LASIK Fundamentals, Surgical Techniques, and Complications - part 1 docx

Azar Massachusetts Eye and Ear Infirmary Schepens Eye Research Institute and Harvard Medical School Boston, Massachusetts, U.S.A.. Cornea and Refractive Surgery Service, Massachusetts Ey

Marcel Dekker, Inc New York•Basel

L A S I K Fundamentals, Surgical Techniques,

and Complications

edited by Dimitri T Azar

Massachusetts Eye and Ear Infirmary Schepens Eye Research Institute and Harvard Medical School Boston, Massachusetts, U.S.A.

Douglas D Koch

Cullen Eye Institute Baylor College of Medicine Houston, Texas, U.S.A.

Copyright © 2002 by Marcel Dekker, Inc All Rights Reserved.

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A catalog record for this book is available from the Library of Congress.

ISBN: 0-8247-0797-4

This book is printed on acid-free paper.

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To my wife, Marcia, who makes it all so much more meaningful—and fun.

DDK

To Nathalie, Alexander, Nicholas, and Lara—for all the joyful moments that we share.

DTA

Preface

Dov’é mio figlio? più non lo vedo:

In te più Alfredo–trovar non so

For decades, the majority of ophthalmologists have been embarrassed by and highlysuspicious of refractive surgery, at times with good justification They have been repelled

by its tactics in patient recruitment, uneasy about its seemingly cavalier use without term data, eager to defend its unsuspecting victims, and deeply concerned that its short-term benefits are outweighed by its burdensome long-term consequences And so the words

long-of Germont, uttered pursuant to the most dramatic moment in Verdi’s La Traviata, echo theextreme and oftentimes passionate contempt expressed by so many colleagues and closefriends towards academically oriented ophthalmologists who marched among the van-guards of laser refractive surgeons Their repudiation was not unlike Germont’s scorn ofhis son Alfredo for offending Violetta and for making himself worthy of disdain:

Where is my son? no more do I see him.

I am unable to see Alfredo in you.

The ophthalmological peer-reviewed publications and textbooks prior to the late1980s reflected this disinterest in or hostility toward refractive surgery Unfortunately, inthe pre-LASIK era, there were relatively few high-quality peer-reviewed reports on refrac-tive surgery Criticisms of the scientific rigor with which clinical studies were conductedwere often justified, which, in turn, discouraged academically bound graduates of ophthal-mology training programs from dedicating their careers to this subspecialty Comprehen-sive textbooks of ophthalmology in the pre-LASIK era also kept refractive surgery at arm’slength, relegating it to a minor chapter on the topic Refractive surgery was viewed as anoutlier of great potential but little practical merit in mainstream ophthalmology But thishas all changed with the advent of LASIK!

This book is the first in a series dedicated to Refractive Surgery by Marcel Dekker,Inc It will most certainly be judged by many as just another LASIK book While there isabundant coverage of the topic in other books, this volume has several unique features Itscoverage of LASIK is relatively comprehensive, in that it is not limited to LASIK history,surgical techniques, complications, and their management New aspects of lasers, optics,refraction, diagnostics, and instrumentation are combined with the science and generalprinciples of LASIK, and indications for its use Although not meant to be encyclopedic,key references abound They are intended to serve as a guide to the literature on the topic.Thus, this book is not so much a chronicle of LASIK, as an attempt to serve as a source ofinformation relevant to clinical practice.

We are indebted to the students, residents, and colleagues who have made valuablecontributions to this book Several have included original work and analysis in their chap-ters It is evident that the authors have attended diligently to their assignments We aregrateful for their effort in integrating the sometimes limited information in peer-reviewedliterature with the knowledge derived from their clinical experiences and interactions withcolleagues We hope that this has resulted in a text that is both clinically relevant and as ev-idence-based as possible

We thank Dr Geoffrey Greenwood and Elizabeth Curione of Marcel Dekker, Inc.,for their commitment to this project and Drs Tsubota, Boxer Wachler, Hoang-Xuan, Ang,and Gatinel for their assistance in future books in this series Special thanks go to LeonaGreenhill, for her editorial assistance, and to Rhonda Harris, who managed this project withcare and precision Her attention to detail and her dedication have enabled us to work co-herently in the face of adversity

We take the opportunity to acknowledge the pioneering surgeons and researchers inthe field of refractive surgery Their work and vision have provided the basis not only forcurrent refractive developments that we can offer to our patients, but also for future ad-vances to be made by the next generation of thoughtful contributors to this important field

Dimitri T Azar Douglas D Koch

Contents

1 Refractive Errors and Their Treatment 1

Liane Clamen Glazer and Dimitri T Azar

2 History of LASIK 21

Ioannis Pallikaris and Thekla Papadaki

3 Lasers in LASIK: Basic Aspects 39

Rodrigo Torres, Robert T Ang, and Dimitri T Azar

4 Microkeratomes 57

Sandeep Kakaria, Thanh Hoang-Xuan, and Dimitri T Azar

5 Adjunctive Instrumentation in LASIK 71

Robert T Ang and Dimitri T Azar

6 LASIK Indications, Contraindications, and Preoperative Evaluation 91

Richard E Braunstein, Marc Winnick, and Kenneth A Greenberg

7 Preoperative Optical Considerations in LASIK: Refractive Errors,

Monovision, and Contrast Sensitivity 101

Balamurali K Ambati, Leon Strauss, and Dimitri T Azar

8 Corneal Topography and LASIK Applications 111

Li Wang, Douglas D Koch, Dimitri T Azar, Robert T Ang,

and Rengin Yildirim

9 Wavefront Technology and LASIK Applications 139

Naoyuki Maeda

10 Preoperative Considerations: Diagnosis, Classification,

and Avoidance of Keratoconus Complications 153

Paul Chung-Shien Lu and Dimitri T Azar

11 Corneal Stability and Biomechanics After LASIK 163

Esen Karamursel Akpek, Rana Altan-Yaycioglu, and Walter J Stark

12 LASIK Techniques 175

Dimitri T Azar, Kathryn Colby, and Douglas D Koch

13 Microkeratomes and Laser Settings 189

William J Lahners and David R Hardten

14 Centration of LASIK Procedures 199

Marsha C Cheung, Chun Chen Chen, and Dimitri T Azar

15 Surgical Caveats for Managing Difficult Intraoperative

Situations 229

Samir G Farah and Dimitri T Azar

16 Bilateral Simultaneous LASIK: Advantages, Disadvantages,

and Surgical Caveats 243

David R Hardten, Elizabeth A Davis, Richard L Lindstrom,

and William J Lahners

17 Postoperative Management Protocols for Uncomplicated

LASIK Procedures 255

Melanie A R Graham and Dimitri T Azar

18 Visual Outcomes After Primary LASIK 265

Samir G Farah and Dimitri T Azar

19 Quality of Vision After LASIK 277

Patrick C Yeh and Dimitri T Azar

20 LASIK for Hyperopia, Hyperopic Astigmatism, and Presbyopia 285

Neal A Sher

21 LASIK Retreatments 297

Ayman F El-Shiaty and Brian S Boxer Wachler

22 LASIK Following Radial Keratotomy and Photorefractive

Keratectomy 313

Natalie A Afshari and Dimitri T Azar

23 LASIK After Penetrating Keratoplasty 319

Glenn C Cockerham and Natalie A Afshari

24 Bioptics: Combined LASIK and Phakic Intraocular Lens Surgery 329

José L Güell, Mercedes Vázquez, Fortino Velasco, and Felicidad Manero

25 LASIK and Intrastromal Corneal Ring Segments (ICRS) 335

Jonathan D Primack, Samir G Farah, and Dimitri T Azar

26 Intraoperative Complications 351

Li Wang, Manjula Misra, and Douglas D Koch

27 Postoperative Complications of LASIK 365

Samir G Farah, Jae Bum Lee, and Dimitri T Azar

28 Optical Aberrations After LASIK 387

Samir A Melki, Cinthia E Proano, and Dimitri T Azar

29 Posterior Segment Complications of LASIK 397

Ron Afshari Adelman and Natalie A Afshari

30 Management of Topographical Irregularities Following LASIK 403

Jeffrey Johnson, Roselyn Jeun, and Dimitri T Azar

31 LASIK and TopoLink for Irregular Astigmatism 421

Michael C Knorz

32 Management of Flap Complications in LASIK 431

Manolette R Roque, Samir A Melki, Dimitri T Azar, and Emily Yeung

33 Management of Interlamellar Epithelium 463

Nan Wang and Douglas D Koch

34 Management of Infections, Inflammation, and Lamellar

Keratitis After LASIK 477

Bilal F Khan, Margaret Chang, Sandeep Jain, Kathryn Colby,

and Dimitri T Azar

35 The Future of LASIK 491

Nan Wang and Douglas D Koch

Dimitri T Azar, M.D. Cornea and Refractive Surgery Service, Massachusetts Eye andEar Infirmary, Schepens Eye Research Institute, and Harvard Medical School, Boston,Massachusetts, U.S.A.

Brian S Boxer Wachler, M.D. Refractive Surgery Service, Jules Stein Eye Institute atUCLA, Los Angeles, California, U.S.A

Richard E Braunstein, M.D. Department of Ophthalmology, Columbia UniversityCollege of Physicians and Surgeons, and Harkness Eye Institute, New York, New York,U.S.A

Margaret Chang Columbia University College of Physicians and Surgeons, New York,New York, U.S.A.

Chun Chen Chen, M.D. Cornea and Refractive Surgery Service, Massachusetts Eye andEar Infirmary, Schepens Eye Research Institute, and Harvard Medical School, Boston,Massachusetts, U.S.A

Marsha C Cheung Massachusetts Eye and Ear Infirmary and Harvard Medical School,Boston, Massachusetts, U.S.A

Glenn C Cockerham, M.D. Department of Surgery, Allegheny Ophthalmology and bital Associates, Pittsburgh, Pennsylvania, U.S.A

Or-Kathryn Colby, M.D., Ph.D. Cornea and Refractive Surgery Service, Department ofOpthalmology, Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute,and Harvard Medical School, Boston, Massachusetts, U.S.A

Elizabeth A Davis, M.D., Ph.D. Department of Ophthalmology, University of nesota, and Minnesota Eye Consultants, Minneapolis, Minnesota, U.S.A

Min-Ayman F El-Shiaty, M.D. Department of Ophthalmology, Faculty of Medicine, CairoUniversity, Cairo, Egypt, and Jules Stein Eye Institute at UCLA, Los Angeles, California,U.S.A

Samir G Farah, M.D. Massachusetts Eye and Ear Infirmary, Boston, Massachusetts,U.S.A

Liane Clamen Glazer, M.D. Massachusetts Eye and Ear Infirmary and Department ofOphthalmology, Harvard Medical School, Boston, Massachusetts, U.S.A

Melanie A R Graham, M.D. Greater Baltimore Medical Center, Baltimore, Maryland,U.S.A

Kenneth A Greenberg, M.D. Department of Ophthalmology, Columbia UniversityCollege of Physicians and Surgeons, New York, New York, U.S.A

José L Güell, M.D., Ph.D. Instituto de Microcirugía Ocular, Barcelona, Spain

David R Hardten, M.D. Department of Ophthalmology, University of Minnesota, andDepartment of Medicine, Minnesota Eye Consultants, Minneapolis, Minnesota, U.S.A

Thanh Hoang-Xuan, M.D. Fondation Ophtalmologique Adolphe de Rothschild, andParis University, Paris, France

Sandeep Jain, M.D. Cornea and Refractive Surgery Service, Massachusetts Eye and EarInfirmary, Schepens Eye Research Institute, and Harvard Medical School, Boston, Mas-sachusetts, U.S.A

Roselyn Jeun, O.D. Massachusetts Eye and Ear Infirmary, Boston, Massachusetts,U.S.A

Jeffrey Johnson, O.D. Massachusetts Eye and Ear Infirmary, and Department of thalmology, Harvard Medical School, Boston, Massachusetts, U.S.A.

Oph-Sandeep Kakaria, M.D. Department of Ophthalmology, Cornell University MedicalCenter, New York, New York, U.S.A

Bilal F Khan, M.D. Massachusetts Eye and Ear Infirmary and Department of mology, Harvard Medical School, Boston, Massachusetts, U.S.A

Ophthal-Michael C Knorz, M.D. University of Heidelberg, Heidelberg, Germany, and BaylorCollege of Medicine, Houston, Texas, U.S.A

Douglas D Koch, M.D. Department of Ophthalmology, Cullen Eye Institute, BaylorCollege of Medicine, Houston, Texas, U.S.A

William J Lahners, M.D. University of South Florida, Tampa, and Center for Sight,Sarasota, Florida, U.S.A

Jae Bum Lee Cornea and Refractive Surgery Service, Massachusetts Eye and Ear mary, Schepens Eye Research Institute, and Harvard Medical School, Boston, Mas-sachusetts, U.S.A

Infir-Richard L Lindstrom, M.D. Department of Ophthalmology, University of Minnesota,and Department of Medicine, Minnesota Eye Consultants, Minneapolis, Minnesota, U.S.A

Paul Chung-Shien Lu, M.D. Department of Ophthalmology, Chang Gung MemorialHospital, Taipei, Taiwan, and Harvard Medical School, Boston, Massachusetts, U.S.A

Naoyuki Maeda, M.D. Departments of Ophthalmology and Medical Robotics and age Sciences, Osaka University Medical School, Osaka, Japan

Im-Felicidad Manero Instituto de Microcirugía Ocular, Barcelona, Spain

Samir A Melki, M.D., Ph.D. Cornea and Refractive Surgery Service, MassachusettsEye and Ear Infirmary, and Boston Cornea Center, Harvard Medical School, Boston, Mas-sachusetts, U.S.A

Manjula Misra, M.D. Cullen Eye Institute, Baylor College of Medicine, Houston,Texas, U.S.A

Ioannis Pallikaris, M.D. Department of Ophthalmology, University of Crete MedicalSchool, and Ophthalmology Clinic, University Hospital of Heraklion, Heraklion, Crete,Greece

Thekla Papadaki, M.D. Refractive Surgery Service, Vardinoyannion Eye Institute ofCrete, University of Crete Medical School, Heraklion, Crete, Greece

Jonathan D Primack, M.D. Massachusetts Eye and Ear Infirmary, Boston, sachusetts, U.S.A

Cinthia E Proano, M.D. Cornea and Refractive Surgery Service, Massachusetts Eyeand Ear Infirmary and Harvard Medical School, Boston, Massachusetts, U.S.A.

Manolette R Roque, M.D. Massachusetts Eye and Ear Infirmary and Department ofOphthalmology, Harvard Medical School, Boston, Massachusetts, U.S.A

Neal A Sher, M.D., F.A.C.S. Department of Ophthalmology, University of MinnesotaMedical School, and Department of Surgery, Phillips Eye Institute, Minneapolis, Min-nesota, U.S.A

Walter J Stark, M.D. Cornea, Cataract and Refractive Services, The Wilmer Eye tute, Johns Hopkins University School of Medicine, Baltimore, Maryland, U.S.A

Insti-Leon Strauss, M.D., Ph.D. The Wilmer Eye Institute, Johns Hopkins University School

of Medicine, Baltimore, Maryland, U.S.A

Rodrigo Torres Massachusetts Eye and Ear Infirmary and Harvard Medical School,Boston, Massachusetts, U.S.A

Mercedes Vázquez, M.D Instituto de Microcirugía Ocular, Barcelona, Spain

Fortino Velasco Instituto de Microcirugía Ocular, Barcelona, Spain

Li Wang, M.D. Cullen Eye Institute, Baylor College of Medicine, Houston, Texas,U.S.A

Nan Wang Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, U.S.A

Marc Winnick, M.D. Department of Ophthalmology, Columbia University College ofPhysicians and Surgeons, and Harkness Eye Institute, New York, New York, U.S.A

Patrick C Yeh Cornea and Refractive Surgery Service, Massachusetts Eye and Ear firmary, Schepens Eye Research Institute, and Harvard Medical School, Boston, Mas-sachusetts, U.S.A

In-Emily Yeung, M.D. Massachusetts Eye and Ear Infirmary, Schepens Eye Research stitute, and Harvard Medical School, Boston, Massachusetts, U.S.A

In-Rengin Yildirim, M.D. Refractive Surgery Department, Cerrahpasa Medical School,University of Istanbul, Istanbul, Turkey

1

Refractive Errors and Their Treatment

LIANE CLAMEN GLAZER

Massachusetts Eye and Ear Infirmary and Harvard Medical School,

Boston, Massachusetts, U.S.A.

DIMITRI T AZAR

Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute,

and Harvard Medical School, Boston, Massachusetts, U.S.A.

A LASIK: DEFINITION AND OVERVIEW

Laser in-situ keratomileusis (LASIK) involves creating a corneal flap so that midstromaltissue can be ablated directly and reshaped with an excimer laser beam (1,2) The proce-dure allows the ophthalmologist to surgically reshape the cornea in an attempt to obviatethe need for corrective lenses (Fig 1.1) LASIK is a modification of Colombian José Bar-raquer’s ingenious innovations In 1949, Barraquer first described his technique, and in

1964 he published clinical results of his attempts to achieve emmetropia by shaving and shaping the cornea (3–5) With Barraquer’s technique of keratomileusis (i.e., carving thecornea), a lamellar button (lenticule) of the patient’s cornea was excised with a manual mi-crokeratome Barraquer then reshaped the lenticule so that the central corneal curvaturewas flattened and the refractive power of the cornea decreased He then replaced the lentic-ule in position, either with or without sutures Barraquer’s specific attempts to correct my-opia were called cryolathe keratomileusis, because they involved freezing and reshapingthe removed lenticule with a cryolathe

re-Troutman and Swinger introduced cryolathe keratomileusis to the United States in

1977 (6) While keratomileusis produced good results when performed by experienced geons, the procedure was technically very difficult and the results were therefore variable(7–11) Innovations to this procedure, however, eventually led to the creation of the morehighly refined procedure of LASIK

sur-The introduction of the “excited dimer” (excimer) 193 nm UV laser allowed for the

development of LASIK The argon–fluoride excimer laser is capable of precise ablation ofcorneal tissue with minimal disruption of adjacent tissue The excimer laser’s effect on thecornea was first studied in animal models in 1983 (12) In 1989, Peyman first used a laser

to remove corneal stroma from a lamellar bed in animals (13) Shortly thereafter, similarattempts were made in human eyes (14,15) This early work supported the theory that insitu keratomileusis was better than surface ablation because it induced less activation andproliferation of stromal keratocytes, thereby avoiding both haze and regression In addition,the excimer laser allowed for more accurate tissue removal, thereby eliminating one of themain deterrents to lamellar surgery (see Chap 2)

The LASIK procedure, in its current refined state, involves increasing the eye’s traocular pressure to at least 65 mmHg with a suction device, and then using a microker-atome to create a corneal flap that is at least 6 mm in diameter and 150 microns thick Thisflap, which allows the Bowman’s-layer epithelial complex to remain intact, is then care-fully lifted to expose the layers of the cornea that will be reshaped by the excimer laser Thesize of the optical zone and the depth and profile of the laser ablation will determine thecorrection achieved (Fig 1.1)

in-B THE EPIDEMIOLOGY OF LASIK

Refractive surgery is a young and rapidly growing field In 1998, 400,000 Americans derwent refractive surgical procedures This represents an increase over 1997 of approxi-mately 100% (16) Despite the many refractive surgeries that have been performed, the tar-get population for laser vision correction at the turn of the century is still an estimated 44million people Currently, approximately 3,000 ophthalmologists have been trained to per-form refractive surgery This number is increasing at a rate of 500 to 1,000 surgeons peryear Industry observers predict that the number of trained surgeons will approach 6,000 to8,000, or approximately 50% of the U.S.-based ophthalmologists (16)

un-LASIK produces good refractive results for low, moderate, and high myopia (seeChap 18) Recent reports for low myopia show postoperative uncorrected visual acuities

of 20/40 or better in 100% of patients, and 20/20 or better in 81% of patients (17)

Potential complications of LASIK include halos, interface inflammation, haze, andregression However, significant complications causing a loss of two or more lines of best

Figure 1.1 Schematic diagram of emmetropia In emmetropia, the far point is optical infinity, and the secondary focal point (F 2 ) is at the retina Parallel rays of light focus on the retina (From Ref 115.)

corrected visual acuity (BCVA) are rare (1,18,19) In addition, with improved tation and increasing surgeon experience, LASIK complication rates continue to fall.

instrumen-C REFRACTIVE ERRORS THAT ARE TREATABLE WITH LASIK

Three components of the eye work together to determine the refractive power of the eye:the shape of the cornea, the power of the lens, and the length of the eye The overall re-fracting power of the eye is approximately 60 diopters: the cornea contributes about 45diopters of power, while the lens provides approximately 15 diopters of refractive power tothe eye When the cornea, lens, and length of the eye combine and produce no refractive er-ror, emmetropia is achieved In the instance of emmetropia, a ray of light parallel to the op-tical axis and limited by the pupil focuses at a point on the retina In other words, the sec-ondary focal point is located on the retina In addition, the “far point” (i.e., the furthest point

at which the eye can see clearly) is optical infinity (Fig 1.2) Because the cornea accountsfor approximately two-thirds of the refractive power of the eye, it is logical that most re-fractive surgeries attempt to change the shape of the cornea Other methods of refractivesurgery include implanting intraocular lenses

Traditionally, four major types of naturally occurring ametropias, or refractive errors,have been described: myopia, hyperopia, astigmatism, and presbyopia Wavefront analysis

of human eyes after correction of these optical abnormalities often show additional larities that have been classified using Zernike polynomials into complex subgroups thatmay be simplified into lower- and higher-order aberrations

irregu-In myopia, the secondary focal point is anterior to the retina irregu-In other words, the fractive power of the eye is greater than that required for emmetropia; parallel rays of lightentering the eye are in focus at a location in the vitreous, rather than on the retina Refrac-tive myopia is due to steep corneal curvature or high lens power Axial myopia is due to aneye that is too long (i.e., longer than 22.6 mm) For every millimeter of axial elongation ofthe globe, there are 3 diopters of myopia When myopia is corrected with spectacles, con-tact lenses, or refractive surgery, the parallel rays of light entering the eye come into focus

re-on the retina, which is cre-onsistent with a far point at infinity The prevalence of physiologicmyopia in the general population is approximately 25% (20) LASIK corrects for myopia

by removing tissue in the center of the cornea, thereby flattening the cornea and decreasingthe refractive power of the eye

A hyperopic eye’s secondary focal point is posterior to the retina In other words, allel rays of light entering the eye come into focus at a location posterior to the retina Re-fractive hyperopia is due to a flat cornea (less corneal refracting power), or a lens withlower power Axial hyperopia is due to a short axial length While hyperopia affects ap-proximately 40% of the adult population, it is clearly less visually significant than myopia(21) This is because accommodation may produce enough additional plus power to bringthe parallel rays of light to focus on the retina Thus young hyperopes may compensate andsee well until their accommodative power weakens and they begin to experience manifesthyperopia in their mid to late 30s LASIK corrects for hyperopia by removing a ring of tis-sue around the center of the cornea, thereby making the cornea steeper

par-Astigmatism refers to a refractive error in which the curvature of the cornea, or lesscommonly the curvature of the lens, varies in different meridians In other words, patientswith astigmatism have two focal lines formed by the convergence of rays of light The firstfocal line, created by the more powerful corneal meridian, is closer to the cornea The sec-ond focal line, formed by the less powerful meridian, is further away Dioptrically midway

between the two focal lines is the circle of least confusion A proper refractive correctionwill place the circle of least confusion on the retina.

While astigmatism is clinically detectable in up to 95% of eyes, astigmatism of lessthan 0.50 diopters rarely requires optical correction (22) However, 10% of the general pop-ulation has naturally occurring astigmatism greater than 1 D Since an astigmatic refractiveerror of 1.00 to 2.00 D can decrease uncorrected vision to the 20/30 to 20/50 level, this de-gree of astigmatism causes an unacceptably poor quality of uncorrected visual acuity(23,24)

Regular astigmatism refers to corneal curvatures that are different but symmetrical,with principal meridians 90 degrees away from each other For regular astigmatism, one

Figure 1.2 Schematic diagrams of myopia and hyperopia In myopia, the far point is in front of the eye (top), and the secondary focal point (F 2 ) is anterior to the retina, in the vitreous (middle) In hyperopia (bottom), the rays of light are in focus at a point behind the retina (From Ref 115.)

can achieve a refractive correction with cylindrical or spherocylindrical lenses LASIK cancorrect regular astigmatism by removing more tissue from the steeper side of the cornea.Irregular astigmatism refers to a condition in which the principal meridians changefrom point to point across the pupil, or in which the amount of astigmatism changes fromone point to another Examples of irregular astigmatism include keratoconus or traumaticcorneal scars LASIK is contraindicated in eyes with irregular astigmatism Often, rigidcontact lenses are the best way to improve the visual acuity for eyes with irregular astig-matism The use of Zernike polynomials to analyze the wavefront in patients with irregu-lar astigmatism has allowed, at least in theory, the incorporation of this information into thelaser treatment algorithm in LASIK and paved the way for custom corneal treatments.Presbyopia refers to the age-related loss of accommodative response Presbyopia typ-ically sets in at approximately 40 years of age The condition results from either a loss oflens elasticity or an anatomic change in the position of the lens equator to the ciliary bodyposition Presbyopia is an important issue to discuss during the informed consent of pa-tients seeking refractive surgery Some physicians give their patient the option of monovi-sion, in which one eye is corrected for near vision and the other for distance vision.

D ETIOLOGY, EPIDEMIOLOGY, AND LASIK ALTERNATIVES

1 Corneal Subtractive Procedures

a Lamellar Procedures

Lamellar refractive keratoplasty refers to the placement of a lenticule on or within thecornea to change its refractive power, typically by altering its anterior curvature As de-scribed above and in Chap 2, Jose Barraquer’s introduction of lamellar refractive surgeryand of cryolathe keratomileusis were followed by Ruiz’ introduction of the concept of insitu keratomileusis (25) He introduced a microkeratome propelled by gears that could cre-ate the corneal flap and allow one to perform intrastromal tissue subtraction When ad-vances in instrumentation allowed the lamellar keratoplasty to be performed more accu-rately, the procedure became known as automated lamellar keratoplasty (ALK) (Figure1.3) The initial results of ALK for myopia showed improvement over previous lamellartechniques Results described postoperative uncorrected visual acuities of 20/40 or better

in 86% of patients, with a 6% loss of two or more lines of best spectacle corrected visualacuity (26,27) However, ALK outcomes were not significantly better than the resultsachieved with contemporary radial keratotomy (RK) techniques (28,29) The accuracy ofALK is limited by the imprecise nature of mechanical cutting

Hyperopic ALK uses a very deep lamellar pass, cutting more than 70% of the stromaldepth (versus the 40% of stromal depth that is used for myopic ALK or for LASIK) The in-traocular pressure causes the thinned cornea to bow forward, and thus the central corneasteepens (Figure 1.4) As one may imagine, this procedure was often unpredictable and oc-casionally led to progressive ectasia Long-term results published in 1998 reported instabil-ity of the postoperative refraction with a progressive myopic shift Even worse, 26% of eyesdeveloped “iatrogenic keratoconus,” and 16.4% required penetrating keratoplasty (30,31).Hyperopic ALK is now considered unsafe due to the risk of progressive corneal ectasia

b Surface Laser Ablation (PRK and LASEK)

Another form of refractive surgery that utilizes corneal subtractive procedures is fractive keratectomy (PRK) PRK is more accurate than ALK because it makes use of the

193 nm argon fluoride (ArF) excimer laser Like LASIK, PRK utilizes the excimer laser toflatten the cornea by ablating the central cornea Unlike LASIK, no corneal flap is made;rather, the central corneal epithelium is simply removed with a spatula or with the laser it-self (Fig 1.5) While there are three distinct techniques for performing excimer laser PRKfor myopia, the most widely used technique applies wide-area surface ablation with a large-diameter beam Most of the published results of PRK are based on PRK performed in thismanner It is primarily in Europe that PRK is performed with the two other techniques:scanning slit and flying spot lasers A modification of PRK, first performed at the Mas-sachusetts Eye and Ear Infirmary by Azar and Abad in 1996, was the use of 20% alcohol

to create an epithelial flap which was repositioned to cover the stromal ablation bed Sincethen, several investigators have started to use this laser epithelial keratomileusis (LASEK)technique, but it is not clear whether LASEK offers any advantages over PRK

PRK has been shown to be safe and efficacious for the treatment of low to moderatemyopia Several national studies of PRK demonstrated that 90 to 100% of patients had20/40 or better uncorrected visual acuity, and 78 to 98% had postoperative refractionswithin 1.0 D of the target outcome (32–34) Refractive results of PRK in patients with morethan 6 D of myopia tend to be highly variable and undergo more regression Another dis-advantage of PRK is a prolonged recovery time: while the corneal epithelium is healing,

Figure 1.3 Myopic automated lamellar keratoplasty (A) A microkeratome is used to create a hinged flap in the cornea (B) A second microkeratome is used to remove a parallel-faced disc of tis- sue from the corneal bed (C) The initial flap is replaced, leaving a flattened central cornea (From Ref 116.)

patients may experience discomfort and blurry vision In addition, potential complicationsinclude postoperative haze, halos, induced astigmatism, diplopia, and keratitis (32,35–37).Although PRK was the first widely accepted laser vision correction procedure, it hasbeen largely supplanted by LASIK as the refractive procedure of choice not just for highmyopia but even for low and moderate myopia This is partly because LASIK providesfaster visual rehabilitation and decreased time for wound healing (38–43).

While LASIK is often the refractive procedure of choice for myopes, PRK may bepreferable to LASIK for patients who are hesitant to undergo incisional surgery, or for pa-tients who have contraindications to LASIK Such contraindications would include corneal

Figure 1.4 Hyperopic automated lamellar keratoplasty (A) A hinged flap, approximately 70% of corneal thickness, is created with a microkeratome (B) Intraocular pressure displaces the very thin posterior layer and overlying flap anteriorly, causing central corneal steepening The original shape

of the cornea is shown with dotted lines (From Ref 116.)

Figure 1.5 Schematic illustration of photorefractive keratectomy The excimer laser is used to move anterior stroma (left), causing corneal flattening (middle) The region of tissue subtraction is designated by the shaded area (From Ref 115.)

re-thinning, in which less than 200 to 250 microns of tissue would be left in the corneal bed,and epithelial anterior basement membrane dystrophy, because this carries the risk of ep-ithelial ingrowth PRK is also competitive with LASIK in the arena of treatment for lowhyperopia A recent review of 65 eyes with
1.00 to
4.00 D of hyperopia documentedthat 92% of the eyes were within 1.00 D of the intended manifest SE at 18 months fol-low-up (44) Other reports support the use of hyperopic PRK (H-PRK) for treating low de-grees of hyperopia (45–48) H-PRK for the treatment of higher degrees of hyperopia is lesspredictable and may result in greater regression (45).

2 Biologic and Synthetic Tissue Addition: Epikeratoplasty and ICRS

One can alter the shape, and thus the refractive index, of the cornea by adding material ther to the surface of the cornea or into the corneal stroma Epikeratoplasty involves adding

ei-a lenticule with power (i.e., cei-arved donor tissue) to the deepitheliei-alized surfei-ace of man’s layer Also called epikeratophakia or lamellar keratoplasty, this technique hasproven problematical First, the refractive results have been disappointing: only 33% ofeyes achieved 20/40 or better uncorrected visual acuity, 15% lost two or more lines of bestcorrected visual acuity (BCVA), and regression was more than 2 D in 17.6% of patients at

Bow-5 months’ follow-up (49–Bow-51) Secondly, 2.Bow-5–3.Bow-5% of patients who underwent this dure had delayed reepithelialization, which in some cases led to infections and graft melt-ing (49,52–55) Almost 8% of grafts had to be removed due to failure of epithelialization,haze, glare, irregular astigmatism, epithelial ingrowth, epithelial interface cysts, melt, in-fection, stromal infiltrates, and wound dehiscence (49,53–54,56–57) After the lenticuleswere removed, many patients were left with central corneal scarring and loss of BCVA(58,59) Because of the disappointing outcomes and unfortunate complications, epikerato-plasty has been abandoned as a technique for the correction of myopia However, epiker-atoplasty is still occasionally used to diminish the myopia and irregular astigmatism of ker-atoconus or other corneal thinning disorders (60) This procedure is also sometimes usedfor the treatment of pediatric and adult aphakia

proce-Keratophakia is a procedure in which a lens is placed within the corneal stroma to ter the cornea’s refractive power While lenses used to be fashioned from donor humancorneas (homoplastic), these lenses had variable outcomes and a high incidence of compli-cations (61) Keratophakic lenses are now fashioned exclusively from synthetic materials(alloplastic)

al-The intrastromal corneal ring segment (ICRS), approved in the United States in April

1999, is a keratophakic technique to correct for low myopia (1.00 to 3.00 D with anastigmatic component of 1.00 D or less) The ICRS consists of two 160 degree polymethylmethacrylate (PMMA) segments placed in two pockets of the peripheral stroma, therebyinducing peripheral steepening and indirectly causing central corneal flattening (Fig 1.6).The procedure is unique in that it retains the potential to be adjusted or reversed

Two-year results of the phase II and phase III FDA trial analyzed 358 patient eyes

At 24 months after surgery, 97% of the eyes had an UCVA of 20/40 or better, 76% were20/20 or better, and 55% were 20/16 or better In addition, 93% were within 1.00 D range,and 73% were within 0.50 D range (62)

Potential disadvantages with the procedure include accidental perforation into the terior chamber, surface perforation of the epithelium anteriorly, and induced astigmatism,which can result from postoperative movement of the intracorneal ring segments (63)

3 Corneal Surface Remodeling: Incisional Surgery, Thermal Surgery, and Orthokeratology

a Incisional Surgery: Radial Keratotomy

Radial keratotomy (RK) is a procedure that treats myopia by making deep and radialcorneal stromal incisions to weaken the peripheral and paracentral cornea and thereby flat-ten the central cornea (Figs 1.7 and 1.8) Experimenting on rabbits’ corneas, Dutch oph-thalmologist Lendert Jan Lans first demonstrated that nonperforating corneal incisions par-allel to the limbus cause peripheral bulging and central flattening (Fig 1.7) (64,65).Japanese ophthalmologist Tsutomu Sato first performed anterior and posterior kerato-tomies to treat myopia after observing that patients with keratoconus who had experiencedbreaks in Descemet’s membrane and hydrops subsequently developed corneal flattening(66–67) Sadly, the role of the corneal endothelium was not understood when Sato was per-forming his keratotomies Later follow-up showed that of 170 eyes that underwent a kera-totomy by Sato’s group, 121 (71%) developed bullous keratopathy, and only 49 eyes (21%)retained clear corneas The average time between the procedure and the onset of edema was

20 years (68) The posterior keratotomy was abandoned for anterior keratotomies once thephysiological importance of the corneal endothelium had been established

In 1960, Svyatoslav Fyodorov of the Soviet Union visited Japan, where Akiyamataught him Sato’s technique of radial keratotomy (Sato himself had recently died) Fyo-dorov improved upon Sato’s techniques by devising improved instrumentation and by de-veloping mathematical formulas to improve the reproducibility of RK (69–70) Fyodorovalso showed that paracentral incisions were more effective than peripheral, scleral inci-sions In addition, he demonstrated that the diameter of the central clear zone is inverselyproportional to the magnitude of refractive correction

In the United States, the first RK was performed by Bores at the Kresge Eye Institute

in November 1978 It was at about this time that the 20 year follow-up data of Sato’s terior keratotomy patients was revealing the 75% incidence of bullous keratopathy Be-cause the newer anterior procedure had scarce follow-up data, the National Eye Institutechose to fund two large nationwide studies to examine RK: the Prospective Evaluation ofRadial Keratotomy (PERK) and the Analysis of Radial Keratotomy

Figure 1.6 Intrastromal corneal ring segments (From Ref 63.)

The PERK study showed that 10 years after surgery, 53% of the patients had 20/20

or better uncorrected vision, 85% had 20/40 or better, and 63% of patients younger than 40years old were spectacle-independent Low myopes did better than moderate myopes: 94%

of patients with low myopia (2.00 to 3.12 D) had an uncorrected visual acuity (UCVA)

of 20/40 or better, while 79% of moderate myopes (3.25 to 4.37) had 20/40 or betterUCVA The 10-year PERK results also revealed a long-term instability of refractive errors:

Figure 1.7 The development of radial and astigmatic keratotomy Illustrations of Lans’s studies showing peripheral corneal bulging (A) and subsequent scarring (B) after astigmatic keratotomy (C) and radial keratotomy (D) (From Ref 117.)

43% of eyes demonstrated a hyperopic shift of one diopter or more (71) Of note, the PERKstudy employed eight-incision surgery (Fig 1.8) In lower myopes, the four oblique inci-sions may be sufficient to achieve the desired result.

The concept of standardized RK was introduced by Casebeer in the early 1990s ing his “cookbook” method, even novice surgeons have been able to perform RK, but thesafety and stability of this approach was controversial, for it was not studied with the samerigor that was used in the PERK study (72–75)

Us-Approximately 250,000 RK operations were performed each year in the UnitedStates in the early 1990s (71–76) There were advantages of RK over more modern forms

of refractive surgery; RK utilized relatively inexpensive equipment, the procedure did notinvolve incisions in the optical zone, and RK had more long-term follow-up data than sev-eral other contemporary refractive procedures In the early days of excimer laser surgery,

RK was generally reserved for patients who had low and moderate myopia (up to 5 D) who

Figure 1.8 Eight-incision radial keratotomy A diagram highlights the optical zone (top) and the central flattening (middle) after performing an RK with eight incisions (bottom) (From Ref 117.)