.35

21.21

14.89

22.15

14.46

23.18

14.06

24.31

13.67

17.75

20.69

15.54

21.58

15.07

22.55

14.63

23.62

14.22

24.79

13.82

18.00

21.03

15.73

21.95

15.25

22.96

14.80

24.06

14.38

25.28

13.98

18.25

21.37

15.92

22.32

15.43

23.37

14.97

24.51

14.54

25.78

14.13

18.50

21.71

16.11

22.70

15.61

23.78

15.14

24.97

14.69

26.28

14.27

18.75

22.06

16.30

23.08

15.79

24.19

15.31

25.42

14.85

26.79

14.42

19.00

22.41

16.49

23.46

15.97

24.61

15.47

25.89

15.01

27.30

14.57

19.25

22.75

16.68

23.84

16.14

25.03

15.64

26.35

15.16

27.82

14.72

19.50

23.10

16.87

24.22

16.32

25.46

15.80

26.82

15.32

28.34

14.86

19.75

23.46

17.06

24.61

16.49

25.88

15.97

27.30

15.47

28.87

15.01

20.00

23.81

17.24

25.00

16.67

26.32

16.13

27.78

15.63

29.41

15.15

*Base d on the e q uation: O R = SR / (1 − [d × SR]), whe re : O R = ocular re fraction SR = sp e ctacle re fraction d = ve rte x d istance (m) The le ns p owe rs e nclose d within the he avy b ord e r re late to the stand ard ve rte x d istance of 12 mm that will ap p ly in most case s. (Courte sy of Ad rian S Bruce .)

APPENDIX

Co rne al Curvat ure – Co rne al Po we r Co nve rsio n

D

Co nve rsio n b e t w e e n Co rne al Fro nt Surface Rad ius o f Curvat ure (r; mm) and Co rne al Po w e r (K; D)* r (mm)

K (D)

r (mm) K (D)

r (mm)

K (D)

r (mm)

K (D)

r (mm)

K (D)

6.20

54.44

6.58

51.29

6.96

48.49

7.34

45.98

7.72

43.72

6.21

54.35

6.59

51.21

6.97

48.42

7.35

45.92

7.73

43.66

6.22

54.26

6.60

51.14

6.98

48.35

7.36

45.86

7.74

43.60

6.23

54.17

6.61

51.06

6.99

48.28

7.37

45.79

7.75

43.55

6.24

54.09

6.62

50.98

7.00

48.21

7.38

45.73

7.76

43.49

6.25

54.00

6.63

50.90

7.01

48.15

7.39

45.67

7.77

43.44

6.26

53.91

6.64

50.83

7.02

48.08

7.40

45.61

7.78

43.38

6.27

53.83

6.65

50.75

7.03

48.01

7.41

45.55

7.79

43.32

6.28

53.74

6.66

50.68

7.04

47.94

7.42

45.49

7.80

43.27

6.29

53.66

6.67

50.60

7.05

47.87

7.43

45.42

7.81

43.21

6.30

53.57

6.68

50.52

7.06

47.80

7.44

45.36

7.82

43.16

6.31

53.49

6.69

50.45

7.07

47.74

7.45

45.30

7.83

43.10

6.32

53.40

6.70

50.37

7.08

47.67

7.46

45.24

7.84

43.05

6.33

53.32

6.71

50.30

7.09

47.60

7.47

45.18

7.85

42.99

6.34

53.23

6.72

50.22

7.10

47.54

7.48

45.12

7.86

42.94

6.35

53.15

6.73

50.15

7.11

47.47

7.49

45.06

7.87

42.88

6.36

53.07

6.74

50.07

7.12

47.40

7.50

45.00

7.88

42.83

6.37

52.98

6.75

50.00

7.13

47.34

7.51

44.94

7.89

42.78

6.38

52.90

6.76

49.93

7.14

47.27

7.52

44.88

7.90

42.72

6.39

52.82

6.77

49.85

7.15

47.20

7.53

44.82

7.91

42.67

6.40

52.73

6.78

49.78

7.16

47.14

7.54

44.76

7.92

42.61

6.41

52.65

6.79

49.71

7.17

47.07

7.55

44.70

7.93

42.56

6.42

52.57

6.80

49.63

7.18

47.01

7.56

44.64

7.94

42.51

6.43

52.49

6.81

49.56

7.19

46.94

7.57

44.58

7.95

42.45

6.44

52.41

6.82

49.49

7.20

46.88

7.58

44.53

7.96

42.40

6.45

52.33

6.83

49.41

7.21

46.81

7.59

44.47

7.97

42.35

6.46

52.24

6.84

49.34

7.22

46.75

7.60

44.41

7.98

42.29

6.47

52.16

6.85

49.27

7.23

46.68

7.61

44.35

7.99

42.24

6.48

52.08

6.86

49.20

7.24

46.62

7.62

44.29

8.00

42.19

6.49

52.00

6.87

49.13

7.25

46.55

7.63

44.23

8.01

42.13

6.50

51.92

6.88

49.06

7.26

46.49

7.64

44.18

8.02

42.08

6.51

51.84

6.89

48.98

7.27

46.42

7.65

44.12

8.03

42.03

6.52

51.76

6.90

48.91

7.28

46.36

7.66

44.06

8.04

41.98

6.53

51.68

6.91

48.84

7.29

46.30

7.67

44.00

8.05

41.93

6.54

51.61

6.92

48.77

7.30

46.23

7.68

43.95

8.06

41.87

6.55

51.53

6.93

48.70

7.31

46.17

7.69

43.89

8.07

41.82

6.56

51.45

6.94

48.63

7.32

46.11

7.70

43.83

8.08

41.77

6.57

51.37

6.95

48.56

7.33

46.04

7.71

43.77

8.09

41.72 Continue d

443

444

APPENDIX D

Co rne al Curvat ure – Co rne al Po we r Co nve rsio n

Co nve rsio n b e t w e e n Co rne al Fro nt Surface Rad ius o f Curvat ure (r; mm) and Co rne al Po w e r (K; D)* (Continue d ) r (mm)

K (D)

r (mm) K (D)

r (mm)

K (D)

r (mm)

K (D)

r (mm)

K (D)

8.10

41.67

8.42

40.08

8.74

38.62

9.06

37.25

9.38

35.98

8.11

41.62

8.43

40.04

8.75

38.57

9.07

37.21

9.39

35.94

8.12

41.56

8.44

39.99

8.76

38.53

9.08

37.17

9.40

35.90

8.13

41.51

8.45

39.94

8.77

38.48

9.09

37.13

9.41

35.87

8.14

41.46

8.46

39.89

8.78

38.44

9.10

37.09

9.42

35.83

8.15

41.41

8.47

39.85

8.79

38.40

9.11

37.05

9.43

35.79

8.16

41.36

8.48

39.80

8.80

38.35

9.12

37.01

9.44

35.75

8.17

41.31

8.49

39.75

8.81

38.31

9.13

36.97

9.45

35.71

8.18

41.26

8.50

39.71

8.82

38.27

9.14

36.93

9.46

35.68

8.19

41.21

8.51

39.66

8.83

38.22

9.15

36.89

9.47

35.64

8.20

41.16

8.52

39.61

8.84

38.18

9.16

36.84

9.48

35.60

8.21

41.11

8.53

39.57

8.85

38.14

9.17

36.80

9.49

35.56

8.22

41.06

8.54

39.52

8.86

38.09

9.18

36.76

9.50

35.53

8.23

41.01

8.55

39.47

8.87

38.05

9.19

36.72

9.51

35.49

8.24

40.96

8.56

39.43

8.88

38.01

9.20

36.68

9.52

35.45

8.25

40.91

8.57

39.38

8.89

37.96

9.21

36.64

9.53

35.41

8.26

40.86

8.58

39.34

8.90

37.92

9.22

36.61

9.54

35.38

8.27

40.81

8.59

39.29

8.91

37.88

9.23

36.57

9.55

35.34

8.28

40.76

8.60

39.24

8.92

37.84

9.24

36.53

9.56

35.30

8.29

40.71

8.61

39.20

8.93

37.79

9.25

36.49

9.57

35.27

8.30

40.66

8.62

39.15

8.94

37.75

9.26

36.45

9.58

35.23

8.31

40.61

8.63

39.11

8.95

37.71

9.27

36.41

9.59

35.19

8.32

40.56

8.64

39.06

8.96

37.67

9.28

36.37

9.60

35.16

8.33

40.52

8.65

39.02

8.97

37.63

9.29

36.33

9.61

35.12

8.34

40.47

8.66

38.97

8.98

37.58

9.30

36.29

9.62

35.08

8.35

40.42

8.67

38.93

8.99

37.54

9.31

36.25

9.63

35.05

8.36

40.37

8.68

38.88

9.00

37.50

9.32

36.21

9.64

35.01

8.37

40.32

8.69

38.84

9.01

37.46

9.33

36.17

9.65

34.97

8.38

40.27

8.70

38.79

9.02

37.42

9.34

36.13

9.66

34.94

8.39

40.23

8.71

38.75

9.03

37.38

9.35

36.10

9.67

34.90

8.40

40.18

8.72

38.70

9.04

37.33

9.36

36.06

9.68

34.87

8.41

40.13

8.73

38.66

9.05

37.29

9.37

36.02

9.69

34.83

*Base d on the e q uation: Surface p owe r (D) = (1.3375 − 1.0) / rad ius (m) (Courte sy of Ad rian S Bruce and Suzanne E Efron.)

APPENDIX

E

Ext e nd e d Ke rat o me t e r Rang e Co nve rsio n

Co nve rsio n o f Ke rat o me t e r Re ad ing (D) t o it s Ext e nd e d Value (D) w he n a + 1.25 D Le ns (fo r St e e p Co rne as) o r a − 1.00 D Le ns (fo r Flat Co rne as) is He ld in Fro nt o f t he Ke rat o me t e r* STEEP CO RNEAS (USING A +1.25 D LENS)* Ke rat o me t e r Re ad ing (D)

Ext e nd e d Value (D)

Ke rat o me t e r Re ad ing (D)

Ext e nd e d Value (D)

Ke rat o me t e r Re ad ing (D)

Ext e nd e d Value (D)

43.00 43.13 43.25 43.38 43.50 43.63 43.75 43.88 44.00 44.13 44.25 44.38 44.50 44.63 44.75 44.88 45.00 45.13 45.25 45.38 45.50 45.63 45.75 45.88

50.13 50.28 50.42 50.57 50.72 50.86 51.01 51.15 51.30 51.44 51.59 51.74 51.88 52.03 52.17 52.32 52.47 52.61 52.76 52.90 53.05 53.19 53.34 53.49

46.13 46.25 46.38 46.50 46.63 46.75 46.88 47.00 47.13 47.25 47.38 47.50 47.63 47.75 47.88 48.00 48.13 48.25 48.38 48.50 48.63 48.75 48.88 49.00

53.78 53.92 54.07 54.21 54.36 54.51 54.65 54.80 54.94 55.09 55.23 55.38 55.53 55.67 55.82 55.96 56.11 56.25 56.40 56.55 56.69 56.84 56.98 57.13

49.25 49.38 49.50 49.63 49.75 49.88 50.00 50.13 50.25 50.38 50.50 50.63 50.75 50.88 51.00 51.13 51.25 51.38 51.50 51.63 51.75 51.88 52.00

57.42 57.57 57.71 57.86 58.00 58.15 58.30 58.44 58.59 58.73 58.88 59.02 59.17 59.32 59.46 59.61 59.75 59.90 60.04 60.19 60.34 60.48 60.63

46.00

53.63

49.13

57.27

FLAT CO RNEAS (USING A − 1.00 D LENS)† Ke rat o me t e r Re ad ing (D)

Ext e nd e d Value (D)

Ke rat o me t e r Re ad ing (D)

Ext e nd e d Value (D)

Ke rat o me t e r Re ad ing (D)

Ext e nd e d Value (D)

36.00 36.12 36.25 36.37 36.50 36.62 36.75 36.87 37.00 37.12 37.25 37.37 37.50 37.62 37.75 37.87 38.00

30.87 30.98 31.09 31.19 31.30 31.41 31.52 31.62 31.73 31.84 31.94 32.05 32.16 32.27 32.37 32.48 32.59

38.12 38.25 38.37 38.50 38.62 38.75 38.87 39.00 39.12 39.25 39.37 39.50 39.62 39.75 39.87 40.00 40.12

32.70 32.80 32.91 33.02 33.12 33.23 33.34 33.45 33.55 33.66 33.77 33.88 33.98 34.09 34.20 34.30 34.41

40.25 40.37 40.50 40.62 40.75 40.87 41.00 41.12 41.25 41.37 41.50 41.62 41.75 41.87 42.00

34.52 34.63 34.73 34.84 34.95 35.06 35.16 35.27 35.38 35.48 35.59 35.70 35.81 35.91 36.02

*Base d on the e q uation: Exte nd e d = (1.166 × ke ratome te r) − 0.005. †Base d on the e q uation: Exte nd e d = (1.858 × ke ratome te r) − 0.014. (De rive d rom d ata in: Mand e ll, R. B., (1988) Diop tral and mm curve s or e xte nd e d ke ratome te r rang e . Ap p e nd ix 7. In Contact Le ns Practice (4th e d . p p . 998–999). Sp ring f e ld , IL: Charle s C. Thomas.) (Courte sy o Ad rian S Bruce .)

445

APPENDIX

F

So ft Le ns Ave rag e Thickne ss

The Ave rag e Thickne ss (mm) o f So ft Le nse s o f Give n Ce nt re Thickne ss (mm) and Le ns Po w e r (D)* Ce nt re Thickne ss (mm)

AVERAGE THICKNESS (mm) FO R VARIO US LENS PO WERS (D) +8.00

+6.00

+4.00

+ 2.00

− 2.00

− 4.00

−6.00

−8.00

−10.00

− 12.00

− 14.00

− 16.00

0.03

0.047

0.062

0.075

0.088

0.099

0.110

0.121

0.131

0.04

0.057

0.073

0.087

0.100

0.113

0.124

0.136

0.147

0.05

0.067

0.084

0.098

0.112

0.125

0.137

0.149

0.160

0.06

0.077

0.094

0.109

0.124

0.137

0.150

0.162

0.174

0.07

0.087

0.104

0.120

0.135

0.148

0.162

0.174

0.186

0.08

0.052

0.097

0.114

0.130

0.145

0.160

0.173

0.186

0.199

0.09

0.062

0.107

0.124

0.141

0.156

0.171

0.184

0.198

0.210

0.10

0.072

0.116

0.134

0.151

0.167

0.181

0.195

0.209

0.222

0.11

0.082

0.126

0.144

0.161

0.177

0.192

0.206

0.220

0.233

0.12

0.091

0.136

0.154

0.171

0.187

0.203

0.217

0.231

0.245

0.13

0.071

0.101

0.145

0.164

0.181

0.198

0.213

0.228

0.242

0.256

0.14

0.082

0.111

0.155

0.174

0.191

0.208

0.223

0.238

0.253

0.267

0.15

0.093

0.121

0.165

0.184

0.201

0.218

0.234

0.249

0.263

0.278

0.16

0.103

0.131

0.174

0.193

0.211

0.228

0.244

0.259

0.274

0.288

0.17

0.078

0.113

0.140

0.184

0.203

0.221

0.238

0.254

0.269

0.284

0.299

0.18

0.090

0.123

0.150

0.194

0.213

0.231

0.248

0.264

0.280

0.295

0.309

0.19

0.101

0.134

0.160

0.203

0.223

0.241

0.258

0.274

0.290

0.305

0.320

0.20

0.112

0.144

0.169

0.213

0.232

0.250

0.268

0.284

0.300

0.315

0.330

0.21

0.123

0.153

0.179

0.223

0.242

0.260

0.278

0.294

0.310

0.326

0.341

0.22

0.094

0.134

0.163

0.189

0.232

0.252

0.270

0.287

0.304

0.320

0.336

0.351

0.23

0.107

0.144

0.173

0.198

0.242

0.261

0.280

0.297

0.314

0.330

0.346

0.361

0.24

0.119

0.155

0.183

0.208

0.251

0.271

0.290

0.307

0.324

0.340

0.356

0.372

0.25

0.131

0.165

0.193

0.218

0.261

0.281

0.299

0.317

0.334

0.350

0.366

0.382

*Base d on the following assump tions: • b ack op tic zone rad ius = 8.7 mm • op tic zone d iame te r = 8.00 mm • re fractive ind e x (n) = 1.48 − 0.0015 × wate r conte nt • wate r conte nt = 55% (Note : variations to 38% and 74% only affe ct ave rag e thickne ss b y ± 0.03 mm) • minimum thickne ss at e d g e of op tic zone for p lus le nse s = 0.03 mm (Base d on the the ory outline d in: Bre nnan, N. A. (1984). Ave rag e thickne ss of a hyd ro g e l le ns for g as transmissib ility calculations. Am. J. O p tom. Physiol. O p t., 61, 627–635.) (Courte sy of Ad rian S Bruce .)

446

APPENDIX

G

So ft Le ns O xyg e n Pe rfo rmance

Co nve rsio n Be t w e e n So ft (hyd ro g e l) Le ns Wat e r Co nt e nt (%), Barre r (Fat t unit s) and ISO O xyg e n Pe rme ab ilit y (Dk) Value s, and Fat t O xyg e n Transmissib ilit y (Dk / t ) Value s fo r Vario us Le ns Thickne sse s Wat e r Co nt e nt (%)

Dk Barre r Unit s* at 35°C

Dk ISO Unit s † at 35°C

Dk / t ‡ AT 35°C (BASED O N BARRER UNITS) FO R VARIO US LENS THICKNESSES (t , mm) 0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.12

0.15

0.20

0.25

0.30

35

6.7

5.0

22.3

16.8

13.4

11.2

9.6

8.4

7.4

6.7

5.6

4.5

3.4

2.7

2.2

36

7.0

5.2

23.2

17.4

13.9

11.6

10.0

8.7

7.7

7.0

5.8

4.6

3.5

2.8

2.3

37

7.3

5.4

24.2

18.1

14.5

12.1

10.4

9.1

8.1

7.3

6.0

4.8

3.6

2.9

2.4

38

7.5

5.7

25.2

18.9

15.1

12.6

10.8

9.4

8.4

7.5

6.3

5.0

3.8

3.0

2.5

39

7.9

5.9

19.6

15.7

13.1

11.2

9.8

8.7

7.9

6.5

5.2

3.9

3.1

2.6

40

8.2

6.1

20.4

16.3

13.6

11.7

10.2

9.1

8.2

6.8

5.4

4.1

3.3

2.7

41

8.5

6.4

21.3

17.0

14.2

12.1

10.6

9.4

8.5

7.1

5.7

4.3

3.4

2.8

42

8.8

6.6

22.1

17.7

14.7

12.6

11.1

9.8

8.8

7.4

5.9

4.4

3.5

2.9

43

9.2

6.9

23.0

18.4

15.3

13.2

11.5

10.2

9.2

7.7

6.1

4.6

3.7

3.1

44

9.6

7.2

23.9

19.2

16.0

13.7

12.0

10.6

9.6

8.0

6.4

4.8

3.8

3.2

45

10.0

7.5

24.9

19.9

16.6

14.2

12.5

11.1

10.0

8.3

6.6

5.0

4.0

3.3

46

10.4

7.8

20.7

17.3

14.8

13.0

11.5

10.4

8.6

6.9

5.2

4.1

3.5

47

10.8

8.1

21.6

18.0

15.4

13.5

12.0

10.8

9.0

7.2

5.4

4.3

3.6

48

11.2

8.4

22.5

18.7

16.0

14.0

12.5

11.2

9.4

7.5

5.6

4.5

3.7

49

11.7

8.8

23.4

19.5

16.7

14.6

13.0

11.7

9.7

7.8

5.8

4.7

3.9

50

12.2

9.1

24.3

20.3

17.4

15.2

13.5

12.2

10.1

8.1

6.1

4.9

4.1

51

12.6

9.5

21.1

18.1

15.8

14.1

12.6

10.5

8.4

6.3

5.1

4.2

52

13.2

9.9

21.9

18.8

16.5

14.6

13.2

11.0

8.8

6.6

5.3

4.4

53

13.7

10.3

22.8

19.6

17.1

15.2

13.7

11.4

9.1

6.8

5.5

4.6

54

14.2

10.7

23.7

20.4

17.8

15.8

14.2

11.9

9.5

7.1

5.7

4.7

55

14.8

11.1

24.7

21.2

18.5

16.5

14.8

12.4

9.9

7.4

5.9

4.9

56

15.4

11.6

22.0

19.3

17.1

15.4

12.9

10.3

7.7

6.2

5.1

57

16.1

12.0

22.9

20.1

17.8

16.1

13.4

10.7

8.0

6.4

5.4

58

16.7

12.5

23.9

20.9

18.6

16.7

13.9

11.1

8.4

6.7

5.6

59

17.4

13.0

24.8

21.7

19.3

17.4

14.5

11.6

8.7

7.0

5.8

60

18.1

13.6

25.8

22.6

20.1

18.1

15.1

12.1

9.0

7.2

6.0

61

18.8

14.1

23.5

20.9

18.8

15.7

12.5

9.4

7.5

6.3

62

19.6

14.7

24.5

21.7

19.6

16.3

13.0

9.8

7.8

6.5

63

20.4

15.3

22.6

20.4

17.0

13.6

10.2

8.1

6.8

64

21.2

15.9

23.5

21.2

17.7

14.1

10.6

8.5

7.1

65

22.1

16.5

24.5

22.1

18.4

14.7

11.0

8.8

7.4

66

22.9

17.2

22.9

19.1

15.3

11.5

9.2

7.6

67

23.9

17.9

23.9

19.9

15.9

11.9

9.5

8.0

68

24.8

18.6

24.8

20.7

16.6

12.4

9.9

8.3

69

25.8

19.4

25.8

21.5

17.2

12.9

10.3

8.6

70

26.9

20.2

26.9

22.4

17.9

13.4

10.8

9.0

71

28.0

21.0

23.3

18.7

14.0

11.2

9.3

Continue d

447

448

APPENDIX G So ft Le ns O xyg e n Pe rfo rmance

Co nve rsio n Be t w e e n So ft (hyd ro g e l) Le ns Wat e r Co nt e nt (%), Barre r (Fat t unit s) and ISO O xyg e n Pe rme ab ilit y (Dk) Value s, and Fat t O xyg e n Transmissib ilit y (Dk / t ) Value s fo r Vario us Le ns Thickne sse s (Continue d ) Wat e r Co nt e nt (%)

Dk Barre r Unit s* at 35°C

Dk ISO Unit s † at 35°C

72

29.1

73

Dk / t ‡ AT 35°C (BASED O N BARRER UNITS) FO R VARIO US LENS THICKNESSES (t , mm) 0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.12

0.15

0.20

0.25

0.30

21.8

24.3

19.4

14.6

11.6

9.7

30.3

22.7

25.2

20.2

15.1

12.1

10.1

74

31.5

23.6

21.0

15.8

12.6

10.5

75

32.8

24.6

21.9

16.4

13.1

10.9

76

34.1

25.6

22.7

17.1

13.6

11.4

77

35.5

26.6

23.7

17.8

14.2

11.8

78

36.9

27.7

24.6

18.5

14.8

12.3

79

38.4

28.8

19.2

15.4

12.8

80

40.0

30.0

20.0

16.0

13.3

• For le nse s ≤± 1.50 D: ave rag e thickne ss = ce ntre thickne ss. • Dk / t value s for thickne sse s b e low a minimum manufacturab le thickne ss are not shown. • Data in this tab le are b ase d on the Morg an–Efron e q uation, which corre cts for b ound ary and e d g e e ffe ct e rrors d uring p olarog rap hic me asure me nt: Dk = 1.67 × 10− 11 e xp 0.0397WC (Morg an, P. B. & E ron, N. (1998). The oxyg e n p e r ormance o conte mp orary hyd rog e l contact le nse s. Contact Le ns Ant. Eye , 21, 3–6.) • The shad e d ce lls re p re se nt Dk / t value s that are consid e re d to b e ad e q uate for op e n-e ye (d aily) le ns we ar; that is, Dk / t < 12 Barre r / cm (Be njamin W. J. (1996). Downsizing o Dk and Dk / L: The d i f culty in using hPa inste ad o mmHg . Int. Contact Le ns Clin., 23, 188–189.) • The d ata in this tab le d o not ap p ly to silicone hyd rog e l le nse s, which are d e scrib e d b y a wate r conte nt–Dk re lationship that is e sse ntially the inve rse of that which is use d he re to d e scrib e hyd rog e l le nse s. *Trad itional units of oxyg e n p e rme ab ility are : × 10− 11 (cm 2 × ml O 2) / (s × ml × mmHg ), or Barre r. †ISO units of oxyg e n p e rme ab ility are : × 10–11 (cm 2 × ml O ) / (s × ml × hPa). 2 ‡Trad itional units of oxyg e n transmissib ility are : × 10− 9 (cm × ml O ) / (s × ml × mmHg ), or Barre r / cm. 2 (Courte sy o Ad rian S Bruce .)

APPENDIX

H

Co nst ant Ed g e Cle arance Rig id Le ns De sig ns

The o re t ical Ed g e Cle arance s fo r a Le ns Fit t e d w it h 10 µm Ce nt ral Cle arance o n a Co rne a w it h a Shap e Fact o r o f 0.85* 8.40 m DIAMETER – 69 µm EDGE CLEARANCE BO ZR 7.00 7.10 7.20 7.30 7.40

7.50

7.60

7.70

7.80

7.90

8.00

8.10

8.20

8.30

8.40

8.50

BO ZD

7.40

7.40

7.40

7.40

7.40

7.40

7.40

7.40

7.40

7.40

7.40

7.40

7.40

7.40

7.40

7.40

BPR1

7.80

7.90

8.05

8.20

8.30

8.45

8.60

8.75

8.90

9.00

9.15

9.25

9.40

9.50

9.65

9.80

BPZD1

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

BPR2

9.80

10.10

10.30

10.50

10.90

11.20

11.40

11.70

12.00

12.40

12.60

13.10

13.40

13.80

14.10

14.40

TD

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.40

8.80 m DIAMETER – 73 µm EDGE CLEARANCE BO ZR 7.00 7.10 7.20 7.30 7.40

7.50

7.60

7.70

7.80

7.90

8.00

8.10

8.20

8.30

8.40

8.50

BO ZD

7.60

7.60

7.60

7.60

7.60

7.60

7.60

7.60

7.60

7.60

7.60

7.60

7.60

7.60

7.60

7.60

BPR1

7.70

7.85

7.95

8.10

8.20

8.35

8.45

8.60

8.75

8.90

9.00

9.15

9.25

9.40

8.55

9.65

BPZD1

8.30

8.30

8.30

8.30

8.30

8.30

8.30

8.30

8.30

8.30

8.30

8.30

8.30

8.30

8.30

8.30

BPR2

8.90

9.10

9.30

9.50

9.80

10.00

10.30

10.40

10.70

10.80

11.10

11.40

11.70

11.90

12.10

12.50

TD

8.80

8.80

8.80

8.80

8.80

8.80

8.80

8.80

8.80

8.80

8.80

8.80

8.80

8.80

8.80

8.80

9.20 m DIAMETER – 80 µm EDGE CLEARANCE BO ZR 7.00 7.10 7.20 7.30 7.40

7.50

7.60

7.70

7.80

7.90

8.00

8.10

8.20

8.30

8.40

8.50

BO ZD

7.80

7.80

7.80

7.80

7.80

7.80

7.80

7.80

7.80

7.80

7.80

7.80

7.80

7.80

7.80

7.80

BPR1

7.60

7.70

7.85

7.95

8.10

8.20

8.35

8.45

8.60

8.70

8.85

8.95

9.10

9.20

9.35

9.47

BPZD1

8.60

8.60

8.60

8.60

8.60

8.60

8.60

8.60

8.60

8.60

8.60

8.60

8.60

8.60

8.60

8.60

BPR2

8.60

8.80

9.00

9.20

9.40

9.60

9.80

10.00

10.20

10.40

10.60

10.90

11.10

11.30

11.60

11.83

TD

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.20

9.60 m DIAMETER – 90 µm EDGE CLEARANCE BO ZR 7.00 7.10 7.20 7.30 7.40

7.50

7.60

7.70

7.80

7.90

8.00

8.10

8.20

8.30

8.40

8.50

BO ZD

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

BPR1

7.60

7.70

7.80

7.95

8.10

8.20

8.30

8.45

8.55

8.70

8.85

8.95

9.10

9.20

9.35

9.50

BPZD1

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

BPR2

8.30

8.50

8.70

8.90

9.00

9.30

9.50

9.70

9.90

10.10

10.20

10.50

10.60

10.90

11.00

11.20

TD

9.60

9.60

9.60

9.60

9.60

9.60

9.60

9.60

9.60

9.60

9.60

9.60

9.60

9.60

9.60

9.60

10.00 mm DIAMETER – 105 µm EDGE CLEARANCE BO ZR 7.00 7.10 7.20 7.30 7.40 7.50

7.60

7.70

7.80

7.90

8.00

8.10

8.20

8.30

8.40

8.50

BO ZD

8.20

8.20

8.20

8.20

8.20

8.20

8.20

8.20

8.20

8.20

8.20

8.20

8.20

8.20

8.20

8.20

BPR1

7.65

7.70

7.80

7.95

8.10

8.20

8.35

8.45

8.65

8.70

8.85

9.00

9.15

9.25

9.40

9.50

BPZD1

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

8.90

BPR2

8.00

8.15

8.35

8.50

8.60

8.80

8.90

9.10

9.15

9.45

9.60

9.75

9.90

10.10

10.25

10.45

BPZD2

9.50

9.50

9.50

9.50

9.50

9.50

9.50

9.50

9.50

9.50

9.50

9.50

9.50

9.50

9.50

9.50

BPR3

8.10

8.30

8.40

8.60

8.80

9.00

9.20

9.30

9.50

9.70

9.80

10.00

10.20

10.40

10.50

10.70

TD

10.00

10.00

10.00

10.00

10.00

10.00

10.00

10.00

10.00

10.00

10.00

10.00

10.00

10.00

10.00

10.00

*(Base d on the d e sig n conce p t of Guillon, M., Lyd on, D. P. M. & Sammons, W. A. (1983). De sig ning rig id g as-p e rme ab le contact le nse s using the e d g e cle arance te chniq ue . J. Br. Contact Le ns Assoc., 6, 19–25.) (Courte sy of Grae me Young .)

449

APPENDIX

I

So ft To ric Le ns Misalig nme nt De mo nst rat o r

Re sid ual Re fract ive Erro r Ind uce d b y Mislo cat io n o f To ric Le nse s o f Give n Cylind rical Po w e rs, Base d up o n an O cular Re fract io n o f Plano / − cyl × 180

Example • • • • •

O cular re fraction (re q uire d re fractive corre ction) is: p lano / −1.00 × 180 A le ns is p lace d on the e ye of b ack ve rte x p owe r: p lano / − 1.00 × 180 An ove rre fracton yie ld s a re sid ual e rror of: + 0.34 / − 0.68 × 35 From the tab le , a mislocation is ind icate d of: − 20° The le ns b ack ve rte x p owe r in situ is: p lano / −1.00 × 160

Mislocation (−)(°)

−1.00 D Cylind e r

−2.00 D Cylind e r

−3.00 D Cylind e r

5

0.09 / − 0.17 × 42.5

0.17 / − 0.35 × 42.5

0.26 / − 0.52 × 42.5

10

0.17 / −0.35 × 40.0

0.35 / −0.69 × 40.0

0.52 / −1.04 × 40.0

15

0.26 / −0.52 × 37.5

0.52 / −1.04 × 37.5

0.78 / −1.55 × 37.5

20

0.34 / −0.68 × 35.0

0.68 / −1.37 × 35.0

1.03 / −2.05 × 35.0

25

0.42 / −0.85 × 32.5

0.85 / −1.69 × 32.5

1.27 / −2.54 × 32.5

30

0.50 / −1.00 × 30.0

1.00 / −2.00 × 30.0

1.50 / −3.00 × 30.0

35

0.57 / −1.15 × 27.5

1.15 / −2.29 × 27.5

1.72 / −3.44 × 27.5

40

0.64 / −1.29 × 25.0

1.29 / −2.57 × 25.0

1.93 / −3.86 × 25.0

45

0.71 / −1.41 × 22.5

1.41 / −2.83 × 22.5

2.12 / −4.24 × 22.5

50

0.77 / −1.53 × 20.0

1.53 / −3.06 × 20.0

2.30 / −4.60 × 20.0

55

0.82 / −1.64 × 17.5

1.64 / −3.28 × 17.5

2.46 / −4.91 × 17.5

60

0.87 / −1.73 × 15.0

1.73 / −3.46 × 15.0

2.60 / −5.20 × 15.0

65

0.91 / −1.81 × 12.5

1.81 / −3.63 × 12.5

2.72 / −5.44 × 12.5

70

0.94 / −1.88 × 10.0

1.88 / −3.76 × 10.0

2.82 / −5.64 × 10.0

75

0.97 / −1.93 × 7.5

1.93 / −3.86 × 7.5

2.90 / −5.80 × 7.5

80

0.98 / −1.97 × 5.0

1.97 / −3.94 × 5.0

2.95 / −5.91 × 5.0

85

1.00 / −1.99 × 2.5

1.99 / −3.98 × 2.5

2.99 / −5.98 × 2.5

90

1.00 / −2.00 × 0.0

2.00 / −4.00 × 0.0

3.00 / −6.00 × 0.0

Rule s of thumb for soft toric mislocation: 1. The sp he rical e q uivale nt is ze ro if the e rror is p ure ly d ue to axis misalig nme nt. 2. The d ire ction of le ns misalig nme nt is always op p osite to the axis of the ove rre fraction, re lative to the p re scrib e d cylind e r axis. 3. LARS: ‘le ft ad d , rig ht sub tract’. Whe n allowing for nasal rotation in the rig ht e ye , the amount of rotation should b e sub tracte d from the re q uire d cylind e r axis and vice ve rsa for the le ft e ye . (Ge ne rate d using the formulae in: Lind say, R. G., Bruce , A. S., Bre nnan, N. A. & Pianta, M. J. (1997). De te rmining axis misalig nme nt and p owe r e rrors of toric soft le nse s. Int. Contact Le ns. Clin., 24, 101–107.) (Courte sy of Ad rian S Bruce .)

450

APPENDIX

J

Dry-e ye Q ue st io nnaire

A Typ ical Dry-e ye Q ue st io nnaire Symp t o m

Fre q ue ncy Ne ve r

Some time s

O fte n

Constantly

Sand y or g ritty e e ling Burning Dryne ss Sore ne ss Scratchine ss Itching Eye d iscom ort on waking Te aring (wate ry e ye s) Fore ig n-b od y se nsation Eye p ain Re d ne ss Mucus d ischarg e Lig ht se nsitivity Blurre d vision In e ction o lid s or e ye Hay e ve r O the r alle rg y Nasal cong e stion Coug h Bronchitis Cold symp toms Sinus cong e stion Dryne ss o throat / mouth / vag ina Troub le swallowing ood Asthma symp toms Joint p ain Muscle p ain DO YO UR EYES SEEM O VERLY SENSITIVE TO : Cig are tte smoke ■ Swimming ■ Smog

■

Alcohol

■

Air cond itioning

■

Sunshine

■

Heater and air blowers ■

Wind

■

Dust

■

Use o VDUs

■

Polle n

Eye d rop s

■

■

451

452

APPENDIX J Dry-e ye Q ue st io nnaire

GENERAL Q UESTIO NS RELATING TO DRY EYE Do you we ar contact le nse s?

Ye s

No

Have you p re viously trie d to we ar contact le nse s? Do you use artif cial te ars or d rop s? Have you e ve r b e e n told you sle e p with your e ye s op e n? Have you e ve r b e e n tre ate d or d ry e ye s? Do you take antihistamine s? Do you take b irth control p ills? Are you taking d iure tics (wate r p ills)? Are you taking any othe r me d ication (p re scrib e d or ove r the counte r)? I ye s, g ive b rand and re ason or taking me d ication Have Yo u o r Any Blo o d Re lat ive Had Any o f t he Fo llo wing ?

Co nd it io n Yourse lf

Re lative

Re lationship

Arthritis Thyroid d ise ase Lup us Sarcoid osis Asthma Sjög re n’s synd rome Skin d isord e rs He art d ise ase Hyp e rte nsion Gout Cataract Diab e te s Glaucoma (Ad ap te d with p e rmission rom: Lowthe r, G. E. (1997). Examination o p atie nts and p re d icting te ar-f lm re late d p rob le ms with hyd rog e l le ns we ar. In Dryne ss, Te ars and Contact Le ns We ar (p p . 23–53). O x ord : Butte rworth-He ine mann) (Courte sy o Ke ith H Ed ward s.)

APPENDIX

K

Efro n Grad ing Scale s fo r Co nt act Le ns Co mp licat io ns

T e grading scales presented on the ollowing two pages were devised by Pro essor Nathan E ron and painted by the ophthalmic artist, erry R arrant. T ese grading scales are designed to assist practitioners to quanti y the level o severity o a variety o contact lens complications. T e eight complications on the f rst page are those that are more likely to be encountered in contact lens practice. Many o these complications are graded routinely by some

practitioners. T e complications on the second page are less commonly encountered in contact lens practice, and represent pathology that is rare or unusual. An explanation as to how to use these grading scales is given in Chapter 39. T e development o these grading scales was kindly sponsored by CooperVision.

453

454

APPENDIX K Efro n Grad ing Scale s fo r Co nt act Le ns Co mp licat io ns

APPENDIX K

Efro n Grad ing Scale s fo r Co nt act Le ns Co mp licat io ns

455

APPENDIX

L

Scle ral Le ns Fit Scale s

To accurately estimate the amount o vaulting (clearance) underneath the posterior sur ace o a scleral lens necessitates a re erence point or comparison. Although some have suggested corneal thickness or the re erence, we pre er the centre

Ce nt ral Vault ing

 

456

thickness (CT) o the lens itsel , which is specif ed by the manu acturer. In each o the examples below, the CT is 0.30 mm (300 µm). In most scleral lens designs, the ideal amount o clearance is about 300 µm.

APPENDIX L

Scle ral Le ns Fit Scale s

Limb al Vault ing None

Good

Moderate

 

Ed g e Re lat io nship

(Imag e s and te xt re p rod uce d with p e rmission from Josh Lotoczky, O D; Chad Rose n, O D; and Craig W. Norman, FCLSA.)

457

This pa ge inte ntiona lly le ft bla nk

INDEX

A

Abbe re ractometer, or rigid lens measurement, 141 Abbreviations, used or describing contact lens, 439t Aberrations rigid lens optics, 133–134 so lens, 69–72, 70 spherical, 70, 71 visual optics and high-order, correction o , 38 ocular depth o ocus, 38, 38 white light, 37, 37 . see also Chromatic aberration Aberrometer, 350 Acanthamoeba keratitis, 399 Accelerated orthokeratology, outcomes o , 297–299 Accessory lacrimal glands, 24 Accommodation demand, spectacle corrections and, 40–41, 41 precision o ocular ocus and, 29–30, 30 Accumulation o mucus, o scleral lenses, 201 Acoustic neuroma, 277, 277 Acuvue lens, 175, 175 Adjustments to lenses, bi ocal and multi ocal contact lenses, 223 common reasons why lenses ail to, 224t Advantages o planned so lens replacement a ercare schedules, enhanced compliance with, 181 avoidance o long-term adverse changes in anterior eye, 178–180 in so lenses, 176–178 higher-water-content hydrogel materials, use o , 180 lens parameters easy to change, 181–182 ready availability o replacement lenses, 181 silicone hydrogel materials, use o , 180 simple lens care regimens, 180–181, 181t single-use trial lenses, 181 trial lens tting with accurate prescription, 181, 182 Advantages o regular planned rigid lens replacement, 189–191 daily wear, 190, 190 extended wear, 190–191 lens binding, 191, 191 A ercare, 364–384.e2 procedures ollowing lens removal, 369–376 corneal topography, 370–373 keratometry, 370–373 lens inspection/veri cation, 374–375 pachymetry, 373, 374 re raction, 369–370 slit-lamp biomicroscopy, 373–374, 375 vision, uncorrected, 369 procedures while lenses are worn, 365–369 external examination, 366 history taking, 365–366 overkeratometry, 366, 367 overre raction, 366 slit-lamp biomicroscopy, 366–369 visual acuity, 366 schedules, enhanced compliance with, 181 solving problems, in contact lens wear, 379–384 visit, preparing or, 364–365 discussion with patient, 377

Against-the-rule corneal astigmatism, rigid toric lens and, 157 Age, accommodation amplitude and, 29, 30 Air Optix Colors, 208, 209 Air Optix Night and Day lens, 57 Air ow, and lens wear, 249 Alignment bitoric lenses, 160 ALK. see Anterior lamellar keratoplasty (ALK) Alternating copolymer, so lens and, 46, 46 Alternating-image (translating) designs, bi ocal and multi ocal contact lenses, 224–226, 225 general principles o lens designs, 224–225, 225 Alternating-vision bi ocal lenses, 383 Altitude, and sport, 247–248 Ametropia basic optics o eye and, 28–30 accommodation and precision o ocular ocus, 29–30, 30 general optical characteristics o , 28–30, 28 –29 model eyes and, 28–29, 29t, 30 high. see High ametropia Amorphous polymer, so lens and, 46–47, 47 Analogue, discom ort, 381, 381 Anatomical measurements, o anterior eye, 347 Anisometropes, spectacle magni cation and, 40 Anterior blepharitis, 388 Anterior corneal shape, assessment o , 337–340 Anterior eye, 10–27.e2 avoidance o long-term adverse changes in, 178–180 discom ort, 179, 179 ocular sur ace pathology, 179–180 reduced vision, 179, 179 conjunctiva, 20–23, 314 cornea, 10–17 digital imaging, resolution in, 411, 411 –412 eyelids in, 17–20 lacrimal system in, 23–25 ocular adnexa in, 17–25 preocular tear lm in, 25 Anterior lamellar keratoplasty (ALK), 288 Anterior segment imaging technologies, in keratoconus assessment, 251 Anterior-segment morphology, assessment o , 337–343 anterior corneal shape, 337–340 posterior corneal shape, 338–340 scleral shape, corneal and, 340–343 Anterior-segment optical coherence tomography (AS-OC ), 375–376, 376 or corneal and scleral shape, 340–343 or corneal thickness, 344, 345 Anti-myopia lenses, 7 Aphakia in children, 268–269, 268 –269 and high-power lens design, 264–265 Apical bearing, 254, 254 Apical clearance, 254–255, 254 Application o lens. see Insertion o lens Aquatic environment, and lens wear, 248 AS-OC . see Anterior-segment optical coherence tomography (AS-OC ) Aspheric curve, 219–220 Aspheric lens designs, 256

bi ocal and multi ocal contact lenses, 219, 220 back-sur ace, 220 ront-sur ace, 219–220, 220 zonal aspheric and spherical designs, 220–221, 221 rigid, 146–147, 146t Aspheric rigid lens manu acture, 128 Association o British Dispensing Opticians, 435 Association o Contact Lens Manu acturers, 361 Astigmatism against-the-rule corneal, 157 corneal, neutralization o , by rigid lens o spherical power, 132–133, 132 high, and high-power lens designs, 265–266 induced, 159–160 irregular, in children, 270, 270 residual, 159, 159 in keratoconus, 256 in so toric lenses degree o , 95 irregular, 102 visual optics and, 29 Autolensometer, 74, 74 Avaira lenses, 57–59 Average thickness o so lenses, 446t Axial edge li , 138, 139 Axial edge thickness, 439t

B

Babies. see Paediatric tting Back optic zone diameter (BOZD), 197, 254, 254 , 439t in rigid lens design, 145 Back optic zone radius (BOZR), 83, 197, 347, 439t rigid toric lens, 157 so lens design, 88 Back peripheral radius, 439t Back peripheral zone diameters, 439t Back scleral radius (BSR), 197 Back scleral size (BSS), 198 Back-sur ace aspheric designs, 220 Back sur ace o lens generation o , 123–124, 125 toric, 128 toroidal, 96 Back sur ace radius, o curvature, in rigid lens measurement, 137–138 Back-sur ace toric lenses, 160–161 Back vertex power (BVP), 74, 439t calculating BVP in situ, 100–101 determination o , in so toric lenses, 98 required, di erent BOZR and, 131, 131 rigid lens, 158 Bacterial keratitis, 405 Bandage lenses, 283 Barnacle-like calcium carbonate deposits, 177, 177 Basal lamina, o cornea, 11, 11 Base curve radius, in so lens design, 88 Bausch & Lomb lens heat unit, 104, 104 Bedewing, corneal endothelium, 406–407, 407 BHVI eye mapper, 311, 312 BHVI scorecard, or myopia, 307, 308 Biconcentric designs, bi ocal and multi ocal contact lenses, 218

Pages ollowed by b, t, or f re er to boxes, tables, or gures, respectively.

459

460

Ind e x

Bi ocal and multi ocal contact lenses, 217–229 alternating-image (translating) designs, 224–226, 225 historical designs o , 218 monovision, 226–229, 227 –228 or myopia, 309 simultaneous image designs, 218–224, 219 Binding lens, planned replacement or rigid lenses and, 191, 191 Binocular vision assessment, 352 Bio nity lenses, 57–59 Biometry, or children examination, 271–272 ‘Biomimetic’ contact lenses, 54 Bitoric rigid lenses, therapeutic applications, 280 Blebs, endothelial, 239, 407–408, 407 –408 E ron grading scale or, 455 Blemish, as non-edge (body) de ects, 66 Blepharitis, 388 E ron grading scale or, 455 Blinking contact lenses and, 385, 386 and tting high-plus lenses, 266 technique, in lens removal, 360, 360 Block copolymer, so lens and, 46, 46 Blood and lymphatic supply, to eyelids, 20 Blood vessels, o conjunctiva, 22, 22 Body contact, extreme, and lens wear, 249 Body movements, extreme, and lens wear, 249 Bowman’s layer, o cornea, 11, 11 Boxing, and lens wear, 249 BOZD. see Back optic zone diameter (BOZD) BOZR. see Back optic zone radius (BOZR) Branched homopolymer, or hydrogel lenses, 46, 46 BSR. see Back scleral radius (BSR) BSS. see Back scleral size (BSS) Bubbles, o scleral lenses, 201 Bull’s eye, orthokeratology and, 301, 302 Burton lamp, 327, 327 BVP. see Back vertex power (BVP)

C

CAB. see Cellulose acetate butyrate (CAB) Capillary orces, rigid lens and, 144, 145 Captive bubble technique, hydrogels and, 50 Carbamate-substituted RIS ( PVC), or silicone hydrogel lenses, 56 , 57 Care products, or contact lenses, 361 Carrier junction thickness, 439t Cast moulding, 63, 64 –65 Cataracts, 316 Caucasian eyes, in so lens tting, 94 CCD. see Charge-coupled device (CCD) Cellulose acetate butyrate (CAB), 116 Central t, o rigid lens, 152–153, 152 –153 Central island, orthokeratology and, 301, 302 Central vaulting, 456, 456 Centration, in so lens tting, 91 Centre, in so lens design, 88 Centre-distance design, bi ocal and multi ocal contact lenses, 218 Centre thickness (C ), 456 in rigid lens design, 145, 145t in rigid lens measurement, 137 Centred cones, 251 Chain polymerization, 47–48 Charge-coupled device (CCD), 411 Chelating agent, 108–109, 109 Chemical bond tinting, o lenses, 208, 208 Chemical injuries, therapeutic applications and, 276, 276 Children. see Paediatric tting Chinese eyes, in so lens tting, 94 Chlorhexidine-preserved system, 105 Chlorine, 105–106

Chromatic aberration, visual optics and, 37, 37 white light, overall optical per ormance o eye, 37, 37 Cicatricial conjunctivitis, therapeutic applications and, 276 CIEs. see Corneal in ltrative events (CIEs) CL-MGD, Contact-lens-associated meibomian gland dys unction (CL-MGD) CLARE. see Contact-lens-associated acute red eye (CLARE) Cleaning o lenses scleral lenses, and conditioning, 201 so contact lenses, 103–104 Cleaning solutions, or rigid lens, 163–164 Clearance, in scleral lens design, 456 ClearKone lenses, or keratoconus, 261, 261 CLEB. see Contact-lens-associated endothelial bedewing (CLEB) CLEK study. see Collaborative Longitudinal Evaluation o Keratoconus (CLEK) study Clinic, compliance enhancement model, 425 Clinical data, optimum rigid lens replacement schedule, 191 CLPC. see Contact-lens-induced papillary conjunctivitis (CLPC) CLR12-70 re ractometer, 80, 80 CLSLK. see Contact-lens-induced superior limbic keratoconjunctivitis (CLSLK) Cobalt blue lter, in illumination system, 333 Cochet-Bonnet aesthesiometer, 335, 337 Codex of the Eye, Manual D, in contact lens, 3 Coef cient o riction (CoF), hydrogels and, 51 CoF. see Coef cient o riction (CoF) Cold environment, and sport, 247 Collaborative Longitudinal Evaluation o Keratoconus (CLEK) study, 255 College o Optometrists, 435 Colour vision, 206 testing, 355 Com ort, in so lens tting, 91 Com ort drops, 357 Commercial digital imaging system, 413 Commercial rigid lens materials, properties o , 119–122 exure, 119–121 hardness, 121 mechanical, 119–121 optical, 121–122 oxygen permeability, 119 re ractive index, 122 sur ace, 121 Compliance, 420–426.e1 with correct prescription, 423, 423 in duration o prescription, 420, 420 enhancement o , 423–424 erroneous procedures, 421–422, 421 with incorrect prescription, 422–423, 423 industry role, 425–426 patient, history taking and, 325–326 Compliance enhancement model, 424–426 Complications, in contact lenses, 385–409.e2 in conjunctiva, 391–394 in corneal epithelium, 397–400, 406–409 in corneal stroma, 400–406 E ron grading scale or, 453, 454 –455 in eyelids, 385–389 in limbus, 394–397 in tear lm, 389–391 Compression, hydrogels and, 48 Cone, morphology o , in keratoconus, 251, 252 Con ocal microscopy, 334–335, 375, 417–418 or corneal thickness, 344–345 ocusing through, 335, 336 slit-scanning, 335 tandem scanning, 334–335

Congenital cataract, surgery or, 268 Conjunctiva anterior eye, 20–23 blood supply o , 22 and diabetes, 314 unctional considerations in, 23 gross anatomy, 20–21, 20 –21 innervation o , 22 microscopic anatomy, 21–22 blanching, o scleral lenses, 201 complications in, 391–394 displacement and thinning, o scleral lenses, 201–202, 202 epithelial ap, 392 epithelium o , 21, 21 hemorrhage, eye redness and, 379, 379 –380 indentation, in so toric lenses, 102 papillary conjunctivitis, 393–394, 393 –394 redness, 392–393, 393 E ron grading scale or, 454 limbal and bulbar, 380 staining, 391–392, 392 E ron grading scale or, 454 stroma, 22, 22 Conjunctivitis, papillary, 245 in children, 274 Constant edge clearance, rigid lens designs, 449t Consulting room, practice, 428 Contact aesthesiometry, 335, 337 Contact angle, hydrogels and, 50, 50 Contact-lens-associated acute red eye (CLARE), 382 Contact-lens-associated endothelial bedewing (CLEB), 406–407, 407 Contact-lens-associated meibomian gland dys unction (CL-MGD), 386–387, 387 Contact lens dispensary, practice, 428 Contact-lens-induced corneal neovascularization, 404 Contact-lens-induced hypoxia, 394–395 Contact-lens-induced papillary conjunctivitis (CLPC), 393–394 Contact-lens-induced superior limbic keratoconjunctivitis (CLSLK), 395–396 Contact lens products, managing o , 433 Contact lens-related papillary conjunctivitis, 236, 236 σ contact lenses, 250 Continuous wear, de nition o , 231 Contracts, 432 Contrast sensitivity, 70, 71 unction, in poor vision, 382 Conventional so extended wear, 231–232 Convergence demand, spectacle corrections and, 41, 41 CooperVision MiSight lens, 7 Copolymer, so lens and, 46, 46 Cornea anterior eye, 10–17 and diabetes, 314, 315t epithelial wound healing o , 16–17 gross anatomy o , 10, 10t microscopic anatomy o , 10–13, 10 transparency o , 15–16 asphericity, 143 collagen cross-linking (CXL), and keratoconus, 262 degenerations involving endothelium, 276 involving epithelium, 276 distortion, 203, 370 E ron grading scale or, 455 dystrophies involving the epithelium, 276 epithelial pain, 275 exhaustion, 240 ront sur ace radius o curvature, and corneal power, conversion between, 443t–444t

Ind e x Cornea (Continued) gra or keratoconus, 262 rejection and ailure, 293–294. see also Post-keratoplasty types o , 288–289 hydration control, in anterior eye, 315–316 hypoxia, in so toric lenses, 102 in ltrates, E ron grading scale or, 455 innervation o , 13–14 unctional considerations in, 14 nerve source and distribution, 13–14, 14 metabolism o , 14–15 oxidative metabolism, 15, 15 oxygen and nutrients in, source o , 14–15, 15 neovascularization, 240–241, 403–404, 403 –404 contact-lens-induced, 404 E ron grading scale or, 454 in so toric lenses, 102 oedema, 236, 236 E ron grading scale or, 454 so toric lenses, 102 physiology, orthokeratology and, 298–299 plana, 275 scarring/thinning, 318 shape, unusual or distorted, 275, 275 staining, E ron grading scale or, 454 stroma, 11–13, 12 complications in, 400–406 deep stromal opacities, 402–403, 403 keratitis, 404–406, 404 –405 neovascularization in, 403–404, 403 –404 oedema, 381, 400–401, 401 organization, in corneal transparency, 15–16 rejection, post-keratoplasty and, 294 thinning, 401–402, 402 warpage, 406, 406 swelling o , 236–237, 237 –238 temperature, in on-eye power changes, 68 therapy. see T erapeutic applications thickness anterior-segment optical coherence tomography (AS-OC ) or, 344, 345 changes in, 237–238, 238 con ocal microscopy or, 344–345 determination o , 343–345 di erences in, 238 LenStar LS 900 biometer or, 345, 345 optical pachymetry or, 343, 343 Scheimp ug scanning in, 344 slit-scanning devices or, 344 specular microscopy or, 345 ultrasonic pachymetry or, 343–344, 344 tomography, 352, 354 topography, 147–148, 147 , 350–352, 353 , 370–373 analysis, 338, 338 –339 irregular, post-keratoplasty and, 289–290, 289 photore ractive procedures and, 282–283 radial keratotomy and, 284–285, 285 visual optics and, 30–31, 31 transparency o hydration control in, 16, 16 in oedema, 16 stromal organization in, 15–16 ulcerations, in ectious, 234–235, 235 wrinkling, 399–400, 399 –400 Corneal curvature-corneal power conversion, 443t–444t ‘Corneal exhaustion syndrome,’ 408 Corneal in ltrative events (CIEs), 173–174, 173 , 235–236, 244

Corneal lenses, or keratoconus, 253–257 designs or, 255–256 aspheric, 256 spherical, 255–256 toroidal, 256–257, 256 –257 tting philosophies, 254–255 Corneal plane, e ective power o plus-and minusprescription spectacle lenses in, 441t–442t Corneal powers, 347 conversion between corneal ront sur ace radius o curvature and, 443t–444t ‘Corneal tomographers,’ 337 Corneal topographers, or rigid lens measurement, 138 Corneo-scleral lenses, or keratoconus, 257, 257 Correction, principles o , 97–101 back vertex power determination, 98 eye relationship, 99 tting, 98 lid anatomy, 98–99 misalignment o lens, 100–101, 101t rotation allowing, 99 e ects o , 98 measurement o , 99–100 predicting, 98, 99t thickness pro le, 99 Corrections to spectacle, 38–41 accommodation demand and, 40–41, 41 convergence demand and, 41, 41 e ectivity and, 39, 39 magni cation, 39–40, 39 –40 Cosmetic tinted lenses, 205, 205 Cosmetics, contact lens wear and, 361 Cost to patient, practice management, 186 reducing, in compliance enhancement, 424. see also Pricing ‘Crab’ louse, 388–389, 389 Crimping, technique o , in toric back sur ace, 128 Cross-linked system, o polymer, 46, 46 Crystalline polymer, so lens and, 46, 47 CSM Instruments, 79 C . see Centre thickness (C ) Cylinders, in so toric lenses axis, 95 oblique, 102 Cylindrical power equivalent toric lenses, 160–161 Cylindrical powers, residual re ractive error induced by mislocation o toric lenses o , 450t

D

da Vinci, Leonardo, 3, 3 Daily disposable so lenses, 165–174.e1, 167 , 168t, 169 –170 advantages rom perspective o lens wearers, 171 o practitioners, 170–171 bimodal distribution o , 170 clinical per ormance o , 170 com ort enhancement strategies, 171 corneal in ltrative events and keratitis, 173–174 disadvantages o , 171 environmental impact o , 172, 173 lens application to assist ametropes in eyewear selection, 174 limitations to more general acceptance, 172–173 manu acturing reliability, 171, 172 patterns o wear, 168–170 Daily wear, planned replacement or rigid lenses and, 190, 190 DALK. see Deep anterior lamellar keratoplasty (DALK) Danalens, 7

461

Decentration, o rigid lens, 153, 153 Decentred lenses, prismatic e ects due to, 134 Decongestant, or eye redness, 379 Deep anterior lamellar keratoplasty (DALK), 12–13, 288 Deep stromal opacities, 402–403, 403 Deliberate non-compliance, 422–423 Deposits on lenses, 176–178, 176 –177 tinted lenses, 213 Descemet’s membrane, 315, 316 in cornea, 13, 13 Descemet’s membrane endothelial keratoplasty (DMEK), 288 Descemet’s stripping automated endothelial keratoplasty (DSAEK), 288 Descemet’s stripping endothelial keratoplasty (DSEK), 288, 289 Designs o lenses bi ocal and multi ocal. see Bi ocal and multi ocal contact lenses general principles o , bi ocal and multi ocal contact lenses, 224–225 used design, 225 solid design, 224–225, 226 rigid. see Rigid lens design so . see So lens design and speci cation, 438 , 439t tinted, 204–207 toric, so , 95–97, 98 toroidal, 256–257, 256 –257 Diabetes, 314–320.e1 and anterior eye, 314–317 and other systemic disease, contact lens wear, 318 cornea, 314, 315t corneal hydration control, 315–316 corneal nerves, 315 endothelium, 315, 316 epithelium, 314–315 eyelids, 314 glucose sensing, 317–318 glucose sensing, in anterior eye, 317–318 iris, 316 lens, 316 microbial keratitis, 316 ocular response to contact lenses, 317 orbit, 314 panretinal photocoagulation (PRP), 317 pupil, 316 tear lm, 314 Diagnostic instruments, 327–345.e5 Diameter o rigid lens, 136–137 o so lens, measurement, 81–82, 81 . see also otal diameter Di raction, visual optics and, 32 Di use wide beam, 348, 348 Di user, 411, 413 –414 Di using lter, in illumination system, 333 Digital imaging, 410–419.e1, 410 bene ts o , in contact lens practice, 410–411 brighter image in, 411–413, 413 –415 commercial digital imaging system in, 413 con ocal microscopy in, 417–418 digital SLR camera slit-lamp imaging system in, 414–415 le back-up and printing in, 416 image editing in, 418–419, 418 –419 in rared imaging in, 416, 417 optical coherence tomography in, 416, 418 recording digital movies in, 416 resolution in, anterior eye, 411, 411 –412 smartphone, 415 video slit-lamp imaging system in, 415–416, 416 Digital SLR camera slit-lamp imaging system, 414–415

462

Ind e x

Dimensional stability, o hydrogel lenses, 52–53 Dimensional tolerance, or so , polymethyl methacrylate (PMMA) and rigid lenses, 440t Dimethyl itaconate, 117, 118 Dimple veiling, 241 Direct ocal illumination, 348–349 optic section, 348, 349 parallelepiped, 348, 349 specular re ection, 348–349, 350 Direct retroillumination, 349–350, 351 Dirt, and lens wear, 248 Disadvantages o planned so lens replacement, 182–183 patient non-compliance, 182 quality and reproducibility issues, 182–183 Discipline, o personnel at the practice, 429, 430t Discom ort, 380–382 characterizing symptoms o , 381, 381 , 381b lens care products associated with, 382, 382 rom rigid lens, 154–155 rom scleral lenses, 201 rom so lens tting, 93 solving symptoms o , 381–382, 382 rom tinted lenses, 213 Disin ection o so contact lenses, 103 solutions, or rigid lens, 163, 163t, 164 Dislodging, so lens, 90 Dismissal, o personnel at the practice, 429 Dispersion, 80 Disposable lenses, 6–7, 175, 175 daily, 7 so lenses. see Daily disposable so lenses so , 232–233 Dk/t, measurement o , in corneal swelling, 238, 238 –239 DMEK. see Descemet’s membrane endothelial keratoplasty (DMEK) Dohlman keratoprosthesis, 288 Dot-matrix printing, o lenses, 209, 210 Drug delivery, in therapeutic applications, 280–281 Dry-eye, 389–390, 390 –391 questionnaire, 451t–452t symptoms, photore ractive procedures and, 283 Dry storage, or scleral lenses, 201 Dryness, rom tinted lenses, 213 DSAEK. see Descemet’s stripping automated endothelial keratoplasty (DSAEK) DSEK. see Descemet’s stripping endothelial keratoplasty (DSEK) ‘Dual Disin ection’ MPS, 109, 109 Dust, and lens wear, 248 Dye dispersion tinting, o lenses, 208 Dynamic stabilization, 97 Dyslexia, 206

E

Ebatco, 79 Eccentric gra , 290 Eccentric optic zone, as non-edge (body) de ects, 66 Edge clearance, in rigid lens design, 145–146, 145 de ects, in so lens manu acture, 66, 66 t, o rigid lens, 153 in lens geometry, 84, 85 li , 138, 139 in rigid lens design, 145–146, 145 polishing, 124–127, 128 pro les, in rigid lens measurement, 138–139 relationship, 457, 457 thickness, in rigid lens measurement, 137 ‘Edge-slicing’ method, 84 Education and history taking, 325. see also Patient education

E ective power, o plus-and minus-prescription spectacle lenses at corneal plane or various vortex distances, 441t–442t Ef cacy, o accelerated orthokeratology, 297–298, 298 Elastomer, or high ametropia, 263 Electrolytes, in preocular tear lm, 26, 26t Electromechanical gauge, or rigid lens measurement, 137, 137 ELK. see Endothelial lamellar keratoplasty (ELK) Employment contract, o personnel at the practice, 429 Endothelial lamellar keratoplasty (ELK), 288 Endothelium o cornea, 13, 13 analysis, 375, 375 complications in, 406–409 bedewing, 406–407, 407 blebs, 407–408, 407 –408 polymegethism, 408–409, 409 and diabetes, 315, 316 rejection, post-keratoplasty and, 294 Engraving, 124, 127 Enhanced monovision, bi ocal and multi ocal contact lenses, 228, 229 Environment constraints on sport, and lens wear, 246–249 protection rom, 277, 277 Envisu S4410, 83–84 Epikeratoplasty, 288 ‘Epithelial plug,’ 397 Epithelium o cornea, 10–11, 11 complications in, 397–400 epithelial staining, 397, 397 –398 microcysts, 398, 398 vacuoles, 399, 399 wrinkling, 399–400, 399 –400 degenerations involving, 276 and diabetes, 314–315 microcysts, 240–241, 240 , 381 E ron grading scale or, 454 response, 244–245 rejection, post-keratoplasty and, 293, 294 wound healing, 16–17 Erroneous procedures, 421–422, 421 ESP. see Eye Sur ace Pro ler (ESP) Eta lcon, 232 Ethnicity, myopia, risk actor or, 307, 308 European Medical Devices Directive, 129 Excess material, as edge de ects, 66 Extended wear, 231–245.e1 adverse e ects o , 234–242, 235t acute physiological e ects, 236–238, 236 –237 chronic physiological changes, 239–241, 240 –241 in ectious corneal ulcerations as, 234–235, 235 mechanical e ects, 241–242 non-in ectious in ammatory events as, 235–236, 236 role o hypoxia in, 238–239, 239 , 239t application o , in practice, 243–245 in clinical practice, 242–243 complications in, management o , 244 de nition o , 231 experiences with, 231–234 conventional so extended wear, 231–232 disposable so lenses, 232–233 non-hydrophilic materials, 232, 232 silicone hydrogel contact lenses, 233, 233 –234 planned replacement or rigid lenses and, 190–191 sa ety o , 242–243 External promotional issues, 431 Eye Sur ace Pro ler (ESP), 340, 341 Eyecup, 3, 3

Eyelash disorders, 388–389, 389 Eyelids, 17–20 action, e ects on lens rotation, 99 anatomy o gross, 17, 17 –18 microscopic, 19, 19 and so toric lenses, 98–99 o anterior eye, and diabetes, 314 blood and lymphatic supply to, 20 complications in, 385–389 blinking, 385, 386 eyelash disorders, 388–389, 389 lid wiper epitheliopathy, 387–388, 388 meibomian gland dys unction, 386–387, 387 ptosis, 385–386, 386 orces, rigid lens and, 145 glands o , 19–20 meibomian, 19, 20 Zeis and Moll, 20, 20 manipulation, 357, 357 movements o , control o , 19 muscles o , 17–18 levator palpebrae superioris, 18, 19 orbicularis oculi, 17–18, 18 superior and in erior tarsal, 18 nerves o , 20 sur acing assessment, or tear lm evaluation, 350 Eyewear selection, lens application to assist ametropes in, 174

F

Fatt oxygen transmissibility (Dk/t) values, or various lens thickness, 447t–448t FBU . see Fluorescein break-up time (FBU ) Fenestrated lenses, 199, 200 Fenestration, 124, 127 in scleral lenses, 260 Fick, Adol Eugene, 4, 4 Fields o view, xation and, 41, 42 Filamentary keratitis, 276 Film, tear. see ear lm Financial management, in planned so lens replacement, 186 Fingertips, roughening o , 317 Fit/ tting lenses assessment o so lens, 91–93 bi ocal and multi ocal contact lenses, 221–222, 222 –223 characteristics, 368–369 children. see Paediatric tting at, orthokeratology and, 301 intrastromal corneal ring segments and, 286 myope, 310–313, 311t optimal, orthokeratology and, 301 photore ractive procedures, 282–284 poor, vision loss related to, 384 post-keratoplasty, 291–293, 292 –293 principles or scleral lenses, 196–199 non-coaxial scleral lenses, 198–199 non-ventilated pre ormed, 196 optic zone sagittal depth and optic zone projection, 198, 198 –200 scleral zone, 197–198, 197 radial keratotomy, 284–286 rigid lens. see Rigid lens tting so lens. see So lens tting tinted lens, 211 toric lenses, so , 96 , 98 trial with accurate prescription, 181, 182 calculation o required sur ace radii rom, 131–132

Ind e x Fit/ tting lenses (Continued) required BVP when the lens to be ordered has a di erent BOZR rom, 131 so , 88–90 tear lens during, 130–131 Fixation, elds o view and, 41 Flash, as edge de ects, 66 Flat- tting lens, orthokeratology and, 301 ‘Flat pack,’ contact lens, 7, 8 Flexure, 366 o commercial rigid lens materials, 119–121 e ects, rigid lens, 134 rigid lens measurement and, 141 Fluid- lled tube, in contact lens, 3 Fluid permeability, o hydrogel so lenses, 52 Fluorescein break-up time (FBU ), 328 instillation, 152 pattern analysis, 418 in rigid lenses, 368, 368 staining, 350 in digital imaging, 411, 413 –414 Fluorescence, rigid lens tting and, 152, 152 Fluorosilicone acrylate lenses, re ractive index, 122 Focimeter, 74, 74 or rigid lens measurement, 139, 139 Focusing, con ocal microscopy through, 335, 336 Food and Drug Administration (FDA) classi cation system, or so lens materials, 60, 60t Forces, acting on so lens, 86 Fourier-domain OC (FD-OC ). see Spectraldomain OC (SD-OC ) FOZD. see Front optic zone diameter (FOZD) FOZR. see Front optic zone radius (FOZR) Fraser’s ve-point grading, in recruitment and selection, 428, 429t Friction hydrogels and, 51 rigid lens measurement and, 140, 140 Friction angle, 79 Friction coef cient, 79 Front optic zone diameter (FOZD), 439t in rigid lens design, 145 Front optic zone radius (FOZR), 83, 439t Front peripheral radius, 439t Front peripheral zone diameters, 439t Front-sur ace aspheric designs, 219–220, 220 Front sur ace o lens, generation o , 124, 126 toric, 127–128 Front sur ace radius, o curvature, in rigid lens measurement, 137–138 Front-sur ace toric lenses, 161, 161 Front vertex power, 439t Frothing, in CL-MGD, 386–387, 387 Fused design, bi ocal and multi ocal contact lenses, 225 lens tting and, 225–226

G

Gas-permeable contact lenses, rigid lenses, singlevision, or myopia control, 307–308 Gaylord patents-harnessing silicone, 116–119 General Optical Council (GOC), 433 Geometric centre thickness, 439t Geometry o lens rigid, 136–139 o so lens measurement, 81–84 diameter, 81–82, 81 edge, 84, 85 radius o curvature, 83–84, 84 sagittal depth, 82 thickness, 82–83, 83 Giant papillary conjunctivitis, o scleral lenses, 202

Glands lacrimal. see Lacrimal gland tarsal. see Meibomian glands o Zeis and Moll, in eyelids, 20, 20 Glare sensitivity test, or poor vision, 382–383 Glass scleral lenses, 4–5 Glaucoma, in children, 274 Glucose sensing, in anterior eye, 317–318 Glyceryl methacrylate (GMA), or hydrogel lenses, 53 , 54 GMA. see Glyceryl methacrylate (GMA) Goblet cells, o eyelids, 21–22, 22 GOC. see General Optical Council (GOC) Gothenburg study, 6, 241 Grading scales, 377–379 design, 377 determinants o , per ormance, 379 how to grade, 377 image size and, 377, 377t, 378 interpretation o , 378, 378t record o , 378 Gra s corneal. see under Cornea polymers, so lens and, 46, 47 pro les, 290 rejection and ailure, 293–294 Gravitational orces rigid lens and, 144, 145 and sport, lens wear and, 249 Green (‘red- ree’) lter, in illumination system, 333

H

Haag-Streit BD900 video slit lamp, 415–416, 416 Hand grooming/hygiene, 357 Handling o lenses paediatric tting, 273–274, 273 , 312–313 problems, 318, 318 Handling tints, tinted lens designs and, 204–205 Handwashing acilities, 357 Hardness, o commercial rigid lens materials, 121 Hartmann-Shack technique, principle o , monochromatic aberrations and, 33, 34 Hartmann-Shack wave ront sensor, 74–75, 75 Hazing, rigid lens sur ace, 368 Heat disin ection, 104 Heidelberg retina tomograph (HR ) III, 335, 337 HEMA. see Hydroxyethyl methacrylate (HEMA) Herschel, Sir John, 3–4, 4 Hexa uoroisopropyl methacrylate monomer, 117–118, 118 High ametropia, 263–267.e1 tting challenges or, 266 high-power lens designs, principles o , 264–266 and aphakia, 264–265 and high astigmatism, 265–266 high-minus, 265, 266 high-plus, 264, 264 –265 lens materials or, 263–264 elastomer, 263 silicone hydrogel, 263–264, 264t and low vision, 266 High-contrast visual acuity chart, 216 or poor vision, 382, 382 High-minus rigid lenses, 265, 266 High-plus rigid lenses, 264, 264 –265 High-power lens designs, principles o , 264–266 High-powered microscopy, 334–335 con ocal microscopy, 334–335 through ocusing, 335, 336 Rostock cornea module (RCM), 335, 337 slit-scanning con ocal microscopy, 335 specular microscopy, 334 tandem scanning con ocal microscopy, 334–335 High-water-content lenses, ionic, 105

463

Higher-order spherical aberrations, 75–76, 76 Higher-order wave ront error (HO-WFE), 350, 352 Higher-water-content hydrogel materials, 180 History o contact lens (listed chronologically), 1–9.e1 early theories (1508-1887), 3–4, 3 –4 glass scleral lenses (1888), 4–5, 4 –5 plastic scleral lenses (1936), 5 plastic corneal lenses (1948), 5 silicone elastomer lenses (1965), 5–6 so lenses (1972), 6, 6 rigid gas-permeable lenses (1974), 6 disposable lenses (1988), 6–7 daily disposable lenses (1994), 7 silicone hydrogel lenses (1998), 7 anti-myopia lenses (2010), 7 contact lens ‘ at pack’ (2011), 7, 8 uture, 7–9 History taking, 321–326.e2 a ercare, 365–366 amily, 325 indications and contraindications or contact lens wear, 323–324 in ormed consent and, 325 medical, 324 medication and, 324–325 ocular, 324 patient compliance and, 325–326 patient education and, 325 reason or visit and, 324 risk/bene t analysis and, 325 social, 325 structure, 324–325 HO-WFE. see Higher-order wave ront error (HO-WFE) Horizontal corneal diameter, 347 Horizontal visible iris diameter (HVID), 86 HR III. see Heidelberg retina tomograph (HR ) III HVID. see Horizontal visible iris diameter (HVID) Hybrid lenses designs, bi ocal and multi ocal contact lenses, 221, 222 or keratoconus, 260–262, 261 Hybrid rigid gas-permeable materials, 116 Hydration control, in corneal transparency, 16, 16 on-eye power changes, 68 Hydrogel lenses or aphasia, speci cations in, 269t or children, 272 so dimensional stability o , 52–53 uid permeability o , 52 ion permeability o , 52 mechanical properties o , 48–49, 48 optical transparency o , 48 oxygen permeability o , 51–52, 52 properties o , 48–53 re ractive index o , 52 sur ace properties o , 49–51 swell actor o , 52–53 water content o , 51. see also Silicone hydrogel lenses Hydrogen peroxide, 106–107, 107 Hydrophilic lenses, 231 Hydrophobic sur aces, o rigid lens materials, 115–116 Hydroxyethyl methacrylate (HEMA), in so lenses, 6 Hydroxyethylcellulose, rigid lens and, 163 Hyperaemia eye redness and, 379 scleral lenses and, 201 Hyperglycaemia, diabetes mellitus and, 314 Hyperopic orthokeratology, 299–300 Hypersecretory meibomian gland dys unction, 386

464

Ind e x

Hyperthin hydrogel lenses, dehydration o , 381, 382 Hypoxia during closed-eye lens wear, 240–241 contact-lens-induced, 394–395 and keratoconus, 262 role o , in extended wear, 238–239, 239 and scleral lenses, 202 ‘Hysteresis,’ hydrogels and, 50

I

Identi cation tints, 207 IEK. see IntraLase-enabled keratoplasty (IEK) Illumination, 411 retroillumination, 349–350, 351 system, o slit-lamp biomicroscopes, 332–333 Image editing, 418–419, 418 –419 Image resolution, 411 Imaging. see Digital imaging Impression arcs, corneal distortion, 371–372, 372 Impression moulding, non-coaxial scleral lenses, 199 In vivo wettability, o hydrogel lenses, 49–50 Indirect illumination, 349–350 uorescein staining, 350 retroillumination, 349–350, 351 sclerotic scatter, 350, 351 Indirect retroillumination, 351 Induced astigmatism, o rigid toric lens, 159–160 In ection risk o , hydrogel lens, 242–243, 243 o scleral lenses, 202 In ectious ulcerative keratitis, 244 In erior steepening, corneal distortion, 371, 371 In ammation, corneal, 235–236 In ormed consent, 325, 362–363, 432 In rared imaging, 416, 417 Injection, eye redness and, 379 Innervation conjunctival, 22 corneal, 13–14 Insertion o lens, 358–359, 359 rigid, 148–149, 149 so , 89, 89 Inspection o lens, 357–358, 358 , 374–375 nal, edge polishing and, 124–127 Internal promotional issues, 431 IntraLase-enabled keratoplasty (IEK), 288 Intraocular lenses (IOLs), 269 Intrastromal corneal ring segments, contact lens tting ollowing, 286 Inverted air bubble technique, 121 IOLs. see Intraocular lenses (IOLs) Ion permeability, o hydrogel lenses, 52 Ionic high-water-content contact lens, 105 Iris, o anterior eye, and diabetes, 316 Iron deposits, on non-replacement basis, 177 Irreversible water loss, 178, 178 ISO 14729:3, ow chart o disin ection, 111 ISO 18369-3, conditioning according to, 73 ISO classi cation system, or so lens materials, 60, 60t ISO oxygen permeability (Dk) values, or various lens thickness, 447t–448t

J

“Jelly bumps,” on non-replacement basis, 177, 177 JENVIS 0-4 scale, 331–332

K

K-values, 347 Kalt, Eugène, 4, 4

KC design lenses, or keratoconus, 261, 261 Keratitis, 173–174, 404–406, 404 –405 contact-lens-associated microbial (in ectious), 405 lamentary, 276 in ectious, 382 microbial, 234–235, 235 and daily wear, 243 incidence o , 235 severe exposure, 277 sterile in ammatory, 236 Keratoconjunctivitis sicca, 318 superior limbic, 382, 395–397, 396 E ron grading scale or, 455 and so toric lenses, 102 Keratoconus, 251–262.e2, 275 , 350, 352 clinical assessment o , 251–252 anterior ocular health, 252 anterior segment imaging technologies in, 251 cone morphology in, 251, 252 progression in, monitoring o , 262 re ractive management o , 252–262, 252b hybrid lenses, 260–262 rigid lenses, 253–260, 253t so lenses in, 252–253 spectacle correction in, 252 Keratocytes, 12, 12 Keratoglobus, 275 Keratology. see Orthokeratology Keratometers, 83, 84 range conversion, extended, 445t or rigid lens measurement, 138, 138 Keratometry, 337–338, 337 , 347, 370–373 in children, examination techniques or, 271, 272 in so lens tting, 86 mires, in so lens tting, 93 Keratoplasty anterior lamellar, 288 deep anterior lamellar, 12–13, 288 endothelial lamellar, 288 IntraLase-enabled, 288 penetrating ull-thickness, 288 suture techniques or, 287 , 290, 290 procedures, lexicon o , 289b. see also Post-keratoplasty Keratoscopy, 338 Kodak Wratten number 12 (yellow) lter, in illumination system, 333, 333

L

Lacrimal drainage system, 24–25, 24 Lacrimal gland accessory, 24 anatomy o gross, 23, 23 microscopic, 23, 23 blood and nerve supply o , 24 unctional considerations in, 24, 24 ‘Lags’ o accommodation, visual optics and, 29–30, 30 Lamellae, stromal, in cornea, 11–12, 12 Lamellar corneal transplantation techniques, 12–13 Laminate constructions, o lenses, 209–210, 210 Large cylindrical components, o so toric lenses, 102 Laser in situ keratomileusis (LASIK), ap damage in, 246 LASIK. see Laser in situ keratomileusis (LASIK) Lathe cutting, 61–62, 61 –62 Layout, practice, 427, 428 Lens edge rubbing, 242

Lens-eye interactions, 369, 369 Lens-eye relationship, in so toric lenses, 99 Lens-holding device, 138, 138 Lens-induced mucin ball, 390 Lens o anterior eye, and diabetes, 316 LenStar LS 900 biometer, 345, 345 Levator palpebrae superioris, in eyelids, 18, 19 Lid. see Eyelids ‘Lid wiper,’ 51 Lid wiper epitheliopathy, 387–388, 388 Limbal vaulting, 457, 457 Limbus/limbal complications, 394–397 hyperaemia, 239, 240 redness, 380, 380 , 394–395, 394 –395 E ron grading scale or, 454 superior limbic keratoconjunctivitis, 395–397, 396 vascularized limbal keratitis, 395, 395 –396 Linear homopolymer, so lens and, 46, 46 Lipids, in preocular tear lm, 26–27, 26t, 27 LipiView inter erometer, 327–328, 328 LOF SEA (location, onset, requency, type, sel -treatment, e ect on patient, associated symptoms), 324 Long-term adverse changes, in so lenses, avoidance, 176–178 irreversible water loss, 178, 178 lens deposits, 176–178, 176 –177 storage contamination, 178, 179 sur ace damage and crazing, 178 Loose- tting lenses, in so lens tting, 91, 93 Loss o lens, rigid, 155 lotra lcon lens, 57 Low-contrast visual acuity chart, 216 or poor vision, 382, 382 Low-water-content lenses, 87 Luminance, variation o pupil size at, 217 Luminance transmittance, in rigid lens measurement, 141 Lymphatics, o conjunctiva, 22, 22

M

MAA. see Methacrylic acid (MAA) Magni cation o spectacle, 39–40, 39 –40 visual optics and, 39–40, 39 –40 Maintenance o tinted lenses, 211 Manu acture o lenses rigid. see Rigid lens manu acture so . see So lens manu acture Manu acturer-driven systems, planned so lens replacement, 183, 183 Marking, rigid lens manu acture, 124, 127 Mass-produced lenses, reproducibility and quality o , 66–67 Mastrota paddle, 331, 331 Material properties or so lens, 77–81 modulus, 77–78, 77 –78 oxygen permeability, 79–80, 80 re ractive index, 80, 80 spectral and luminous transmittance, 80–81, 81 sur ace riction, 79 tolerance, 440t water content, 77 wettability, 78–79, 79 Materials or so lenses, 43–60, 87 classi cation o , 59 hydrogel or, 48–55, 53 , 55t properties o , 48–53 polymers or, 45–48 silicone hydrogel or, 55–59, 56 –57 , 58t, 59 Mechanical properties, o commercial rigid lens materials, 119–121 Medication, 324–325

Ind e x Meibomian Gland Evaluator, 331, 331 Meibomian glands dys unction (MGD), 367–368, 386–387, 387 E ron grading scale or, 455 examination o , 331, 331 –332 in eyelids, 19, 20 Menicon Z, 118 Methacrylic acid (MAA), or hydrogel lenses, 53 , 54 Methacrylic acid monomer, 117, 117 Methyl methacrylate (MMA) or hydrogel lenses, 53 , 54 polymerization o , 115, 116 MGD. see Meibomian glands; dys unction (MGD) Michelson inter erometry, 340–341 Microbial keratitis, in anterior eye, 316 Microcysts, 398, 398 Microwave irradiation, 105 Mid-peripheral t, o rigid lens, 153 Miniscleral lenses ront-sur ace toric orms, 259 or keratoconus, 258 optical coherence tomography in, or tting o , 258 speci cation o , 258–259 Mirror, 357 Misalignment o so toric lenses demonstrator or, 450t determining, 100–101, 101t Misight®,309, 310 MMA. see Methyl methacrylate (MMA) Modern orthokeratology, 296, 296 Modi cation code, so lens materials and, 60 Modi ed monovision, 228–229 Modulus o material properties, 77–78, 77 –78 rigid lens measurement and, 141 Moiré de ectometry, 74, 75 or rigid lens measurement, 137–138 Money management, in practice management, 433, 434 Monochromatic aberrations, visual optics and, 33–37, 33 –34 , 35t–36t, 36 –37 Monovision correction, 383 enhanced, 228 general tting principles, 227 modi ed, 228–229 partial, 227–228 presbyopia, 226–229, 227 –228 problem-solving approaches or tting, 229 Movement, in so lens tting, 91–92, 92 Mucin ball-induced uid- lled pits, 391, 391 Mucins balls, 390–391, 391 in preocular tear lm, 26, 26t Müller, August, 4, 5 Multicurve lens, 255 Multi ocal contact lenses. see Bi ocal and multi ocal contact lenses Multiple pieces, as non-edge (body) de ects, 66 Multipurpose solutions, 107–109 active agents, 108 constituents o , 108t sodium chlorite containing, 109, 109 Munnerlyn ormula, 282 Myope, contact lenses and, 310–313 Myopia high, in children, 269, 269 visual optics and, 29 Myopia control, 306–313.e2 continuing care and complications, 313 tting myope with contact lenses, 310–313, 311t handling o lenses, 312 lens selection and tting, 312 measurements and examination, 311, 312

Myopia control (Continued) lenses, types o , 307–310, 308t bi ocal/multi ocal contact lenses, 309 extended depth o ocus, lenses with, 310 peripheral hyperopic retinal de ocus, contact lens management o , 309–310 positive spherical aberration, lenses with, 310 simultaneous de ocus or dual- ocus contact lenses, 309 single-vision rigid gas-permeable contact lenses, 307–308 single-vision so contact lenses, 308–309 orthokeratology and, 303–304, 304 patient selection and, 306–307, 307 risk actors or, 306–307, 308 Myopic creep, 241

N

N-vinyl pyrrolidone (NVP), or hydrogel lens, 53–54, 53 Neovascularization, 202–203, 202 in children, 274 Nerves o conjunctiva, 22 corneal alteration to structure and unction, 202 o anterior eye, and diabetes, 315 source and distribution o , 13–14, 14 o eyelids, 20 Neutral-density lter, in illumination system, 333 Neutralization, o corneal astigmatism, by rigid lens o spherical power, 132–133, 132 NIBU . see Non-invasive tear lm break-up time (NIBU ) Nick, as edge de ects, 66 Non-coaxial scleral lenses, 198–199 Non-compliance consequences in, 420 extent and pattern o , 420 history taking and, 325–326 predicting o , 424 reasons or, 422, 422 Non-contact aesthesiometry, 335–336 Non-disposable planned replacement lens, 176 Non-edge (body) de ects, in so lens manu acture, 66–67 Non-hydrophilic materials, 232, 232 Non-in ectious in ammatory events, 235–236, 236 Non-invasive tear lm break-up time (NIBU ), 328–329 Non-ventilated pre ormed scleral lenses, 196 Nutrients, source o , in corneal metabolism, 14–15, 15 NVP. see N-vinyl pyrrolidone (NVP)

O

Objective ocular redness assessments, 331–332 Objective re raction, 346–347 Oblate shape, corneal distortion, 370–371, 370 Oblique bitoric lenses, 161 Observation o eye, 327–335 Burton lamp or, 327, 327 high-powered microscopy or, 334–335 meibomian glands in, 331, 331 –332 objective ocular redness assessments and, 331–332 slit-lamp biomicroscopy or, 332–334, 333 tear lm in, 327–330 Obstructive meibomian gland dys unction, 386 Occupational sa ety, in lens manu acture, 67 OC . see Optical coherence tomography (OC )

465

Ocular adnexa, in anterior eye, 17–25 conjunctiva, 20–23 eyelids, 17–20 lacrimal system, 23–25 Ocular depth o ocus, aberrations and, 38, 38 Ocular ocus, precision o , accommodation and, 29–30 Ocular history or contact lens wear, 324 Ocular motility disorders, in children, 269–270 Ocular side-e ects o therapy, in contact lens wear, 318, 318 Ocular sur ace pathology, 179–180 Ocular topography or cornea, 143–144 e ect o , on so lenses, 94 or ethnic variations, in ocular dimensions, 144 or lids, 144 or rigid lens design and tting, 143–144, 143t OCULUS Keratograph 5M or meibomian glands, 331, 332 or objective ocular redness assessments, 331, 332 or tear break-up, 328–329, 328 –329 or tear meniscus height, 330, 330 Oculus keratograph 5M, 416 Oedema, corneal, 236, 236 , 400–401, 401 E ron grading scale or, 454 so toric lenses, 102 transparency in, 16 On-eye power changes, 68–69, 69 Opacities, deep stromal, 402–403, 403 Opaque backing, 210, 210 Opaque tints, 209–210 Ophthalmic disease, in children, 274 Ophthalmoscopy, 352 Optic section direct illumination, 348, 349 Optic zone diameter, in rigid lens measurement, 136–137 Optic zone projection, 198, 198 –200 Optic zone sagittal depth, 198, 198 –200 Optical coherence tomography (OC ), 82, 251, 330, 330 , 416, 418 anterior-segment, 258 Anterior-segment Optical Coherence omography (AS-OC ), 340–343 or keratoconus, 259 spectral-domain, 342 swept-source, 342–343, 343 time-domain, 342, 342 Optical distortions, 140 Optical pachymetry, 343, 343 Optical parameters, o so lens, 73–77 optical quality, 76, 77 power and power pro les, 74–76 scattering, 76–77 Optical quality, 76, 77 o rigid lens measurement, 140 Optical tolerance, or so , polymethyl methacrylate (PMMA) and rigid lenses, 440t Optical transparency, o hydrogel materials, 48 Opticians Act 1989, 433 Optics, so lens, 68–72.e1 aberration, 69–72, 70 on-eye power changes, 68–69, 69 Optimal lens t, orthokeratology and, 301 Optimec JCF instrument, 83–84 Optimum lens t, 150, 151t Optimum replacement schedule, rigid lens, 191 Orbicularis oculi, in eyelids, 17–18, 18 Orbit, o anterior eye, and diabetes, 314 Orbscan II, 416–417 Orbscan instrument, or posterior corneal elevation, 372–373 Ordering lens, 433 scleral, 200–201, 200 Ordinal, discom ort, 381

466

Ind e x

Orthokeratology, 296–305.e2 accelerated, outcomes o , 297–299 corneal physiology, 298–299 determinants o success, 299 ef cacy, 297–298, 298 regression, 298, 298 sa ety, 299 a ercare, 302–303 assessment o lens t, 302–303 ocular health, 303 re raction, 303 visit schedule, 302 history o , 296 hyperopic, 299–300 lens tting in, 300–302 approaches, 300–301 base curve, 300 indications and contraindications o , 300 lens delivery, 301–302 post-wear assessment, 301, 302 modern, 296, 296 myopia control, 303–304 myopia progression, mechanism or, 303–304, 304 research and, 303 overnight, 297 unwanted, 242 Osmometry, 329, 329 Oval cones, 251 Overkeratometry, 366, 367 Overre raction, in contact lenses, 366 Oxidative metabolism, in cornea, 15, 15 Oxygen consumption, 239t in closed-eye oxygen tensions, 239, 239 in hypoxia, 239 need or, in rigid lens materials, 115 source o , in corneal metabolism, 14–15, 15 Oxygen ux, 439t Oxygen permeability, 79–80, 80 , 439t o commercial rigid lens materials, 119 o hydrogel lenses, 51–52, 52 rigid lens measurement and, 141 o so lens materials, 60t Oxygen transmissibility, 439t lens material and, 308–309 through corneo-scleral lenses, 257 through miniscleral lens, 259 through scleral lenses, 260 tinted lenses, 211

P

Pachymetric mapping, 352 Pachymetry, 373, 374 Paediatric tting, 268–274.e1 common a ercare problems in, 274 elective lens wear or, 270, 271 examination techniques or, 271–272 anterior segment examination, 271, 271 –272 biometry, 271–272 keratometry, 271, 272 handling o lenses in, 273–274, 273 , 312–313 indications or, 268–270 aphakia as, 268–269, 268 –269 high myopia, 269, 269 irregular astigmatism as, 270, 270 ocular motility disorders, 269–270 pseudophakia as, 269 lens selection or, 272–273 hydrogel lenses in, 272 rigid lenses in, 273 silicone hydrogel lenses in, 273 silicone rubber lenses in, 272–273, 272

Paediatric tting (Continued) ocular response to, 270–271 therapeutic lenses, 270 tinted and prosthetic lenses, 270 Panretinal photocoagulation (PRP), in anterior eye, 317 Pantographic system or lens engraving, 127 Papillae, 393, 394 Papillary conjunctivitis, 245, 393–394, 393 –394 in children, 274 E ron grading scale or, 454 Parallel bitoric lenses, 160 Parallelepiped direct illumination, 348, 349 Partial monovision, bi ocal and multi ocal contact lenses, 227–228 Patient in compliance enhancement model, 425, 425 cost to, planned so lens replacement, 186 expectations, presbyopia and, 216 non-compliance, planned so lens replacement, 182 and practice management processes, 431 scheduling, 432 selection, presbyopia and, 214 , 216 wearing vision correction across the age range versus the proportion wearing contact lenses, 215 Patient education, 356–363.e1 in compliance enhancement, 423–424 and history taking, 325 instruction, 357–362 care products and, 361 cosmetics and, 361 emergency and, recognizing, 362 hand grooming/hygiene as, 357 lens inspection as, 357–358, 358 lens recentring, 359, 359 lens removal, 360 lid manipulation as, 357, 357 patient discharge and, 362 wearing schedules and, 361–362 objectives o , 356 optimum teaching environment or, 356–357 timing o , 356 PDMS. see Polydimethyl siloxane (PDMS) Penetrating keratoplasty (PKP) ull-thickness, 288 suture techniques or, 287 , 290, 290 Pentacam, 339–340, 339 –340 , 416–417 Per lcon A, 231 Per ormance-enhancing tinted lenses, 206 Peri-ballast, 97 Periodic sel -review, in compliance enhancement, 424 Peripheral corneal desiccation, o rigid lens, 154, 154 Peripheral corneal mechanical trauma, in rigid lens, 154, 154 Peripheral t, in so lens tting, 92–93, 92 Peripheral hyperopic retinal de ocus, contact lens management o , 309–310, 310 Peripheral junction thickness, 439t Peripheral radii, o rigid toric lens, 157 Permeability coef cient (Dk), 79 Phase-shi ing Schlieren, 75 pHEMA. see Poly(hydroxyethyl methacrylate) (pHEMA) Photore ractive procedures, contact lens tting ollowing, 282–284 Physiological stress, extended wear and, 244–245, 245 Piggy-back tting, 266 PKP. see Penetrating keratoplasty (PKP) Placido-based keratoscopes, 338, 339 Placido disc corneal topography, 350–352 or keratoconus, 256–257

Planned replacement rigid lenses, 187–192.e1 li e expectancy o rigid contact lenses, 187–188, 188 –189 optimum replacement schedule, 191 regular, 188–189, 189 –190 , 189t advantages o , 189–191 schemes available, 191 Planned so lens replacement advantages o , 175–182 disadvantages o , 182–183 practice management issues relating to cost to patient, 186 nancial management, 186 lenses available or planned replacement, 184, 184 –186 , 184t manu acturer-driven systems, 183, 183 practice-driven systems, 183–184 practice logistics, 184–186 practice management issues relating to, 183–186 supply routes, alternative, 186 toric, 101–102 Plasma coating, silicone hydrogel materials and, 118–119 Plastic corneal lenses, 5 Plastic scleral lenses, 5 PMMA lenses. see Polymethyl methacrylate (PMMA) lenses Polarizing lter, in illumination system, 333 Polarographic electrode technique, 119 Poly (4-methyl pent-l-ene), 116 Polydimethyl siloxane (PDMS), or silicone hydrogel lenses, 56, 56 Polyhexanide-based MPS, 107–108, 108 Poly(hydroxyethyl methacrylate) (pHEMA), hydrogel lenses and, 53 Polymegethism, endothelial, 316 , 408–409, 409 E ron grading scale or, 455 Polymerization, so lens and, 47–48 Polymers, or so lens, 45–48 structure o , 46–47 Polymethyl methacrylate, 115 re ractive index, 122 Polymethyl methacrylate (PMMA) lenses, 147, 231, 260, 296 dimensional tolerances in, 440t optical tolerances or, 440t Polyquaternium-1-based MPS, 108–109 Polyvinyl alcohol (PVA) or hydrogel lenses, 55 rigid lens and, 163 Polyvinyl pyrrolidone (PVP), or hydrogel lenses, 55 Post-keratoplasty, 287–295.e2, 287 –288 contact lens and results, 294 wear, indications and contraindications or, 291, 291 continuing care and complications, 293 corneal topography ollowing, irregular, 289–290, 289 general concerns in, 291 gra pro les, 290 gra rejection and ailure, 293–294 indications in, 287–288 lens- tting techniques and, 291–293, 292 –293 management issues, 294 rigid lenses, 291–292 suture techniques and, 287 , 290, 290 types o corneal gra , 288–289 Post-re ractive surgery, 282–286.e1 contact lens tting ollowing intrastromal corneal ring segments, 286 photore ractive procedures, 282–284 radial keratotomy, 284–286 Posterior blepharitis, 388 Posterior corneal elevation, 372–373, 373

Ind e x Posterior corneal shape, 338–340 Post tting care, o contact lens, 364 Power and power pro les, 74–76 Practice-driven systems, planned so lens replacement, 183–184 Practice location and accommodation, 427–428 Practice logistics, planned so lens replacement, 184–186 Practice management, 427–436.e1 personnel at the practice in, 428–430 planned so lens replacement cost to patient, 186 nancial management, 186 lenses available or planned replacement, 184, 184 –186 , 184t manu acturer-driven systems, 183, 183 practice-driven systems, 183–184 practice logistics, 184–186 practice management issues relating to, 183–186 supply routes, alternative, 186 practice location and accommodation in, 427–428 pricing in, 430–431, 431 processes in, 431–433 products and services provided in, 430 pro essional regulation in, 433–435 promotional issues in, 431 Practice newsletters, in promotional issues, 431 Practitioner, in compliance enhancement model, 425 Precorneal mucin layer, 390 Pre x, or classi cation o so lens materials, 60 Preliminary examination, or contact lenses, 346–355.e1, 346 anterior eye or, areas o , 347 binocular vision assessment and, 352 corneal tomography in, 352, 354 corneal topography in, 350–352, 353 keratometry or, 347 objective re raction in, 346–347 ophthalmoscopy and, 352 pachymetric mapping in, 352 slit-lamp biomicroscopy or, 347–350, 347 subjective re raction in, 346–347 supplementary tests in, 354–355 tear lm and, evaluation o , 350 wave ront re raction, 350 Preocular tear lm, 25–27 unction and properties o , 25, 25t sources and composition o , 25–27, 25 electrolytes, 26, 26t lipids, 26–27, 27 mucins, 26 proteins, 26, 26t structure o , models o , 27, 27 tear production, 25 Presbyopia, 214–230.e1 bi ocal and multi ocal contact lenses, 217–229 contact lens correction o , 383 contact lens options or, 215 , 217t correction o , patient’s options or, 214 initial measurements o , 216–217 lens tting or advantages and disadvantages o , 230t approach depending on, 216b clinical pearls or, 230t patient selection, 214 , 216 pupil size, 217, 217 –219 Prescription correct, compliance with, 423, 423 duration o , 420 incorrect, compliance with, 422–423, 423 Pricing policy, in contact lens industry, 425 in practice management, 430–431, 431

Printing, o lenses, 208–209, 209 Prion, trial lens sets and, 164 Prism ballast, 161 in so toric lenses, 96–97 Prismatic e ects, due to decentred or tilted lenses, 134 ‘Problem-solving’ tool, 173 Product support, in contact lens industry, 425 Products and services provided, in practice management, 430 Pro essional model, 431 Pro essional regulation, 433–435 Pro le o lens, myopia control and, 309 Projection systems, or rigid lens measurement, 136 Promotional issues, 431 Prophylactic tints, 207 Prosthetic tinted lenses, 205, 206 Protection rom lids, therapeutic applications and, 277 Protein removal solutions, or rigid lens, 164 Proteins, in preocular tear lm, 26, 26t Proud gra , 289–290, 289 –290 PRP. see Panretinal photocoagulation (PRP) Pseudokeratoconus, 371, 371 Pseudophakia, in children, 269 Ptosis, 314, 385–386, 386 ‘Ptosis crutch,’ 386 Pupil o anterior eye, and diabetes, 316 diameter o , 347 visual optics and, 31–32, 32 , 32t size o , presbyopia, 217, 217 , 219 variation o , 218 Push-up test, in so lens tting, 92, 92 PVA. see Polyvinyl alcohol (PVA) PVP. see Polyvinyl pyrrolidone (PVP)

Q

‘Quad-map’ ormat, o corneal tomography, 352, 354 Quadrant-speci c lenses, or keratoconus, 256 Questionnaire, dry-eye, 451t–452t

R

Radial edge li , 138, 139 Radial edge thickness, 439t Radial keratoneuritis, 405, 405 Radial keratotomy, contact lens tting ollowing, 284–286 Radius o curvature, 83–84, 84 Radiuscope, or rigid lens measurement, 137, 137 Random copolymer, 46, 47 RCM. see Rostock cornea module (RCM) Rebamipide, 388 Recall letter, in promotional issues, 431 Recentring lens, 359, 359 Reception area and ront desk, practice, 427 Recruitment and selection, in personnel at the practice, 428–429 Recurrent erosion syndrome, 275–276 Red eye, 379–380 characterization o , 379 diagnosing and solving, 379–380, 379 –380 in children, 274 Re erees, ametropic, 250 Re raction, 369–370 orthokeratology and, 303 o rigid toric lens, 158–159 surgery. see Post-re ractive surgery Re ractive end-points, 222 Re ractive error changes in, 370 vision loss related to, 383

467

Re ractive index, 80, 80 o commercial rigid lens materials, 122 o hydrogel, or so lens, 52 rigid lens measurement and, 141 Re ractive surgery types o , 282, 282t or vision correction, and sport, 246 Re ractometer, or so contact lens, 51 Regression, o accelerated orthokeratology, 298, 298 Removal o lens, 360 rigid, 149, 150 so , 89, 90 . see also A ercare. Research and development, in contact lens industry, 425–426 Residual astigmatism, o rigid toric lens, 159, 159 Retina, blur circles and pupil diameter, 31–32, 31 Retinopathy, 314, 315 Retinoscopy, in so lens tting, 93 Retroillumination, 349–350, 351 Reusable so lenses, 175–186.e1, 176 advantages o planned replacement, 175–182 avoidance o long-term adverse changes in anterior eye, 178–180 avoidance o long-term adverse changes in contact lenses, 176–178 enhanced compliance with a ercare schedules, 181 lens parameters easy to change, 181–182 ready availability o replacement lenses, 181 simple lens care regimens, 180–181, 181t single-use trial lenses, 181 trial lens tting with accurate prescription, 181, 182 use o higher-water-content hydrogel materials, 180 use o silicone hydrogel materials, 180, 180 determining appropriate lens replacement requency, 183 disadvantages o planned replacement, 182–183 practice management issues relating to planned so lens replacement, 183–186 Reverse-geometry lens manu acture, 128–129 Reverse-geometry rigid lens designs, 283–284, 284 Rewetting o scleral lenses, 201 solutions, 109–110 improved dryness symptoms, 110 RGP lenses. see Rigid gas-permeable (RGP) lenses Rigid gas-permeable (RGP) lenses, 6, 147 Rigid lens care systems, 163–164.e1 cleaning solutions, 163–164 disin ection solutions, 163, 163t, 164 protein removal solutions, 164 trial lens sets, disin ection o , 164 wetting solutions, 163, 164 Rigid lens design, 143–155.e1, 145–147 back optic zone diameter in, 145 centre thickness in, 145, 145t constant edge clearance, 449t edge orm in, 146, 146 edge li and edge clearance in, 145–146, 145 ront optic zone diameter in, 145 photore ractive procedures and, 283–284 PMMA versus RGP, 147 spherical versus aspheric, 146, 146t Rigid lens tting, 143–155.e1, 347 assessment o , 150–153 characteristics o , 150 corneal topography, 147–148, 147 empirical, 147 uorescein assessment o , 152–153 lens insertion, removal and settling or, 148–150

468

Ind e x

Rigid lens tting (Continued) optimum, 150, 151t problem, 153–155 satis actory, 150 selection o initial lenses or, 148, 148t trial tting options or, 147–148 trial tting set or, 147 white light assessment o , 151. see also Rigid lens design Rigid lens manu acture, 123–129.e1 aspheric, 128 crazing/cracking, 126 edge polishing, 124–127, 128 engraving, 124, 127 enestration, 124, 127 nal inspection, 124–127 industry regulation, 129 lens back sur ace, generating, 123–124, 125 lens ront sur ace, generating, 124 marking, 124, 127 raw materials, 123 reverse-geometry lens, 128–129 rigid lens prescribing data, 124 specialty, 127–129 toric, 127–128 Rigid lens materials, 113–122.e1, 120t commercial, properties o , 119–122 development, essential structural, 118 Gaylord patents-harnessing silicone, 116–119 hybrid rigid gas-permeable materials, 116 search or better, 116 Rigid lens measurement, 136–142.e1 back and ront sur ace radius o curvature in, 137–138 centre and edge thickness in, 137 edge pro les in, 138–139 riction and, 140, 140 lens and optic zone diameter in, 136–137 lens geometry in, 136–139 luminance transmittance in, 141 material properties in, 140–142 modulus and exure in, 141, 141 optical properties o , 139–140 optical quality and sur ace de ects in, 140 oxygen permeability and, 141 power and power pro les in, 139–140, 139 re ractive index and, 141 sur ace hardness in, 141–142 wettability in, 140 Rigid lens optics, 130–135.e1 aberrations, 133–134 basic tear lens properties, 130 BVP, required, 131 corneal astigmatism, neutralization, 132–133, 132 exure e ects, 134 prismatic e ects due to decentred or tilted lenses, 134 sur ace radii, calculation o , 131–132 visually disturbing e ects, 134–135, 134 Rigid lenses, 187, 187 –188 , 362 buttons, 124 or children, 273 dimensional tolerances in, 440t orces acting on, 144–145, 144 –145 high-minus, 265, 266 high-plus, 264, 264 –265 or keratoconus, 253–260, 253t li e expectancy o , 187–188, 188 –189 non-wetting o , 367–368, 368 optical tolerance or, 440t planned replacement, 187–192.e1 li e expectancy o rigid contact lenses, 187–188, 188 –189 optimum replacement schedule, 191

Rigid lenses (Continued) regular, 188–189, 189 –190 , 189t schemes available, 191 post-keratoplasty and, 291–292 radial keratotomy and, 285, 286 single-vision gas-permeable contact lenses, 307–308 therapeutic applications o , 279, 279 –280 toric. see Rigid toric lens Rigid toric lens alignment bitoric lenses in, 160 back-sur ace toric lenses in, 160–161 criteria or use o , 156–157, 157 cylindrical power equivalent, 160–161 design and tting, 156–162.e1 design consideration or, 157–158 e ect o lens rotation in, 161, 162 orms o , 156 ront-sur ace, 161 induced astigmatism o , 159–160 oblique bitoric lenses in, 161 optical consideration or, 158–161 re raction o , 158–159 residual astigmatism o , 159, 159 spherical power equivalent bitoric lens in, 160 Risk/bene t analysis, 325 Rockwell scale hardness test, 121 Roger’s seven-point plan, in recruitment and selection, 428 Rostock cornea module (RCM), 335, 337 Rotation, in so toric lenses allowing or, 99 e ects o , 98 measurement o , 99–100, 100 predicting, 98, 99t Roughness, as edge de ects, 66 Rules on the Fitting o Contact Lenses 1985, 433

S

Sa ety extended wear, 242–243 tinted lenses, 212 Sagittal depth, lens geometry, 82 SAI. see Sur ace asymmetry index (SAI) Saline solutions, 110 or lling non-ventilated RGP scleral lenses, 201 improved dryness symptoms, 110 Satis actory lens t, 150 Scanning slit-beam imaging, 416–417 Scattering, optical parameters, 76–77 Scheimp ug imaging devices or corneal shape, 339–340, 339 –340 or corneal thickness, 344 Schlieren principle, 75, 76 Scleral GP lenses, 292–293, 293 Scleral lenses, 193–203.e1, 368–369 advantages and disadvantages o , 195 t scales, 456, 456 tting principles o , 196–199 glass, 4–5, 4 indications or, 196, 196 or keratoconus, 260 criteria or, 260 with or without enestrations, 260 lens hygiene and maintenance, 201 modi cation, 200 non-coaxial, 198–199 ordering, 200–201, 200 photore ractive procedures and, 284 plastic, 5 radial keratotomy and, 285–286 therapeutic applications o , 279–280 wear, problems and complications with, 201–203 Scleral shape, corneal and, 340–343

Scleral zone, 197–198, 197 alignment, 195 Sclerotic scatter, 350, 351 Scuba diving, and lens wear, 248, 249 SD-OC . see Spectral-domain OC (SD-OC ) SEALs. see Superior epithelial arcuate lesions (SEALs) Seasonal cycles, and lens wear, 250 Secretion tests, or tear lm evaluation, 350 Secretory cells, o eyelids, 21–22, 22 Semiscleral lens designs, bi ocal and multi ocal contact lenses, 221, 222 Sensitivity, corneal, 354 measurement o , 335–336 contact aesthesiometry or, 335, 337 non-contact aesthesiometry or, 335–336 photore ractive procedures and, 283 Series suf x, or classi cation o so lens materials, 60 Service agreements, 432 Sessile drop technique, 50, 50 Settling time, or rigid lens, 150 SF. see Shape actor (SF) Shape actor (SF), 143 SHS Ophthalmic OmniSpect, 83–84 SICS. see Solution-induced corneal staining (SICS) Silicone acrylate material, re ractive index, 122 Silicone elastomer lenses, 5–6 poor wetting in, 232, 232 Silicone hydrogel or high ametropia, 263–264, 264t or so lens, 55–59, 56 –57 , 58t, 59 Silicone hydrogel lenses, 7, 87, 367, 374 or children, 270, 273 experiences with, 233, 233 –234 risk o in ection with, 242–243, 243 Gaylord patents or, 116–119 high-modulus, 369 multipurpose solutions and, 109 sport and, 249 therapeutic applications o , 278–279 Silicone hydrogel materials, 180, 180 Silicone hydrogel polymers, as material, 119 Silicone rubber, structure o , 116 Silicone rubber lenses, or children, 272–273, 272 Simple lens care regimens, 180–181, 181t Simultaneous image designs, bi ocal and multi ocal contact lenses, 218–224, 219 aspheric, 219, 220 hybrid and semiscleral lens designs, 221, 222 lens adjustments, 223, 223b lens tting, 221–222, 222 –223 meeting expectations, 224 Simultaneous-vision bi ocal lenses, 383–384 Simultaneous-vision tting distance vision adjustment options during, 223b near vision adjustment options during, 223b Single-use disposable so trial lenses, 218 Single-use trial lenses, 181 Single-vision rigid gas-permeable contact lenses, or myopia control, 307–308 Single-vision so contact lenses, or myopia control, 308–309 Size o eye, apparent, visual optics and, 41–42 Slit-lamp biomicroscopy, 332–334, 333 , 347–350, 347 ollowing lens removal, 373–374, 375 or tear lm evaluation, 350 while lenses are worn, 366–369 Slit-scanning con ocal microscopy, 335 Slit-scanning devices or corneal shape, 339 or corneal thickness, 344 Slit section, in digital imaging, 411, 413 –414 Smartphone digital imaging, 413 –414 , 415 ‘Smile’ patterns, in impression arcs, 371, 372

Ind e x Smiley ace, orthokeratology and, 301, 302 SOCRA ES (site, onset, character, radiation, association, time course, exacerbating/ relieving actors and severity), 324 Sodium uorescein tting pattern, or miniscleral lens, on keratoconus, 258, 258 So lens care systems, 103–112.e2 chlorhexidine-preserved system, 105 chlorine, 105–106 disin ection, 103 evolution o , 104 hydrogen peroxide, 106–107 lens cleaning, 107 e ects o , 104 rationale or, 103–104 lens rinsing, e ects o , 104 multipurpose solutions, 107–109 silicone hydrogel lenses and, 109 physical methods, 104–105 relative per ormance measures, 110–111 rewetting solutions, 109–110 saline solutions, 110 storage case, 111–112, 112 thiomersal-preserved system, 105 So lens design, 87–88 back optic zone radius, 88 back vertex power, 88 centre, 88 lens material and water content, 87 thickness, 88 total diameter, 88 So lens tting assessment o , 91–93 basic principles o , 86–87 orces acting, 86, 87 ideal t, 86–87 Caucasian versus Chinese eyes, 94 characteristics o , 90–91, 91t actors a ecting, 90t introduction, 86 ocular measurement, 86 optimum t, 90 options, 88 problems, 93–94 requirements o , well tting, 87t trial lens tting, 88–90 So lens manu acture, 61–67.e1 edge de ects in, 66, 66 methods o , 61–63, 87–88 cast moulding, 63, 64 –65 lathe cutting, 61–62, 61 –62 spin casting, 62–63, 63 –64 non-edge (body) de ects in, 66–67 occupational sa ety in, 67 reproducibility and quality o , 66–67 So lens measurement, 73–85.e2 conditioning according to ISO 18369-3, 73 lens geometry, 81–84 diameter, 81–82, 81 edge, 84, 85 radius o curvature, 83–84, 84 sagittal depth, 82 thickness, 82–83, 83 material properties, 77–81 modulus, 77–78, 77 –78 oxygen permeability, 79–80, 80 re ractive index, 80, 80 spectral and luminous transmittance, 80–81, 81 sur ace riction, 79 water content, 77 wettability, 78–79, 79 optical parameters, 73–77 optical quality, 76, 77 power and power pro les, 74–76 scattering, 76–77

So lens optics, 68–72.e1 aberration, 69–72, 70 on-eye power changes, 68–69, 69 So lens oxygen per ormance, 447t–448t So lenses, 6, 6 , 361–362 corneal shape changes due to, 371 daily disposable, 165–174.e1, 167 , 169 –170 bimodal distribution o , 170 clinical per ormance o , 170 com ort enhancement strategies, 171 corneal in ltrative events and keratitis, 173–174 disadvantages o , 171 environmental impact o , 172, 173 lens application to assist ametropes in eyewear selection, 174 lens wearers, advantages rom perspective o , 171 limitations to more general acceptance, 172–173 manu acturing reliability, 171, 172 patterns o wear, 168–170 practitioners, advantages rom perspective o , 170–171 dimensional tolerances in, 440t disposable, 232–233 hydrogel, therapeutic applications o , 278–279, 279 or keratoconus, 252–253 material property tolerances or, 440t materials or. see Materials or so lenses optical tolerances or, 440t photore ractive procedures and, 283 planned replacement or, 187 radial keratotomy and, 285, 285 single-vision, or myopia control, 308–309 toric. see So toric lenses water content, in various lens thickness, 447t–448t So toric lenses, 95–102.e1 correction, principles o , 97–101 criteria or use, 95 design o , 95–97, 98 limitations o , 102 misalignment demonstrator, 450t power measurement, 74 replacement o , planned, 101–102 So ab chlorine system, 105–106, 106 So Perm (CIBA Vision) lens, or keratoconus, 260–261 Solid design, bi ocal and multi ocal contact lenses, 224–225, 226 Solution-induced corneal staining (SICS), 374, 375 Special mailings, in promotional issues, 431 Speciality-commodity continuum, 430, 431 Spectacles or aphakia, 268 corrections, 38–41 accommodation demand and, 40–41, 41 convergence demand and, 41, 41 e ectivity and, 39, 39 or keratoconus, 252 magni cation, 39–40, 39 –40 dispensary, practice, 428 Spectral-domain OC (SD-OC ), 342 Spectral transmittance, 80–81, 81 Specular microscopy, 334 or corneal thickness, 345 Specular re ection direct illumination, 348–349, 350 Spherical aberrations, 70, 71 higher-order, 75–76, 76 Spherical components, low, in so toric lenses, 102 Spherical designs or bi ocal and multi ocal contact lenses, 221 or rigid lenses, 146, 146t, 255–256

469

Spherical lenses, power measurement, 74 Spherical power equivalent bitoric lens, 160 Spin casting, 62–63, 63 –64 technique, in so lenses, 6, 6 Split, as non-edge (body) de ects, 66 Sport, 246–250.e1 environmental and physical constraints in, 246–249 environmental conditions, 246–249 physical conditions, 249 general considerations in, 249–250 participation in, 247t sporting per ormance in, enhancement by contact lenses, 246 tinted lenses, 206 vision correction, 246, 247t re ractive surgery and, 246 Spot diagram, 133 Spring-back test, 92 SQI ARS (site and radiation, quality, intensity, timing, aggravating actors, relieving actors, secondary symptoms), 324 Squeeze pressure, 86 SRI. see Sur ace regularity index (SRI) SS-OC . see Swept-source OC (SS-OC ) Stabilization techniques or so toric lens, 96–97 Staining conjunctival, 391–392, 392 corneal, E ron grading scale or, 454 epithelial, 397, 397 –398 uorescein, 350, 411, 413 –414 Staphylococcal anterior blepharitis, 388, 389 Steep- tting lens, orthokeratology and, 301 Steep scleral zone, 197 Stem, or classi cation o so lens materials, 60 Stereopsis, 354 Sterile in ammatory keratitis, 236 Sterile keratitis, 405 Stevens-Johnson disease, 277 Sticky eye, in children, 274 Storage contamination, 178, 179 Stress, physiological, extended wear and, 244–245, 245 Stress-strain curve, hydrogel materials and, 48–49, 48 Subaquatic environment, and sport, lens wear, 248, 249 Subjective re raction, 346–347 Subscription schemes, in practice management, products and services provided in, 430 Suction holders, in lens removal, 361 Suf x groups, or so lens materials, 60, 60t Sunglasses, and glare relie , 250 Sunken gra , 290 Sunshades, and glare relie , 250 Superior epithelial arcuate lesions (SEALs), 242, 242 , 245 Superior limbic keratoconjunctivitis, 382, 395–397, 396 E ron grading scale or, 455 and so toric lenses, 102 Superior tarsal muscles, in eyelids, 18 Supply routes, alternative, 186 Sur ace asymmetry index (SAI), 406 Sur ace de ects, o rigid lens measurement, 140 Sur ace o lens assessment, 366–368, 368 degradation o , 203 riction, 79, 242 Sur ace optics, in so toric lenses, 96 Sur ace properties, o commercial rigid lens materials, 121 Sur ace radii, required, rom a trial lens t, calculation o , 131–132 Sur ace regularity index (SRI), 406

470

Ind e x

Sur ace tension orces, rigid lens and, 144 Surgery re ractive. see also Post-re ractive surgery types o , 282, 282t or vision correction, and sport, 246 Suture techniques, post-keratoplasty and, 287 , 290, 290 Swell actor, o hydrogel lenses, 52–53 Swept-source OC (SS-OC ), 342–343, 343 Symbols, used or describing contact lens, 439t SynergEyes hybrid lenses, 292, 292 or keratoconus, 260–261 Systemic disease, 318. see also Diabetes

T

anaka patent, 117–118 andem scanning con ocal microscopy, 334–335 angential maps, 352 arsal gland. see Meibomian glands D-OC . see ime-domain OC ( D-OC ) eaching area, in patient education, 356, 356 ear lm in anterior eye, and diabetes, 314 assessment, in digital imaging, 411, 413 –414 complications in, 389–391 dry eye in, 389–390, 390 –391 mucin balls in, 390–391, 391 . see also Dry-eye dys unction, 382 evaluation o , 350 examination o , 327–330 osmometry, 329, 329 tear break-up, 328–329, 328 –329 tear meniscus height ( MH), 329–330, 330 tear quality and thickness, 327–328, 328 ‘ ear map,’ 328 ear meniscus height ( MH), 329–330, 330 earLab osmometer, 329, 329 ears break-up, 328–329, 328 –329 de ciency, therapeutic applications and, 277 as edge de ects, 66, 66 lm. see ear lm layer thickness analysis, 419, 419 lens power o , ormula, 130 properties o , 130, 130 during trial lens ts, 130–131, 131 quality and thickness, 327–328, 328 stability tests, 350 earscope, 390 earscope-plus, 327, 328 . see also Mucins erms, symbols and abbreviations, used or describing contact lens, 439t etronic 1107, hydrogel lenses and, 55 T eatric tinted lenses, 207 T eatric ‘wol -eye’ lens, 207 T eoretical model, optimum rigid lens replacement schedule, 191 T erapeutic applications, 275–281.e1 chemical injuries, 276, 276 complications, 281 conjunctivitis, cicatricial, 276 cornea degenerations involving the endothelium, 276 degenerations involving the epithelium, 276 dystrophies involving the epithelium, 276 drug delivery in, 280–281 environment, protection rom, 277, 277 epithelial pain, relie o , 275 erosion syndrome, recurrent, 275–276 lamentary keratitis, 276 indications or, 275–278 keratitis, severe exposure, 277

T erapeutic applications (Continued) lens types, 278–280 rigid, 279, 279 –280 scleral, 279–280 so hydrogel and silicone hydrogel lenses, 278–279, 279 lids, protection rom, 277, 277 medication in, concurrent, 280 precorneal tear reservoir, maintenance o , 277–278 shape, unusual or distorted, 275 spontaneous per oration, 278 tear de ciency, 277 tinted, 205–206 trauma or surgery, 278, 278 T ickness o cornea, determination o , 343–345 geometry o lens, 82–83, 83 o rigid lens, 137 o so lens, 82–83, 88 average, 446t toric, 99 o tear, 327–328, 328 T iomersal-preserved system, 105 T ree-point touch, 254 , 255 ight lenses, in so lens tting, 90–91, 94 ilted lenses, prismatic e ects due to, 134 ime-domain OC ( D-OC ), 342, 342 ime settling, in so lens tting, 89–90 int distribution, o lenses, 211, 211 inted lenses, 204–213.e2 basic options o , 204 clinical considerations o , 211–213 discom ort and dryness, 213 lens deposits, 213 lens tting, 211 lens maintenance, 211 multiple pairs, care o , 212 ocular e ects, 212 oxygen transmissibility, 211 replacement requency, 212 sa ety, 212 tint distribution, 211, 211 visual e ects, 211–212 designs and applications, 204–207 cosmetic, 205, 205 handling tints, 204–205, 204 per ormance-enhancing, 206 prosthetic, 205, 206 therapeutic, 205–206 manu acture, 208–210 sporting per ormance enhancement and, 246 sur ace characteristics o , 210 theatric, 207 issue ablation, photore ractive procedures and, 282–283 issues, box o , 357 MH. see ear meniscus height ( MH) olerance o lenses, 440 omey Casia SS-100, 416 onometry, 354 opography. see under Cornea; Ocular topography oric lenses, so , 95–102.e1, 381 correction, principles o , 97–101 criteria or use o , 95 design o , 95–97, 98 limitations o , 102 replacement o , planned, 101–102 oric rigid lens manu acture, 127–128 oroidal back sur ace, 96 oroidal lenses, 198 designs, 256–257, 256 –257 otal diameter, 439t in so lens design, 88 tting, 93

PX. see Poly (4-methyl pent-l-ene) raining sta , in recruitment and selection, 429–430 ranslucent tints, o lenses, 208–209 ransmissibility, 79–80 ransparency, corneal, 15–16 rial lens sets, disin ection o , 164 rial pack o solutions, 357 ri uoroethyl methacrylate monomer, 117–118, 118 1,1,9-trihydroper uoro-nonyl methacrylate monomer, 116–117, 117 RIS. see ris(trimethylsiloxy)-methacryloxypropylsilane ( RIS) ris(trimethylsiloxy)-methacryloxy-propylsilane ( RIS), 116–117, 117 or silicone hydrogel lenses, 56, 56 runcation, 97, 97 in ront-sur ace toric lenses, 161, 161 wo-handed technique, in lens removal, 360–361, 360

U

UltraHealth design lenses, or keratoconus, 261 Ultrasonic pachymetry, 343–344, 344 , 373 Ultrasound, or lens disin ection, 105 Ultrasound pachymeters, or rigid lens measurement, 137 Ultraviolet light, and lens wear, 248–249 Unintentional non-compliance, 422–423 Uninterrupted wear, risks in, 243 Unwanted orthokeratology, 242

V

V-gauge, or rigid lens measurement, 136, 136 Vacuoles, 399, 399 Vascularity, eye redness and, 379 Vascularization, 245 Vascularized limbal keratitis, 395, 395 –396 Vat dye tinting, o lenses, 208, 208 Vaulting, 456, 456 central, 456, 456 limbal, 457, 457 Vertex distance correction, 441t–442t Vertex power, back, in so lens design, 88 Vickers Microhardness test, 121 Video slit-lamp imaging system, 415–416 Videokeratoscopy, 147–148, 147 in so lens tting, 93 Virtual contact-lens- tting so ware, or keratoconus, 257, 257 Vision binocular, assessment, 352 distance, adjustment options or improving, during simultaneous-vision tting, 223b loss, a er lens removal, 383, 383 low, and ametropia, 266 measurement o , 346 near, adjustment options or improving, during simultaneous-vision tting, 223b poor, 382–384 and rigid lens, 155 in so lens tting, 93 uncorrected, 369 unstable, and rigid lens, 155 variable, in so lens tting, 94 Visit schedules, based on lens type, 364, 365 , 365t Visual acuity with contact lenses, 366 poor, in so lens tting, 94 Visual e ects, in tinted lenses, 211–212 Visual elds, 354

Ind e x Visual optics, 28–42.e2 aberrations and, 32–38 chromatic, 37, 37 correction o high-order, 38 monochromatic, 33–37, 33 –34 , 35t–36t, 36 –37 accommodation demand and, 40–41, 41 ametropia and, 28–30, 29t, 30 apparent size o eye, 41–42 convergence demand and, 41, 41 corneal topography and, 30–31, 31 di raction and, 32 eye and in white light, per ormance o , 37 elds o view and, 41, 42 xation and, 41 pupil diameter and retinal blur circles, 31–32, 31 –32 , 32t. see also Aberrations; Ametropia spectacle magni cation and, 39–40, 39 –40 Vitrodyne 2000, 78, 78

W

Waiting area, practice, 428 Warpage, 406 Water content o hydrogel lenses, 51 measurement, o so lens, 77 o so lens, 87 wetting/wettability o hydrogels, 49–50 poor, o silicone elastomer lenses, 232, 232 rigid lens measurement and, 140 so lens measurement, 78–79, 79 Wave ront error (WFE), 350 Wave ront re raction, 350 Wearing schedules, in contact lens wear, 361–362 Wettability. see under Water Wetting solutions, or rigid lens, 163, 164 WFE. see Wave ront error (WFE) White light, anterior eye, 37, 37

471

X

Xanthelasma, 314, 315 Xerogel lens, 63

Y

Young, T omas, 3, 4

Z

Zeiss visante, 416 Zernike aberrations, RMS wave ront errors and, 35, 36t Zernike coef cients, monochromatic aberrations and, 34–35, 34 , 37 Zernike polynomials, monochromatic aberrations and, 35, 36 Zonal aspheric designs, bi ocal and multi ocal contact lenses, 221

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