Ординатура / Офтальмология / Английские материалы / Wavefront Customized Visual Correction The Quest for Super Vision II_Krueger, Applegate, MacRae_2003
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xContributors
Michael Mrochen, PhD
IROC AG, Institut für Refractive und Ophthalmo Chirurgie Swiss Federal Institute of Technology, Institute of Biomedical Engineering
Zurich, Switzerland
Zoltan Z. Nagy, MD, PhD
1st Department of Ophthalmology Semmelweis University Budapest, Hungary
Daniel R. Neal, PhD
WaveFront Sciences
Albuquerque, NM
Ioannis Pallikaris, MD, PhD
University of Crete Medical School
Vardinoyannion Eye Institute of Crete
Heraklion, Crete, Greece
University Hospital of Crete
Department of Ophthalmology
Heraklion, Greece
Sophia I. Panagopoulou, PhD
University of Crete Medical School
Vardinoyannion Eye Institute of Crete
Heraklion, Crete, Greece
Seth Pantanelli, BS
Department of Biomedical Engineering
University of Rochester
Rochester, NY
George H. Pettit, MD, PhD
Alcon Orlando Technology Center
Orlando, Fla
Patricia Piers
Department of Applied Research
Pfizer Groningen BV
Groningen, Netherlands
Sotiris Plainis, PhD
Research Associate
Optometry and Neuroscience
Manchester, United Kingdom
Jason Porter, MS
Center for Visual Science
University of Rochester
Rochester, NY
Dan Z. Reinstein, MD, MA (Cantab), FRCSC
London Vision Clinic
St. Thomas’ Hospital
Kings College London
University of London London, United Kingdom
Weill Medical College of Cornell University New York, NY
Centre Hospitalier National d’Ophtalmologie des Quinzes Vingts Paris, France
Austin Roorda, PhD
University of Houston College of Optometry
Houston, Tex
Christian A. Sandstedt, PhD
Calhoun Vision, Inc
Pasadena, Calif
Leisa Schmid, PhD
The Laservision Centre
Southport Qld
Australia
Eckhard Schroeder, PhD
Carl Zeiss Meditec AG
Jena, Germany
Daniel M. Schwartz, MD
University of California
San Francisco Medical School
Department of Ophthalmology
San Francisco, Calif
Jim Schwiegerling, PhD
Department of Ophthalmology and Optical Sciences
University of Arizona
Tucson, Ariz
Theo Seiler, MD, PhD
IROC AG, Institut für Refractive und Ophthalmo Chirurgie Zürich, Switzerland
Mario G. Serrano, MD
Scientific Director
Bogota Laser Refractive Institute
Bogota, Colombia
Steven Slade, MD
Private and Clinical Practice Faculty
University of Texas at Houston
Houston, Tex
Erin D. Stahl
Durrie Vision
University of Kansas Medical Center
Kansas City, Kan
P. Randall Staver, MS
Emory Vision
Atlanta, Ga
R. Doyle Stulting, MD, PhD
Emory Vision
Atlanta, Ga
N.E. Sverker Norrby, PhD
Department of Applied Research
Pfizer Groningen BV
Groningen, Netherlands
Contributors xi
Gustavo E. Tamayo, MD |
Stefan Tuess, Dip Eng |
President |
VisuMed AG |
Bogota Laser Refractive Institute |
Cologne, Germany |
Aruba Laser Refractive Institute |
|
Bogota, Colombia |
Hartmut Vogelsang, PhD |
|
Carl Zeiss Meditec AG |
Natalie Taylor, PhD |
Jena, Germany |
SensoMotoric Instruments GmbH |
|
Teltow, Germany |
Liliana Werner, MD, PhD |
|
John A. Moran Eye Center |
Winfried Teiwes, Dr Ing |
University of Utah |
SensoMotoric Instruments GmbH |
Salt Lake City, Utah |
Teltow, Germany |
|
|
David R. Williams, PhD |
Larry N. Thibos, PhD |
Center for Visual Science |
School of Optometry |
University of Rochester |
Indiana University |
Rochester, NY |
Bloomington, Ind |
|
|
Geunyoung Yoon, PhD |
Keith P. Thompson, MD |
Department of Ophthalmology |
Emory Vision |
University of Rochester |
Atlanta, Ga |
Rochester, NY |
Daniel Topa |
|
WaveFront Sciences |
|
Albuquerque, NM |
|
FOREWORD
Twenty years ago, a few groups across the world started to investigate the application of excimer lasers in ophthalmology.
There was a group in New York, which included Steve Trokel, Dobly Srinivasan, and a young student named Ron Krueger, that did basic research regarding photoablation of the cornea. (As a side note, the term photoablation was not yet coined.) Other groups sprung up in London (John Marshall) and in Berlin (Josef Wollensak and Theo Seiler). Very soon, we started using the excimer laser for therapeutic ablations in 1986 and we commenced pilot trials on photorefractive keratectomy in 1988.
The focus of this photorefractive approach was to correct myopic refractive errors. Based on the refractive experience with radial keratotomy, the expectations of doctors and patients regarding quality of vision were low. The typical criterion of success was that 60% of the operated eyes were within ±1 diopter (D) from emmetropia, and the criterion of safety was that less than 5% of the eyes lost two Snellen lines or more. Patients suffered severe pain during the first days after surgery and it took a month before normal visual acuity was obtained.
The next step in the evolution was to make the operation more tolerable for the patient. Laser in-situ keratomileusis (LASIK) came along, offering a much faster visual rehabilitation within days and significantly less pain after surgery. The early dreams of an extended refractive range of correction (up to -20 D), however, did not come true. Also, the quality of vision was still not a high priority; most of the scientific reports did not even mention glare and low contrast visual acuity.
Human vision has many dimensions, among others are form recognition, movement recognition, and color vision. In the standard ophthalmology practice, only a tiny part of this multidimensional network called vision is examined. High contrast visual acuity tested by means of Snellen charts is the most common method of examination, but it is descriptive only for a small portion of vision, even of form recognition. Patient complaints about deteriorated vision after ablative refractive surgery led us to other alternative examination methods—such as contrast sensitivity testing, low contrast visual acuity, glare testing, and aberrometry—to quantify these complaints. During these evaluations, we learned that we have to individualize the ablation profile in order to not deteriorate vision, especially mesopic vision.
This individualization of ablation is the next step in the evolution of refractive surgery and is the main topic of this book. We have entered a new era in refractive surgery and have to perform much basic research, not only in physiologic optics but also including neuronal processing of optical information. Such basic research involves natural scientists and, therefore, we should not be surprised to find many PhDs among the authors in this book.
A serious bottleneck of any new development in medicine is the education of medical practitioners who are handicapped by the conventional medical education that is based only marginally on natural science and more on a "medicine as an experience" science. This book is an excellent approach to overcoming this problem; it merges basic science and clinical experience.
I read this book with great pleasure (and learned a lot) and wish the reader the same pleasure in understanding the "new" refractive surgery to the benefit of our patients.
Theo Seiler, MD, PhD
Professor
Institut für Refraktive und Ophthalmo-Chirurgie (IROC)
Zurich, Switzerland
INTRODUCTION
In the span of 20 years, excimer refractive surgery has developed from a hotly debated hypothesis based on a handful of animal experiments into a widely accepted technology that has been successfully applied to millions of patients. This success has been due to the many people who devoted extraordinary efforts to understanding the nature of the interaction of far ultraviolet (UV) light with the cornea and to developing the complex technology that has been so successful in correcting human refractive errors. Ronald Krueger has been one of the few people who was and is present for this technical evolution and symbolizes the struggle of the early years and the movement toward maturation, success, and acceptance of the technology. At the outset, Ron, as a medical student, explored the tissue interactions of the high-powered pulsed UV lasers. He helped develop data measuring ablation thresholds and rates and explored the ultra-structure of both the acute lesions and the healed ablated areas in the corneas of rabbit and then monkey eyes.
These experiments gave increasing confidence that tissue could be removed from the cornea with a predictable pattern. The recognition of tissue removal of layers less thick than a quarter of the wavelength of light was strong evidence that this would be a potent technology for the reshaping of the cornea and the controlled modification of the optical properties of the eye. It was the successful ablation of a 3.5 mm series of circles in monkey corneas that drove the investment into commercial development of the instrumentation. However, the barriers to successful expression of the technology appeared formidable. It was evident from considerations of tissue ablation rates that exposure times of 15 seconds to almost 1 minute would be necessary for adequate tissue removal to achieve the bulk of optical corrections. It was also evident that, for this technology to succeed, it would have to be extremely safe with a biological and optical complication rate that was very, very low.
In spite of the difficulties that were immediately apparent, the limitations of existing technology for the correction of refractive errors were widely recognized and alternate solutions were avidly sought. The ametropic patient was frustrated by the technical limitations of his or her spectacles and contact lenses and the ophthalmic surgeon was inhibited by the complication rate, unpredictability, and optical imperfections of existing refractive surgical techniques. The energy generated by this desire by patients for alternate surgical technology and the surgeon's frustration with the status quo became the driving force behind the enormous investment of time, energy, and capital into the development of clinically practical excimer laser systems.
The first commercial prototype designed to reshape the cornea was shown at the academy exhibition floor in 1987 and generated interest, resistance, and disbelief. There was enthusiastic interest in an alternate technology to radial keratotomy, marked resistance to the idea that anyone would touch the center of a normal cornea, and general disbelief in the idea that it could ever be made safe enough to engender wide acceptance.
However, early success led to the development of increasingly accurate excimer laser systems that forced the development of increasingly accurate optical analyzers of the eye. First came marked improvements in corneal topography and a widely distributed clinical instrument evolved from what had been an esoteric laboratory tool present in only a handful of centers. The increasing necessity of precision alignment during the protracted ablation drove the development of precision trackers and alignment technologies. Finally, as the accuracy of the clinical results approached our ability to measure the manifest refraction, improvements in refractive technology were sought. This has resulted in the development of aberrometers using wavefront technology and has produced clinical devices that have become simple to use yet allow precision analysis of the optical details of the eye. This development has been the first major change in refractive technology since the manifest refraction evolved in the mid-19th century.
The importance of this development is reflected in this new book, which covers the optical theories underlying aberration analysis and the practical application of the technology. Raymond Applegate has been a pioneer in optical analysis of the eye and Scott MacRae an avid clinical investigator of the details of excimer laser refractive surgery. They explore in detail the basis of this new technology. The results show that when the sphere and cylinder and also the higher-order symmetrical and asymmetrical aberrations are corrected, not only has the resulting visual acuity improved but, equally important, the quality of vision has also improved. We have seen the virtual disappearance of unwanted optical effects as we have seen an overall improvement in visual acuity. We can anticipate that understanding aberration patterns will also allow development of patterns that will increase the depth of field of the final refraction and do much to ease the discomfort of the presbyopic eye.
Wavefront customized ablation is also a tool that allows us to handle complex and difficult problems that have had no ready surgical solutions in the past. Not only can aberrations be reduced in normal eyes, but the technology allows us to improve vision in injured or previously operated eyes that had been beyond our technical abilities.
Wavefront customized visual correction is one of those rare technical advances that not only makes the technique more effective in terms of quality of vision, but also has increased its safety. It is this latter perception that will drive acceptance of excimer laser surgery to become an alternative to the 1000-year-old technology of spectacles and the 65-year-old technology of contact lenses. The detailing of the refractive measurement of the eye as well as the maturation of the laser technology have gradually eroded the objections of ophthalmic surgeons and have made refractive surgery a legitimate alternative standard of care to spectacles and contact lenses. It is important to understand new technology as we use it clinically. This book brings together—from many diverse sources— all the elements needed to understand and apply this essential new technology.
Stephen Trokel, MD
Professor of Clinical Ophthalmology
Columbia University Medical Center
New York, NY
December 19, 2003
PREFACE
What is this book about? Is this book a second edition to Customized Corneal Ablation: The Quest for Super Vision, which was published 3 years ago, or the second volume in a series of wavefront customization books with new material and new ideas that are nonrepetitive, but additive to the previous volume? In some respects, it is a little of both—a second edition and a second volume. Although some of the chapters are of similar content, the bulk of the material in this book is newer and broader than the material in the first book. Even the title, Wavefront Customized Visual Corrections: The Quest for Super Vision II, talks about a more general overview of customization of vision rather than specific corneal ablation. Much of this broad coverage of the field of wavefront customization is well outlined by the various sections of the book, which expand upon the previous book. Yet in a similar way, it still covers the basic science as well as the clinical science of each section.
In Section I, the first five chapters of the book cover introductory concepts of wavefront customization. These introductory concepts begin with the introduction of customization in the first chapter, followed by a review of where we have been in Chapters 2 and 4. Finally, we outline the concepts of where we are going with customization in Chapters 3 and 5.
In Section II, we deal with the measurement and reporting of wavefront aberrometry. In the Basic Science subsection, a great deal of important information is presented regarding the assessment of optical quality and visual performance in Chapters 6 and 7, as well as new metrics for predicting wavefront aberration impact on vision and improvements in sampling and fitting in Chapter 8 and 9, respectively. The impact of chromatic aberration, aging changes, and other temporal aspects of aberrations are covered in Chapters 10, 11, and 12. Finally, the impact of accommodation dynamics and pupil size is covered in Chapters 13 and 14, respectively.
In the Clinical Science subsection, the commercially available aberrometers are conceptually described in terms of their diagnostic parameters and capabilities. Shack-Hartmann aberrometry, which is the most popular method of recording wavefront error, is covered in Chapters 15 and 16, while retinal imaging aberrometry using Tscherning and ray tracing principles are covered in Chapters 17 and 18, respectively. Finally, the unique methods of retinoscopic double pass aberrometry as well as spatially resolved refractometry are covered in Chapters 19 and 20, with a closing evaluation of the comparative reproducibility of multiple wavefront devices in Chapter 21.
Section III reviews similar concepts to the information in the previous book, however with modification and in-depth expansion based on new information over the past 3 years. The physics and technology requirements of customized corneal ablation, including eye tracking and alignment, are covered in Chapters 22, 23, and 24. Chapter 25 replaces the chapter on corneal biomechanics that appeared in the previous book and covers the important potential limitation of predictability in customized corneal ablation outcomes. Finally, in the Clinical Science subsection, data of the customized ablation platforms and outcomes are reported for each of the laser systems, including Alcon, VISX, Bausch & Lomb, Carl Zeiss Meditec, WaveLight, and Nidek in Chapters 26 through 31, respectively.
Beyond customized corneal ablation, new concepts of customization regarding ocular lenses are covered in Section IV. The four chapters in this section cover the following: biomaterials for wavefront customization in Chapter 32, customized contact lenses in Chapter 33, aspheric profile intraocular lenses in Chapter 34, and laser adjustable intraocular lenses in Chapter 35.
Finally, in Section V, customization extends beyond that which we measure with just an aberrometer. In the Basic Science subsection, corneal topography in customization and synchronizating corneal surface aberrations with total ocular aberrations are covered in Chapters 36 and 37, while vector compensation for wavefront and topographic astigmatism is covered in Chapter 38. In the last three clinically oriented chapters, customized topographic ablation using the VISX CAP method and a surgeon-guided retreatment of irregular astigmatism are addressed in Chapters 39 and 40, with Chapter 41 probing into customized presbyopia correction—a new and exciting development of clinical customization used for expanding the dynamic range of vision correction.
The final section and chapter of the book covers the future of customization, which is a forward look into where we are going in this field. In all, the concepts addressed in this book cover a broad view of the many aspects of customization and reveal a central theme and purpose for pursuing and expanding our knowledge in this area—the quest for super vision. We welcome the reader to participate in this journey with us, as the insights we cover in this book and new concepts revealed in future books are all part of a collective sharing and expansion of knowledge for which not only the authors, but also the readers play an important part.
Ronald R. Krueger, MD, MSE
Raymond A. Applegate, OD, PhD
Scott M. MacRae, MD
Section I
Introduction
Chapter 1
An Introduction to
Wavefront-Guided Visual Correction
Scott M. MacRae, MD; Raymond A. Applegate, OD, PhD; and Ronald R. Krueger, MD, MSE
We live in a time of wondrous change. Customized correction of refractive errors using wavefront technology is revolutionizing the way we think of and treat refractive errors. There are two reasons for this paradigm shift. The first is that wavefront technology has allowed us to treat not only second-order aberrations, sphere and cylinder, but also higher-order aberrations such as coma and spherical aberrations which exist in normal individuals as well as postrefractive surgery patients. There is an important transformation occurring in ophthalmic optics.
We have been correcting second-order aberrations such as myopia, hyperopia, and astigmatism for the past 200 years and are now on the verge of being able to detect and correct higher-order aberrations with laser refractive surgery, contact lenses, and intraocular lenses (IOLs).
The second major shift is perhaps even more fundamental. Until now, eye care practitioners have worked primarily to preserve vision in their patients but have not seen it as their role to enhance vision beyond what nature has designed. With the advent of adaptive optics, it seems possible to correct most of the eye’s aberrations and improve contrast beyond what we viewed as normal previously. For some patients, this is a trivial improvement, but in some cases, visual performance can be improved considerably. Thus, there is a subtle but important shift in our view of ophthalmic optics and the potential of the human visual system. This book explores the intricate shift in thinking that wavefront technology and adaptive optics have introduced to this exciting field.
WAVEFRONT CORRECTION STRATEGY:
NORMALIZATION VS CUSTOMIZATION
Normalization
There are two major strategies taken in correcting higherorder aberration. The first is to take wavefront data from a population, such as the amount of positive spherical aberration in postmyopic laser in-situ keratomileusis (LASIK) eyes, and apply a correction factor that minimizes the positive corneal spherical aberration noted in subsequent treatments. This same strategy is
used when one measures the spherical aberration in preoperative cataract surgery eyes and compensates for this by incorporating negative spherical aberration in the IOLs used to treat this population. This strategy uses information from an average (normalized) population and applies it to the population being treated using wavefront sensing to minimize higher-order aberration. It is not a “customized” correction but a “normalized” correction based on an average in a population of similar age. This strategy tends to be used with radially symmetric higher-order aberration, such as spherical aberration. It is not, however, a customized correction.
Normalization is different from customization because it uses a normalized correction based on an average in the population rather than an individual customized wavefront to determine the treatment.
Customization
The second strategy is to use wavefront measurements to guide higher-order aberration correction for each eye on a “customized” basis. This strategy is being employed by most of the excimer laser companies and surgeons who are attempting to correct lowerand higher-order aberrations using customized ablation.
In reality, laser manufacturers use a combination of these two strategies even in customized ablation. Eyes are measured for spherical aberration preoperatively, but empirical “correction factors” are introduced by laser manufacturers to compensate for the increase in spherical aberration, which is introduced by the laser procedure itself. This combination strategy is employed to optimize the result for each eye.1
WHAT IS CUSTOMIZATION?
Webster’s Dictionary defines customize as “to build, fit, or alter according to individual specifications or needs.”2
We all customize. In a sense, our lives are customized based on our genetics, background, and environment. In a similar way, customized corneal ablation is based on our patients’ underlying genetics, which largely determine their anatomy and in turn their refractive error. Corneal customization is based on our ability to
4Chapter 1
detect significant optical abnormalities or wavefront errors and correct them. This chapter will outline the various ways we as clinicians and surgeons can customize treatment to optimize our patients’ vision while maximizing safety.
ONE SIZE DOES NOT FIT ALL
Customized ablation attempts to optimize the eye’s optical system using a variety of spherical, cylindrical, aspherical, and asymmetrical treatments based on an individual eye’s optics and anatomy, as well as the patient’s needs and preferences. Customization can be used to improve optical quality in normal eyes, as well as eyes with atypical optical aberrations caused by corneal scarring, penetrating keratoplasty, central islands, decentered ablations, lenticular abnormalities, and spherical IOL implants.
Customized correction involves three forms of customization: functional, anatomical, and optical. All three need to be utilized to optimize the patient’s results.
WAYS TO CUSTOMIZE
Vision is a complex process and can be divided into at least two broad subsections: optics and neural processing. Customization of corneal laser treatment to correct the refractive errors of the eye falls under the optics section. Translating the retinal image into a neural precept is neural processing. The focus of this book is on new methods to create an optimal retinal image. Creating an optimal retinal image requires consideration of several interactive factors. These factors can be broken into three classes: 1) functional, 2) anatomical, and 3) optical (Table 1-1).
Functional Factors in Customization
Functional factors require an understanding of the patient’s individual needs and circumstances, including the patient’s age, refraction, occupation, and personal optical requirements (eg, monovision), as well as adaptability. Let us explore two of these areas in more detail.
Age Considerations
Experience has taught us that age is an important consideration when treating patients with laser refractive surgery. A number of studies have shown that myopic patients over the age of 40 to 45 are more susceptible to hyperopic overcorrection.3-5 Further, in our experience, treating younger patients with myopia more aggressively and hyperopia less aggressively than recommended with current age adjusted nomograms results in higher patient satisfaction. The reason is that young eyes generally have a large range of accommodation, so a slight hyperopic overcorrection is not as devastating. Conversely, we tend to treat older patients more aggressively for hyperopia and less aggressively for myopia to leave them emmetropic or slightly myopic since they have limited accommodative amplitudes. On the other hand, older myopic eyes that are overcorrected would be blurred at both distance and near. Further, a slight undercorrection for the older myope can provide functional near vision for many near tasks.
Monovision and Mini-Monovision
In addition to appropriate age adjustments, presbyopic patients may also benefit from monovision, which renders one eye mildly myopic (typically the nondominant eye) by 1 to 1.5 diopters (D), or mini-monovision (0.25 to 0.75 D). There are many factors affecting whether or not monovision is appropriate for a particular patient, including his or her occupational and recreational needs, and perhaps most importantly, motivation. If he or she has never experienced monovision, fitting the patient with disposable contact lenses to simulate monovision is very helpful in guiding him or her in his or her decision for or against monovision.6,7
Anatomical Factors in Customization
Anatomical factors require the consideration of individual anatomical variations of each eye, including the patient’s pupil size in bright and dim light conditions,8-13 and the corneal diameter and thickness prior to surgery. LASIK flap considerations are dependent on ablation design, and corneal thickness limits the amount of refractive error that can be safely treated.
LASIK Flap Considerations
LASIK flaps are often customized depending on ablation design,14-16 thickness,13 and diameter,14 and whether one is treating myopia, hyperopia, or astigmatism. One of the authors prefers a flap size 0.5 to 1 millimeter (mm) larger than the total diameter of the ablation. Since most surgeons prefer to avoid recutting a flap on retreatment, it is preferable to create a larger diameter flap (9.5 mm) when possible (even in myopes) to allow for room to perform a hyperopic retreatment if overcorrection occurs. Studies done on flap thickness and diameter indicate there is considerable variation in flap thickness as well as diameter with commonly used microkeratomes.17-19 For instance, patients with thin corneas and high refractive error require special consideration. These eyes may require the surgeon to use a thinner flap (120 to 160 as opposed to 180 microns [µm]) and may require reducing the ablation optical zone based on the patient’s mesopic pupil size, age, and functional needs. Intraoperative pachymetry may also be more accurate in defining the amount of residual bed after the flap is created with the microkeratome.20 One study evaluated the Chiron 160 microkeratome (Chiron Vision Corp, Claremont, Calif) fixed plate and found that the mean corneal cap thickness measured 124.8 ± 18.5 µm, indicating the corneal flaps were thinner than predicted by the manufacturers’ plate depth measurements.17 Another study showed progressive thinning/thickening of the flap in the direction toward the hinge, as well as variation in flap diameter, depending on the microkeratome used.18
Ablation Diameter Considerations
Hyperopic ablations require larger total diameter ablations (8.5 to 10 mm) compared to myopic ablations (6 to 9.5 mm).15,21 Individuals with higher astigmatism (> 1.5 D) require larger flaps if they are to have round ablation optical zones with minimal aberrations. Ablation optical zones and their transition to normal corneas that encompass the entire pupil under scotopic conditions are preferable to ablation optical zones that do not encompass the pupil under all physiological pupil diameters.8-12,22 Larger astigmatic errors require larger transition zones to ensure a smooth transition and avoid regression.23 Individuals with large amounts of myopic or mixed astigmatism may also benefit from crosscylin-
An Introduction to Wavefront-Guided Visual Correction |
5 |
Table 1-1
Interactive Factors to Consider for Customized Correction
FUNCTIONAL CUSTOMIZATION FACTORS BASED ON THE NEEDS OF THE PATIENT
•Age
•Presbyopia
•Patient’s occupational and recreational needs
•Refraction
•Psychological tolerance
ANATOMICAL FACTORS TO CONSIDER FOR CUSTOMIZED CORRECTION
•Corneal diameter and thickness
•Pupil size (also important for optical customization)
•Anterior chamber depth
•Anterior and posterior lens shape
•Axial length
OPTICAL FACTORS TO CONSIDER FOR CUSTOMIZED CORRECTION
•Customization based on corneal topography
•Customization based on wavefront measurements
–Shack-Hartmann wavefront sensor
–Crosscylinder aberrometer (developed by Howland and Howland)
–Tscherning aberrometer
–Tracey system
–Slit-light bundle
–Spatially resolved refractometer
–Others
der ablation that reduces the amount of cylinder to be corrected in each meridian by 50%. This is because one can reduce tissue removal by 20% to 30%, minimize coupling effects, and perhaps improve the optics of the ablation.24,25 In short, one should consider the ablation design before determining the optimal flap design.
Corneal Thickness Considerations
In corneas that are too thin to safely leave at least a 250-µm bed after LASIK, some surgeons recommend photorefractive keratectomy (PRK), laser epithelial keratomileusis (LASEK), the use of a phakic IOLs, or a combined refractive surgery-IOL procedure.
We anticipate that the ocular anatomy (eg, posterior cornea shape, anterior chamber depth, the anterior and posterior lens shape as well as thickness) will be interactive and relevant factors when determining which optical designs are optimal for each patient.
For instance, a 18 D myope may be a better candidate for a bioptics procedure, which combines a phakic IOL with LASIK, rather than having LASIK alone.26,27 The advent of customized contact lenses opens the possibility that thinner kerataconic corneas may be treated with a customized contact lens, as discussed in Chapter 33. Tamayo and coworkers have attempted to correct some kerataconics with customized ablation using the CAP method described in Chapter 39. It remains to be seen which methods are preferable for this patient group.
Optical Customization
Corneal Topographic-Guided Ablation
Corneal first surface aberrations and/or shape can be calculated from corneal elevation data derived from corneal topography measurements12,28-38 and used along with a standard refraction to design ablative corrections. Using such an approach should reduce aberrations in highly aberrated corneas, but may be detrimental (ie, induce more aberrations) in normal eyes. That is, the potential for visual enhancement beyond the 20/20 level is unknown because formulating the ideal shape for the cornea is not dependent on corneal first surface aberrations alone. Instead, an optimal compensating optic (one that reduces the aberrations of the normal eye) must be designed to negate the aberrations of the whole eye. Corneal topographic-guided ablation has the greatest potential in patients with visual loss known to be related to large corneal topographic abnormalities.
Corneal topography-guided ablation has been attempted on patients with regular and irregular astigmatism, decentered ablations, and central islands.39-41 The results have been encouraging with regular astigmatism and decentered ablations but require refinement with irregular astigmatism.38 The irregular astigmatism group is more challenging but may ultimately benefit more from corneal topographic-guided ablation as the systems become more refined.
In a large study on quality of life,42 it was noted that the second strongest indicator (after visual acuity) of improved quality of life after penetrating keratoplasty was the amount of postoperative astigmatism. Many patients after corneal transplantation