Customized Laser Vision Correction
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This book addresses customized laser vision correction, an integral management option for the treatment of irregular corneas. This type of treatment reshapes the corneal surface in order to improve both the quality and the quantity of vision by reducing high order aberrations. Beginning with an introduction to the basics of this science, each type of customized laser vision correction is discussed in a clear and didactic format for rapid attainment of information. Throughout this practical clinical guide, examples are supported with the most recent scientific material and a step-by-step systematic methodology is included to fit all levels of ophthalmologists.
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Customized Laser Vision Correction - Mazen M. Sinjab
Editors
Mazen M. Sinjab and Arthur B. Cummings
Customized Laser Vision Correction
../images/437781_1_En_BookFrontmatter_Figa_HTML.pngEditors
Mazen M. Sinjab
Damascus University, Damascus, Syria
Arthur B. Cummings
Wellington Eye Clinic, Dublin, Ireland
ISBN 978-3-319-72262-7e-ISBN 978-3-319-72263-4
https://doi.org/10.1007/978-3-319-72263-4
Library of Congress Control Number: 2018942221
© Springer International Publishing AG, part of Springer Nature 2018
This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed.
The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.
The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This Springer imprint is published by the registered company Springer International Publishing AG part of Springer Nature.
The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland
Foreword
Laser refractive surgery has come of age. Refractive surgery delivers benefits across many dimensions—productivity, safety, convenience, lifestyle, economics and quality of life—and the impact of refractive surgery on the human experience can hardly be overstated. Several professions, ranging from first responders to athletes to military and television personalities, have adopted refractive surgery as a standard. Tens of millions of people have been treated. The elegance and precision of modern refractive surgery technologies are amazing.
Routine laser vision correction to treat refractive errors makes up the lion’s share of corneal laser refractive surgery. Yet there are many eyes that may benefit from customized treatments that go beyond simply improving the refractive outcome. Sometimes these treatments are done to attempt optimizing optical performance; other times they are performed to eliminate irregularities in the corneal surface, and in some cases, customized treatments are performed to provide added depth of focus for presbyopia.
Customized laser vision correction has an interesting history starting with topography-guided treatments using the Bausch and Lomb Keracor 117 excimer laser to treat corneal irregularities in the early 1990s. Whole-eye aberrometer-guided treatments came into clinical use with the WaveLight laser platform, the Visx platform and the Bausch and Lomb Zy-wave treatments in the early 2000s. Early claims of achieving super-vision
with aberrometer-guided treatments quickly gave way to the recognition that the main benefit of these treatments was that they generally induced less spherical aberration due to improved optical designs. A significant contribution to the aberrometry-guided technologies was to improve ablation profiles in the form of wavefront-optimized
treatments with the WaveLight and other platforms, which have performed well and have stood the test of time.
Over the past decade, there is a trend for most laser platforms to migrate towards topography-guided treatments. Nidek, Schwind, Zeiss and Alcon WaveLight all have commercial platforms in current use for topography-guided treatments. When used for primary treatments, topography-guided treatments are most commonly used to reduce coma resulting from corneal asymmetry. When used for therapeutic treatments, topography-guided treatments are used to improve optics after prior surgery, or to treat pathology such as keratoconus.
Customized laser vision correction seems simple in concept—just regularize the cornea and leave it with a final curvature that will deliver the desired refractive outcome. In practice, the goal of simultaneously improving corneal shape while achieving reliable refractive outcomes has been elusive.
There are many considerations and these treatments can be complex. There are several steps involved in customized laser vision treatments. Challenges exist at nearly every step. Designing customized treatments requires understanding of the diagnostic equipment, potential artefacts, corneal physiology, depth limits, optics and laser parameters, placing a significant burden on the surgeon during surgical planning. Technologies that support customized treatments are still evolving and have not yet been fully automated.
Refractive surgery represents a turning point in the human experience; it provides the first example where a congenital defect of fundamental importance can be corrected on a mass scale. The past decades have seen refractive surgery evolve from concept into practice, with improvements in predictability, safety, scope and impact. The next era will see refractive surgery proliferate and assume the role as primary care for vision correction. Challenges exist—affordability, delivery systems, personnel, acceptance and others—yet each of these challenges will be met as the field scales to meet the demand. The question is not if, but when.
To reach full adoption, refractive surgery must establish safety levels comparable to the airline industry. In the rare instances where complications occur, customized laser vision correction will provide a key solution.
This book describes the essential concepts behind customized treatments. The evolution of thought in these treatments is a testament to the brilliance, creativity and determination of those who have contributed to the field, with the editors and authors of this book among them. We owe them a debt of gratitude for their ongoing work and commitment to ongoing innovation in refractive surgery.
Guy M. Kezirian
Arizona, USA
Preface
This book with contributions from across the globe by authors who are passionate about refractive surgery and specifically customized LASIK is designed to hopefully ignite your passion, increase your knowledge and understanding and fuel your curiosity. As the saying goes, the more we learn, the less we know. This field is standing on the shoulders of giants and is going to grow more than any of us realize currently. In years to come, refractive surgery may become a rite of passage as do orthodontic braces for misaligned teeth. It is our job to make LVC so safe that it is no longer questioned and so effective that everybody wants it, and we need to make it available to more people. We are immensely grateful to our colleagues who shared their expertise in this book.
When I hear a colleague say that LASIK or PRK is easy and anyone can do it, I am reminded that our patients deserve more. They deserve a surgeon who takes this very seriously indeed. A surgeon who knows that they have good vision with their spectacles and realizes that this is an area where surgical complications are simply not tolerated. If you are not nervous doing a refractive procedure, including something as controlled as LASIK, you are not taking it seriously enough. We are treating people who have healthy eyes and who have other options. If we decide that laser vision correction is the best option for them, we had better do the very best job that we can.
There are many things that I am grateful for: my wife and my sons, my late parents and my immediate family and friends. I’m grateful for good health. Among all the other things in my life that I am grateful for is the fact that I am an ophthalmologist by profession. Even more so, I am grateful that I got into the area of refractive surgery. As ophthalmologists, we have a wonderful opportunity to improve people’s lives daily. Restoring sight, preserving sight and, for refractive surgeons, correcting sight.
Customized Laser Vision Correction underlines the fact that we now have tools to improve vision to beyond what nature gave us, even with the help of glasses and contact lenses. It has also given us the tools to improve on outcomes where things did not go perfectly well with vision correction surgery and restore the quality of vision once more. I hope that you enjoy this book as much as we enjoyed writing and editing it. I hope that you learn as much as we did too in the process.
Arthur B. Cummings
Dublin, Ireland
Making a Difference
In our life, there is always a difference: a difference between being beautiful and being captivating and between being good and being outstanding. That is simply the difference between science and art; however, joining both is mastery.
Correcting vision is a science but drawing vision is an art. Amongst options of vision correction, laser vision correction (LVC) is the most popular. Over the last few years, laser ablation profiles were developed to achieve very good vision, but this is not the mastery today. The mastery today is how to treat corneal irregularities and higher order aberrations (HOAs) to improve the quantity (science) and quality (art) of vision and that is what is known by customized LVC.
Artists look at a scene from different angles and create different dimensions for the scene, and so is customized LVC. There are different subtypes of this type of treatment, and they all aim at reducing corneal irregularities and patient’s symptoms. Corneal wavefront-guided treatments manipulate corneal HOAs. Ocular wavefront-guided treatments manipulate the whole-eye HOAs. Topography-guided treatment and Contoura Vision correction deal with irregularities in terms of corneal elevations. Q-guided treatment deals with corneal asphericity. Raytracing-guided treatment is the latest promising technology that deals with all the previous aspects in addition to eye dimensions and refractive error.
Since I started practising ophthalmology in 1996, I decided to add something to ophthalmology, not only as a physician who is keen to bring the best technology to his patients, but also as a colleague who is keen to bring the best knowledge to his colleagues. This dream became a reality when I published my first book on corneal topography in 2008. I cannot describe how much happiness I felt when I saw my colleagues could read and understand topography accordingly. That motivated me to publish more books about refractive surgery and keratoconus management, and here, I must stop with respect for the support given by my wife and my children for the time they give me, and sure will not forget the virtue of my parents who implanted in my soul tenderness and helping others.
This book is different in many ways. Mainly, it is thanks to the big names of the contributors who are all regarded as global experts in this field. This book is the only book currently available that addresses this topic of customized laser vision correction. It follows a systematic and academic step-by-step methodology. Each subtype is discussed in terms of indications, contraindications, principles of the relevant laser ablation profile and, most important, how to build the laser profile for each case.
We tried to make this book a practical guide in clinical daily practice by drawing scientific guidelines in this art of treatment. We are very grateful to our fellow authors for contributing to this book and sharing their knowledge and experience for the benefit of us physicians and our patients, thereby enhancing our vision and our lives.
Mazen M. Sinjab
Damascus, Syria
Abbreviations
μm
Micrometer (micron)
AB/IS
Asymmetric bowtie inferior steep
AB/SRAX
Asymmetric bowtie with skewed radial axis index
AB/SS
Asymmetric bowtie superior steep
ATR
Against-the-rule
BFE
Best fit ellipsoid
BFS
Best fit sphere
BFTE
Best fit toric ellipsoid
BVD
Back vertex distance
CCT
Central corneal thickness
CDVA
Corrected distance visual acuity
CR
Cycloplegic refraction
CTK
Central toxic keratopathy
CTSP
Corneal thickness spatial profile
Custom-Q
Asphericity-guided
CWF
Corneal wavefront
CWG
Corneal wavefront-guided
CXL
Corneal cross linking
D
Diopter
DEq
Dioptric equivalent
DLK
Diffuse lamellar keratitis
ECD
Ectatic corneal disease
EKR
Equivalent K-reading
Epi-LASIK
Epipolis laser in situ keratomileusis
FDA
Food and drug administration
Femtolasik
Femtosecond laser in situ keratomileusis
FFKC
Forme fruste keratoconus
HOA
High order aberration
I
Inferior
IOL
Intraocular lens
IS
Inferior steep
K 1
Keratometric reading (K-reading) on the flat meridian
K 2
Keratometric reading (K-reading) on the steep meridian
K c
Central K-reading
KC
Keratoconus
KG
Keratoglobus
K max
Maximum K-reading
K ref
Reference K-reading
LASEK
Laser subepithelial keratomileusis
LASIK
Laser in situ keratomileusis
LKP
Lamellar keratoplasty
LOA
Low order aberration
LVC
Laser vision correction
MFIOL
Multifocal intraocular lens
MR
Manifest refraction
MRc
Corrected manifest refraction
MTF
Modulation transfer function
ODP
Objective spherocylindric dioptric power
OWF
Ocular wavefront
OWG
Ocular wavefront-guided
PIOL
Phakic intraocular lens
PKP
Penetrating keratoplasty
PLK
Pellucid-like keratoconus
PMD
Pellucid marginal degeneration
PMT
Post-mydriatic test
PRK
Photorefractive keratectomy
PSF
Point spread function
PTI
Percentage thickness increase
PVA
Potential visual acuity
QS
Quality specification
RGP
Rigid gas permeable
RI
Refractive index
RK
Radial keratotomy
RLE
Refractive lens exchange
RMS
Root mean square
RS
Reference surface
RT
Ray tracing
S
Superior
SA
Spherical aberration
SB
Symmetric bowtie
SB/SRAX
Symmetric bowtie with skewed radial axis index
SBK
Sub-Bowman keratomileusis
SD
Standard deviation
SE
Spherical equivalent
Simk
Simulated K-reading
SimLC
Simultaneous laser correction
SMILE
Small incision lenticule extraction
SR
Strehl ratio
SS
Superior steep
T-CAT
Topographic computer-assisted treatment
TCRP
Total corneal refractive power
TCT
Thinnest corneal thickness
TE TG-PRK
Trans-epithelium topography-guided photorefractive keratectomy
TE-PRK
Trans-epithelium photorefractive keratectomy
TG
Topography-guided
TG-PRK
Topography-guided photorefractive keratectomy
TL
Thinnest location
TMR
Topography-modified refraction
TNP
True net power
TransPRK
Trans-epithelial photorefractive keratectomy
WFG
Wavefront-guided
WFO
Wavefront-optimized
WTR
With-the-rule
Contents
1 Introduction to Astigmatism and Corneal Irregularities 1
Mazen M. Sinjab
2 Introduction to Wavefront Science 65
Mazen M. Sinjab and Arthur B. Cummings
3 Optical Physics of Customized Laser Ablation Profiles 95
Michael Mrochen, Nicole Lemanski and Bojan Pajic
4 Topography-Guided and Contoura™ Laser Vision Correction 115
Arthur B. Cummings
5 Corneal Wavefront-Guided Ablation 167
Shady T. Awwad, Sam Arba Mosquera and Shweetabh Verma
6 Ocular Wavefront-Guided Treatment 185
Mohamed Shafik Shaheen, Ahmed Shalaby Bardan and Hani Ezzeldin
7 Custom Manipulation of Corneal Asphericity (The Q Factor) 207
Fernando Faria-Correia, Renato AmbrósioJr, José Ferreira Mendes and Arthur B. Cummings
8 Ray Tracing Profiles 219
Arthur B. Cummings
© Springer International Publishing AG, part of Springer Nature 2018
Mazen M. Sinjab and Arthur B. Cummings (eds.)Customized Laser Vision Correctionhttps://doi.org/10.1007/978-3-319-72263-4_1
1. Introduction to Astigmatism and Corneal Irregularities
Mazen M. Sinjab¹
(1)
Damascus University, Damascus, Syria
Mazen M. Sinjab
Abstract
A good knowledge of the geometry of the human eye in general and the cornea, is important for customized laser vision correction (CLVC). The difference between optical, visual, pupillary, and achromatic axes, in addition to line of sight, angles kappa, alpha and lambda, is important for understanding the basics of CLVC. The same can be said about corneal dimensions, zones, shape and power.
CLVC aims at improving both quality and quantity of vision by correcting the lower order aberrations (refractive errors) and the higher order aberrations (HOAs). The HOAs are induced by irregularity and asymmetry in the optical system of the eye. To understand the HOAs and their role in the management, definitions, classifications, and etiology of astigmatism, particularly the irregular type, should be understood.
Irregular astigmatism is evaluated subjectively and objectively. The evaluation starts from suspicion and goes through subjective refraction before it ends with ancillary tests, the most important being corneal topography/tomography and aberrometry. The former is essential to confirm the diagnosis, study the tomographic patterns of corneal maps and define ectatic corneal diseases (ECDs).
Objective corneal dioptric power (ODP) is a new concept. It measures the potential power of the cornea in reference to an average K reading of the normal population. This concept is based on understanding the factors affecting corneal power measurement and the types of corneal power maps. Calculating the ODP helps in understanding how the laser ablation profile works.
Keywords
Optical axisVisual axisPupillary axisAchromatic axisLine of sightAngle kappaAngle lambdaAngle alphaAstigmatismTopographyTomographyKeratoconusPellucid marginal degenerationPellucid-like keratoconusKeratoglobusEctasiaForme fruste keratoconusKeratoconus suspectPosterior keratoconusEnantiomorphism
1.1 The Optical System of the Human Eye
The optical system of the human eye is composed of four main non-coaxial optical elements (anterior and posterior corneal and lens surfaces), the pupil, and the retina, which is aplanatic to compensate for the native spherical aberrations and coma through its non-planar geometry [1]. Although, the optical surfaces are aligned almost co-axially, the deviations from a perfect optical alignment results in a range of axes and their inter relationships (Fig. 1.1). This leads us to the following definitions [1]:
../images/437781_1_En_1_Chapter/437781_1_En_1_Fig1_HTML.pngFig. 1.1
Optical surfaces and axes in the human eye
The optical axis: It is the axis containing the center of curvatures of the optical surfaces of the eye. It can be recognized by the Purkinje images I, II, III, and IV namely of the outer corneal surface (I), inner corneal surface (II), anterior surface of the lens (III) and the posterior surface of the lens (IV). If the optical surfaces of the eye were perfectly coaxial, these four images would be coaxial, which is seldom observed.
The visual axis: It is the line connecting the fixation point with the foveola, passing through the two nodal points of the eye, but not necessarily through the pupil center.
The pupillary axis: It is the normal line to the corneal surface that passes through the center of the entrance pupil and the center of curvature of the anterior corneal surface.
The line of sight: It is the ray from the fixation point reaching the foveola via the pupil center.
The achromatic axis: It is defined as the axis joining the pupil center and nodal points.
Angle Alpha: Angle formed at the first nodal point by the eye’s optical and visual axes.
Angle Kappa: Angle between pupillary and visual axes.
Angle Lambda: Angle between pupillary axis and the line of sight.
The refractive power of the human eye emerges mainly from the cornea and the crystal lens. In emmetropia, corneal power ranges from 39 to 48 diopters (D) (average 43.05D) [2], while the power of the crystalline lens is between 15 and 24D (average 19.11) [2]. The refractive media in the human eye are [2]: tear film (n = 1.336), cornea (n = 1.376), aqueous humor (n = 1.336), crystalline lens (n = 1.406), and vitreous humor (1.336); where n is the refractive index of the media measured relatively to air (n = 1.000). The important features determining the dioptric power of these media are the radius of curvature, the refractive index, and the distance between various interfaces.
1.2 Corneal Geometry
The cornea is composed of two surfaces separated by corneal substance. The anterior surface is coated with the tear film, and together form one refractive surface separating air from corneal substance. The posterior surface separates corneal substance from aqueous humor. The shape of both surfaces is defined as: An aspheric prolate, toric, asymmetric conoidal shape. Each of the previous expressions will be explained in detail in the following paragraphs.
1.2.1 Corneal Dimensions
Corneal dimensions include diameters, meridians, radii of curvature, corneal zones, corneal thickness, corneal shape, corneal power, and geometric landmarks.
(a)
Diameters:
The cornea is not a part of a perfect sphere. The sclero-corneal junction (base of the cornea) is an ellipse. The vertical corneal diameter is 10.6 mm on average, whereas the average horizontal corneal diameter is 11.7 mm [3].
(b)
Meridians:
The normal adult cornea has two meridians that are 90° apart. Due to