Manual of Gas Permeable Contact Lenses PDF
Preview Manual of Gas Permeable Contact Lenses
B H An Imprint ofElsevier 11830 Westline Industrial Drive St. Louis, Missouri 63146 MANUAL OF GAS PERMEABLE CONTACT LENSES ISBN 0-7506-7335-4 Copyright © 2004, Elsevier Science (USA). All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, elec- tronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permissions may be sought directly from Elsevier’s Health Sciences Rights Department in Philadelphia, PA, USA: phone: (+1) 215 238 7869, fax: (+1) 215 238 2239, e-mail: [email protected]. You may also complete your request on-line via the Elsevier Science homepage (http://www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’. NOTICE Optometry is an ever-changing field. Standard safety precautions must be followed, but as new research and clinical experience broaden our knowledge, changes in treatment and drug therapy may become necessary or appropriate. Readers are advised to check the most current product information provided by the manufacturer of each drug to be administered to verify the recommended dose, the method and duration of administration, and contraindications. It is the responsibility of the licensed prescriber, relying on experience and knowledge of the patient, to determine dosages and the best treatment for each individual patient. Neither the publisher nor the author assumes any liability for any injury and/or damage to persons or property arising from this publication. Previous edition copyrighted 1986 International Standard Book Number 0-7506-7335-4 Publisher:Linda Duncan Acquisitions Editor:Christie Hart Publishing Services Manager:Pat Joiner Project Manager:David Stein Designer:Amy Buxton Printed in the United States of America Last digit is the print number: 9 8 7 6 5 4 3 2 1 Contributors EDWARDS. BENNETT, OD, MSED ROBERTM. GROHE, OD Director of Student Services Assistant Professor of Clinical Co-Chief of the Contact Lens Service Ophthalmology University of Missouri St. Louis Department of Ophthalmology, College of Optometry Director, Contact Lens Service St. Louis, Missouri Northwestern University Medical School ADRIANS. BRUCE, BSCOPTOM, PHD Chicago, Illinois Senior Fellow Department of Optometry and Vision DAVIDHANSEN, OD Sciences Private Practice The University of Melbourne; Des Moines, Iowa Senior Optometrist Clinical Vision Research VINITAALLEEHENRY, OD Victorian College of Optometry Clinical Associate Professor Melbourne, Victoria, Australia Director of Residencies Co-Chief of the Contact Lens Service HELENJ. CHANDOHA, OD University of Missouri St. Louis Staff Optometrist College of Optometry Cornea and Refractive Surgery St. Louis, Missouri Department Northeastern Eye Institute MILTONM. HOM, OD Scranton, Pennsylvania Private Practice Azusa, California SHELLEYCUTLER, OD Adjunct Faculty URSULALOTZKAT Contact Lenses Granger, Indiana Pennsylvania College of Optometry Elkins Park, Pennsylvania BELINDAMINGWAILUK, BSC, OD Private Practice MICHAELD. DEPAOLIS, OD Columbia, Missouri Clinical Associate Department of Ophthalmology BRUCEW. MORGAN, OD University of Rochester Medical Chief of Cornea and Contact Lens Center Service Rochester, New York Michigan College of Optometry Ferris State University Big Rapids, Michigan v vi CONTRIBUTORS JOHNM. RINEHART, OD LORETTAB. SZCZOTKA-FLYNN, OD, MS Private Practice Associate Professor Peoria, Arizona Department of Ophthalmology Case Western Reserve University; JOSEPHP. SHOVLIN, OD Director, Contact Lens Service Director of Contact Lens Services University Ophthalmologists, Inc. Senior Optometrist University Hospitals of Cleveland Northeastern Eye Institute Cleveland, Ohio Scranton, Pennsylvania JEFFREYJ. WALLINE, OD, MS IVETTASIEDLECKI, OD Research Scientist Assistant Professor The Ohio State University Cornea and Contact Lens Service College of Optometry University of Missouri St. Louis Columbus, Ohio College of Optometry St. Louis, Missouri Preface G as permeable (GP) lenses are an important and underutilized contact lens option in eye care practices today. The benefits of good quality of vision, astigmatic correction, bifocal designs, irregular cornea management, and myopia control make them a “must use” modality in today’s practice if patients are to receive the best possible vision management. However, for several reasons GP lenses are not prescribed for a large percentage of those patients who would benefit most from them. Certainly, the initial comfort issue is an important concern. The ease of fitting disposable lenses makes them a simple option to use. In addition, many practitioners do not feel confident in their GP fitting and problem-solving skills. The goals of this text are to address all of the aforementioned con- cerns and describe a management strategy to optimize the initial com- fort while emphasizing the ease of fitting both spherical and specialty designs. This text is clinical in nature and will help the practitioner understand the basics of material selection, fitting, and problem-solv- ing GP lenses, with the assistance of the video images on the accom- panying CD-ROM. Contemporary presbyopic management with GPs—a much underutilized corrective option—is emphasized as well. With the current interest in corneal reshaping as an alternative to refractive surgery with myopic patients, the accompanying chapter on this topic provides a “How to” approach for the successful incorpora- tion of this modality into contact lens practice. In addition, manage- ment of young people, keratoconus, and postsurgical patients with GP lenses is provided. The authors would like to acknowledge the contributors to this text including: Adrian Bruce, Shelley Cutler, Dave Hansen, Vinita Henry, Ursula Lotzkat, Belinda Luk, Bruce Morgan, John Rinehart, Joe Shovlin, Loretta Szczotka-Flynn, and Jeff Walline. We would also like to acknowledge the contributions of Bob Grohe to this and the first edi- tion as well as a lifelong friendship with the first author. The assistance provided by Maria Taylor via inputting information is also appreciated. The support provided by Elsevier was essential to the publication of this manuscript. We would like to especially thank Karen Oberheim, former medical publisher for Butterworth-Heinemann, for her sup- port and enthusiasm. This book would not exist today if it were not for her. Also, Managing Editor Christie Hart’s persistence in seeing the project completed is greatly appreciated. Likewise, the editorial vii viii PREFACE assistance provided by Project Manager David Stein was invaluable in improving the quality of this text. The assistance of Managing Editor Kathy Falk and Editorial Assistant Colin Odell was also appreciated. The authors would like to thank Drs. John Mountford and Rajni Singh for their review of sections of this text. It is our sincere belief that, in every practice that prescribes contact lenses, GP lenses should be an important part of the practitioner’s fit- ting regimen. We hope this text will be a valuable guide in both moti- vating practitioners to fit GP lenses on a regular basis and increasing their knowledge about the many beneficial applications of this modality. Edward S. Bennett Milton M. Hom 1 Corneal Physiological Response and Consequences of Hypoxia ADRIANS. BRUCE S ome 50 years ago, Smelser et al1,2 demonstrated that oxygen is necessary for normal corneal function. They showed that con- tact lenses made from oxygen-impermeable polymethyl- methacrylate (PMMA) led to marked structural and optical changes in the cornea. Thirty-four years later, when the first edition of this text was published in 1986, Andrasko and Bennett3commented: “In order to predict whether a prospective contact lens wearer will achieve success or failure, the practitioner must have a thorough under- standing of corneal physiology. Specifically, the practitioner must understand corneal metabolism and its effect on corneal hydration, oxy- gen demand and waste production under both normal and adverse conditions.” Today, it is pleasing to see that the technological advances of rigid lens materials have greatly decreased the physiologic impact of daily (open- eye) rigid lens wear. Most patients wearing rigid lenses for daily wear show minimal or no corneal signs that could be attributed to hypoxia. For certain hydrogel lens designs, such as higher refractive errors and toric prescriptions, rigid lenses can offer markedly superior gas exchange characteristics. Despite these advances, it remains important for the clinician to have knowledge of corneal physiology in relation to hypoxia. There are still some patients wearing older, low-oxygen permeability materials, and it is important to detect corneal changes when they are present to form a basis for refitting. There are still some patients needing refit- ting from the oxygen-impermeable PMMA lenses. Another situation is patients who may wear their lenses on an extended-wear basis. Suitable rigid lens materials for extended wear did not exist 15 years ago; however, with the remarkable advances in material biochemistry, there are now a number of suitable materials. However, if suitable materials have not been prescribed or if the 2 Chapter1 Corneal Physiological Response and Consequences of Hypoxia 3 patients have themselves chosen to wear their lenses overnight, then hypoxic corneal changes may occur. Ocular changes related to hypoxia and hypercapnia tend to affect only the cornea, and there are few associated symptoms, unlike those related to other causes.4 Thus it is essential for the patient to have a compre- hensive slit-lamp examination at aftercare visits, including evaluation of epithelial integrity, stromal transparency, and endothelial regularity. HOW MUCH OXYGEN IS NEEDED A decreased availability of oxygen (hypoxia) and an increase in carbon dioxide (hypercapnia) may occur during contact lens wear. Normally there is a flow of oxygen (O ) into the anterior corneal surface and an 2 efflux of carbon dioxide (CO ). The partial pressure gradients for O 2 2 and CO are the key to gas movement across the cornea (Table 1-1 and 2 Figure 1-1). When the eyelid is closed during sleep, the palpebral tarsal vascular supply becomes the site of gas exchange. More than 10 to 15 years ago there was agreement among researchers regarding the precorneal partial pressure of O required to 2 avoid corneal edema. Approximately 10% O is required to avoid stro- 2 mal edema during lens wear.5,6 However, the consensus regarding the O transmissibility of rigid 2 lens materials has been slower in coming, with agreement in pub- lished values of a larger range of lenses occurring only in recent years.7-11 Published values for rigid lens materials by these authors, using Fatt’s ISO/DIS 8321-2 method, are shown in Table 1-2. O transmissibility (Dk/t) of a contact lens material is directly pro- 2 portional to the O permeability of the lens material (Dk, Barrer) and 2 inversely related to the lens thickness (t, cm). The Dk unit Barrer, (cm2/sec)(mlO /[ml ×mm Hg]) ×10−11, is also known as Fatt units.12If 2 metric units are desired, using hectopascals rather than mm Hg and then multiplying the Barrer/Fatt unit values by 0.75006 will give the appropriate value.13 Table1-1 Oxygen and Carbon Dioxide Partial Pressures Oxygen Partial Carbon Dioxide Partial Pressure (PO ) Pressure (Pco ) 2 2 Corneal surface, open eye 155 mm Hg ~ 0 mmHg Corneal surface, closed eye 55 mm Hg 40 mm Hg Anterior chamber (open 55 mm Hg 40 mm Hg and closed eye) 4 SECTION I INTRODUCTION 160 Stroma 140 120 Endothelium g) H m m100 n ( o si Open n e n t 80 Epithelium e g y x O 60 Closed 40 20 Anoxic Covered 0.10 0.20 0.30 0.40 0.50 Distance from aqueous humor (mm) Figure 1-1. Oxygen tension profiles for various conditions at the epithelial surface. (From Fatt I: Steady-state distribution of oxygen and carbon dioxide in the in vivo cornea, Exp Eye Res7(3):413-430, 1968.) CO transmissibility is directly related to the O transmissibility for 2 2 rigid lenses, being numerically seven times greater for gas permeable (GP) lenses.14 Therefore, if a lens has acceptable O transmissibility, 2 then the CO transmissibility will also usually be sufficient. However, 2 the CO efflux from the eye is substantially greater than the ingress 2 ofO (Figure 1-2). 2 Lenses with a Dk/t of 22 to 24 × 10−9 Barrer/cm should meet the corneal O requirement when worn in open-eye conditions.6,15,16 This 2 corresponds to a rigid lens material of Dk = 35, for a lens thickness of 0.15 mm. Holden and Mertz15found an ideal Dk/t value for extended wear to be 87 ×10−9Barrer/cm, although a clinically acceptable value was suggested to be 34 ×10−9Barrer/cm. Chapter1 Corneal Physiological Response and Consequences of Hypoxia 5 ymer hnologyrp (1998) 0 88 12 1819242425 38 Continued olecCo PT n mi a8) 0 0 2 6 9 nj9 1 1 2 3 e9 B(1 &5) s 9 noudion (19 781012 2324 303638 ar TrEf n e b u R & 0 8 tt 93) 1 2 a9 F1 ( * al als et Materi olden 990) 12 39 s H(1 n e L † d er gi ur Ri ct of ufa all 522 1 11123 212 ) n Dk Ma ( y t bili rmea Type A S/AS/AS/AS/AS/A F-S/AS/AF-S/AF-S/AF-S/A S/AF-S/AF-S/A e P n ge 0) 0 0 1-2OxyTable Zero Dk PMMA Low Dk (<15) Polycon IIOptacryl 60Paraperm O2AlbertaBoston II Medium Dk (15-3 Boston ESBoston IVBoston RXDFluoroperm 3Fluorex 500 High Dk (31-60) Paraperm EWQuantum 1Fluoroperm 6 6 SECTION I INTRODUCTION 1-2Oxygen Permeability (Dk) of Rigid Lens Materials*—cont’dTable Polymer Holden et alFatt & RubenTranoudis &BenjaminTechnology†TypeManufacturer(1990)(1993)Efron (1995)(1998)Corp (1998) Paragon HDSF-S/A241Equalens IF-S/A149485147Boston 7F-S/A149Boston EOF-S/A15858 Super Dk (61-100) Menicon EXF-S/A4646258Fluoroperm 92F-S/A257646361Fluoroperm 151F-S/A27488Equalens IIF-S/A185Quantum 2F-S/A1999395Boston XOF-S/A1100 Hyper Dk (>100) Menicon SF-PF-S/A4126134125Menicon ZF-S/A4163 *Measured using ISO/Fatt (ISO/DIS 8321-2) method and categorized according to the criteria listed by Benjamin (1996). In the ISO/Fatt (ISO/DIS8321-2) method, oxygen transmissibility (Dk/t) of a contact lens is directly proportional to the oxygen permeability of the lens material (Dk, Barrer) andinversely related to the lens average thickness (L, cm). For comparison with corneal critical oxygen values, use nominal center thickness of 0.15 mm (e.g.,××−−119divide by 1.5 to obtain Dk/t value; need Dk >18 10Barrer, for Dk/t >12 10Barrer/cm).†Manufacturers:1,Polymer Technology Corporation, Bausch & Lomb; 2,Paragon Optical, USA; 3,GT Laboratories, Illinois; 4,Menicon, Japan;5,Wesley-Jessen/CibaVision.S/A,Silicone/acrylate; F-S/A,fluoro-silicone/acrylate.
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