Corneal Topography: Measuring and Modifying the Cornea

194 Pages·1992·9.9 MB·English

by  Steven E. Wilson, Jian-Yi Wang, Stephen D. Klyce (auth.), David J. Schanzlin MD, Jeffrey B. Robin MD (eds.)

Corneal Topography
David J. Schanzlin Jeffrey B. Robin
Editors
Corneal Topography
Measuring and Modifying
the Cornea
With an Introduction by A.E. Reynolds
With 171 Illustrations in 258 Parts, 85 in Full Color
Springer-Verlag
New York Berlin Heidelberg London Paris
Tokyo Hong Kong Barcelona Budapest
David J. Schanzlin, MD Jeffrey B. Robin, MD
Department of Ophthalmology Department of Ophthalmology
St. Louis University University of Illinois
Bethesda Eye Institute College of Medicine at Chicago
St. Louis, MO 63110, USA UIC Eye Center
Chicago, IL 60612, USA
Library of Congress Cataloging-in-Publication Data
Corneal topography: measuring and modifying the cornea / [edited by]
David J. Schanzlin, Jeffrey B. Robin.
p. cm.
Includes bibliographical references.
ISBN-13: 978-1-4612-7659-3
l. Cornea-Measurement. I. Schanzlin, David J. II. Robin.
Jeffrey B.
[DNLM: l. Corn.::a-anatomy & histology. 2. Cornea-surgery. WW
220 C8I38]
RE336.C69 1991
DNLMIDLC
for Library of Congress 91-4876
Printed on acid-free paper.
© 1992 Springer-Verlag New York Inc.
Softcover reprint of the hardcover 1st edition 1992
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ISBN-13: 978-1-4612-7659-3 e-ISBN-13: 978-1-4612-2766-3
DOl: 10.1007/978-1-4612-2766-3
This book is dedicated to our wives, Nancy
Schanzlin and MaryAnn Robin, and to our children,
Meredith, Michael, and Matthew Schanzlin, and
Zachary David and Matthew Joshua Robin.
Introduction: History of
Corneal Measurement
A. E. Reynolds
The cornea provides most of the refractive power of the eye's optical
system. For the last 150 years, ophthalmologists have tried to deter
mine its topographic characteristics. The oldest method of topo
graphic analysis used the mirror quality of the anterior corneal sur
face. Several types of images, including straight lines, squares, and
concentric rings, were reflected off the corneal "mirror" enabling
qualitative assessments of the topography of the anterior corneal
surface.
In the 1820s, the French ophthalmologist Ferdinand Cuignet pro
vided the first description of a method for studying images reflected
off the anterior corneal surface, calling the technique "kerato
scopy." In his system, a light was projected onto a target that was
held in front of the patient's eye. The light, target, patient, and
observer were positioned so that the observer could visualize the
image of the target on the patient's cornea. Distortion of the reflected
image, indicating abnormal corneal topography, could then be quali
tatively interpreted by the observer. There were several problems
with this prototypic technique of keratoscopy. Prime among these
was that it was very difficult to properly align patient, observer,
target, and lighting so that the image of the target was centered on the
patient's visual axis. Additionally, early observers viewed the re
flected image on a one-on-one ratio making it very difficult to see
minor distortions in corneal shape.
In 1880, A. Placido devised a keratoscopy target that is still in use
today. The target was a disk with alternating black and white rings.
The disk had a hole in its center through which the observer could
visualize the patient's cornea. This was crucial for improving target
alignment with the patient's visual axis. Not only could the observer
grossly center the target on the patient's cornea, but the patient was
able to directly align his or her visual axis with the center of the target
disk. If the cornea was grossly spherical, the reflected images (which
in reality were the white rings) would appear circular and concentric.
Gross distortions of corneal topography could be interpreted as devi
ations either in the shape of the rings or their concentricity. Although
Placido's disk certainly ameliorated the problems with target align-
vii
VIII Introduction: History of Corneal Measurement
ment, the observer still had no magnification capabilities and was
thus incapable of detecting small degrees of corneal topographic
distortion.
The issue of magnification was addressed in the late 1880s by E.
Javal, a pioneer in the development of the keratometer as well as an
astute pioneer of corneal topography. He observed that keratoscopy
would be greatly benefited by the development of a method for
diagrammatically or photographically representing the corneal to
pography. He stated in his System of Diseases of the Eye (p. 152):
The keratoscopic image would furnish a complete record both of the corneal
astigmatism and of the decentration of the visual axis. If only it were prac
tical to fix them by instant photography and measure them under the micro
scope. Until this shall have been accomplished, we must depend upon the
ophthalmometer to obtain an approximate idea of the form of the anterior
surface of the cornea. But, whereas the keratoscopic images show the com
plete topography of the portion of the cornea on which it is reflected, a single
measurement made with the ophthalmometer gives information regarding
only two points lying in some particular corneal section.
In order to extend the range of observation beyond the area that his
ophthalmometer covered, Javal attached to the instrument small
disks like Placido's. One of the advantages of this system was that the
ophthalmometer had an eyepiece telescope system that magnified
the observed images. In 1889, Javal further addressed this problem
by placing a large enameled disk behind the arc that carried the
ophthalmometric mires. The disk had a black background and con
centric white circles. The radius of each of these circles represented
the tangent measured from the middle point of the arc that carried the
mires. Javal had his subjects look into the center of the ophthalmo
meter tube; he would then observe the keratoscopic image over the
central portion of the cornea. The subject would then look up, down,
to the left, and to the right; at each point of gaze, the reflected images
would be observed.
A. Gullstrand, realizing the importance of Javal's goal of de
veloping a system for representing the cornea topography, was the
first to apply photography to keratoscopy. Keratoscopic photo
graphs ("keratographs") were used by ophthalmologists much as the
civil engineer uses photogrammetry. Photogrammetry, simply, can
be used to compare the contours of any two objects. For example, to
observe a mountain with many cliffs and steep slopes in its entirety,
all of its sides must be seen at the same time. A small circle is made on
the very apex of the mountain and, at 50-ft intervals, lines are drawn
around the mountain. An airplane flying over the mountain takes a
photograph of the lines drawn around the mountain; this, in fact,
represents a contour map of the mountain. The topography of the
mountain can then be determined by examining the photograph.
Where the contour lines lie close together, the surface is steep; where
they are farther apart, the terrain has a flatter slope.
The cornea is, in many ways, similar to a mountain and its topogra
phy can be determined in the same manner. The anterior corneal
Introduction: History of Corneal Measurement ix
surface is an ideal reflecting surface and the concentric white rings
from the Placido disk can be easily photographed. The rings of the
keratograph are interpreted in the same manner as the lines on the
contour map. The closer the lines, the steeper the surface; the farther
apart the lines, the flatter the surface.
Despite the recognition of Javal and other pioneers that examina
tion of the entire corneal surface was essential to the understanding
of corneal topography, keratoscopy never gained significant popular
ity. The main reason was the purely qualitative nature of the tech
nique. In 1896, following Javal's suggestion, Gullstrand developed a
method of using a "dividing engine," a measuring microscope, to
determine the distance between two points on a keratoscopic photo
graph. The measurement was then converted to the radius of curva
ture. However, the method was very time consuming for the prac
ticing ophthalmologist and not really practical. This is the main
reason that, until quite recently, the assessment of corneal topogra
phy was almost totally confined to keratome try .
In the 20th century, several attempts have been made to develop
methods to quantitatively analyze keratographs. In the late 1940s and
early 1950s, keratographs were quantitated by comparing them to
photographs of spheres of known radius of curvature. The photo
graph of the sphere was cut in half and placed over the keratograph.
If the corresponding rings matched, that portion of the keratograph
had the same radius as the sphere. By using an appropriate number of
standards, most keratographs could be quantitated.
Charles Hendricks, in 1961, developed a device known as compa
rator, in which a keratograph could be analyzed quantitatively. The
comparator uses black rings of known radii of curvature, comparing
them to the corresponding keratograph rings. By projecting a kerato
graph on the screen of the comparator and changing the magnifica
tion of the projected image to superimpose the rings on the compa
rator screen, a radius of curvature for each keratograph ring can be
determined. A similar instrument was developed by International
Diagnostic Instruments in 1972, mainly for the fitting of contact
lenses. This instrument enables the practitioner to determine the
curvature of approximately 43% ofthe corneal area; by comparison,
a keratometer determines only 6%.
Another method of quantifying corneal curvature from kerato
graphs is to use a method similar to that employed in civil engineering
to determine the profile of the earth's surface. Each ring of the
keratograph is assigned a given elevation and the rings are placed one
upon the other. Using an elevation of 0.1 mm, lines are drawn from
each ring to the corresponding elevation. This method can be used to
determine the shape of any meridian of the cornea. By connecting the
ends of each cord, the profile shape of the cornea can be determined.
This method has been helpful for orthokeratology and refractive
surgery. In radial keratotomy, profile studies have determined that
the meridian receiving the first incisions change shape the most,
whereas those receiving the last incisions change least.
Recent developments in contact lenses and corneal surgery have
x Introduction: History of Corneal Measurement
placed even greater emphasis upon knowing the topography of the
entire cornea. Most photokeratoscopes on the market today project
from 8 to 12 concentric rings onto the cornea. These rings can cover
an area from 4 mm to the limbus; the average is a chord of 7 mm to
9 mm. The aspheric contact lens design is based upon the curves in
the intermediate portion of the cornea (rings 3 through 5), the area
just outside that measured by the keratometer. The aspheric portion
ofthe contact lens must clear the intermediate and peripheral (rings 8
through 12) portions of the cornea in order to be worn comfortably.
Additionally, contact lens fitting after radial keratotomy and other
refractive surgical procedures is greatly dependent upon knowing the
topography of the intermediate cornea.
Perhaps the factor most responsible for the resurgence of interest
in corneal topography has been the introduction and development of
refractive surgery. As is thoroughly discussed in this book, many of
the keratorefractive procedures directly involve the intermediate and
peripheral zones of the cornea. Additionally, evaluating the refrac
tive effect of these procedures involves assessing the topography of
the cornea far beyond the central 3 mm covered by the keratometer.
For example, the keratograph can be used to observe the changes
following radial keratotomy. Lines are drawn from the presurgical
keratograph rings to arbitrarily selected elevation points. Using the
postoperative keratograph, similar lines are drawn to the same eleva
tion points, always using the outer edge of the ring. These changes,
although not quantifiable, are easy to observe and graphically illus
trate the alterations in topography induced by this procedure.
Surgeons have realized, however, that simple demonstrations of
corneal topographic changes in corneal and keratorefractive proce
dures do not provide enough information; quantification of the topog
raphy of these frequently complex corneal surfaces is necessary.
With the explosion of computer technology, it has recently become
possible to accurately quantify the curvature of thousands of points
on the corneal surface. The details of and applications for these
sophisticated instruments are thoroughly described in other
chapters.
Because of new contact lens designs, keratorefractive surgical
procedures, and sophisticated technology, there is, of late, a great
deal of interest in corneal topography. As this chapter has demon
strated, this is actually a renaissance. It appears that the dream of E.
Javal is finally being realized.
Contents
Introduction: History of Corneal Measurement.................... Vll
A. E. Reynolds
Contributors. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
Chapter 1 Quantification and Mathematical Analysis of
Photokeratoscopic Images .............................
Steven E. Wilson, Jian-Yi Wang, and
Stephen D. Klyce
Chapter 2 The Computerized Corneal Topographer
(EH-270) .................................................... 11
Sami G. El Hage
Chapter 3 Corneal Topography Using a Projected Grid ...... 25
Joseph W. Warnicki, Paul G. Rehkopf,
Robert C. Arffa, and John C. Stuart
Chapter 4 Normal Corneal Topography .......................... 33
Colman R. Kraff and Jeffrey B. Robin
Chapter 5 Topography of Corneal Disease Processes ........ 39
Colman R. Kraff and Jeffrey B. Robin
Chapter 6 Classification of Corneal Topography
with Videokeratography ................................ 47
George O. Waring III, Sadeer B. Hannush,
Stephen J. Bogan, and Robert K. Maloney
Chapter 7 Corneal Topography in the Diagnosis and
Management of Keratoconus .......................... 75
Patrick J. Caroline and Craig W. Norman
Chapter 8 Corneal Topography Following
Traumatic Lacerations .................................. 95
Timothy T. McMahon
Xl
xii
Chapter 9 Radial Keratotomy and Corneal Topography ..... 105
Joseph F. Fleming
Chapter 10 The Corneascope-Comparator Method of
Hard Contact Lens Fitting ............................. 117
Maureen K. Lundergan
Chapter 11 Corneal Topography in Management of
PK Astigmatism ........................................... 129
Richard F. Beatty and David J. Schanzlin
Chapter 12 The Corneal Modeling System ........................ 145
Peter J. McDonnell
Chapter 13 Photogrammetric Index Method (PIM) System
of Astigmatism Analysis and Its Use in Surgery
for Astigmatism ........................................... 165
Kenneth L. Cohen and Nancy K. Tripoli
Index........................................................................... 181