Ophthalmology Yanoff - C. 4.2 Corneal Topography and Wavefront Imaging

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CORNEAL TOPOGRAPHY
Corneal topography is a method of measuring and quantifying the
shape and the curvature of the corneal surface. The standard corneal
topographer consists of three components: a placido disc made up of
multiple circles which can be projected onto the corneal surface, a video
camera capable of capturing the reflected image of these rings, and a
computer with software to digitize the resultant captured images.1–3
The digitized image is broken down to individual points around each
circle. The distance of every point is measured from the center of the
placido disc image (Fig. 4-2-1). Each point is assigned a position in
space and a corneal power relative to a predetermined reference axis.
The two most common methods of display are called the axial solution
(most commonly used) and the instantaneous radius of curvature
(tends to accurately highlight peripheral corneal changes). The result-
ing data are displayed as a corneal curvature map, consisting of colors
corresponding to corneal power and curvature. Steep contours are dis-
played as warm colors (e.g., red), while flat contours correspond to cool
colors (e.g., green, blue).
Both absolute and normalized maps can be displayed. Absolute
maps always assign the same color to the same power, and normalized
maps take into account the range of power over a given cornea, ascrib-
ing red and yellow colors to the steepest contours and blue and green
colors to the flattest contours for that particular cornea. When inter-
preting maps, it is important to be cognizant of the scale used. A
standard way to present the power of the corneal surface, suggested by
Klyce et al., is with the axial power map solution using a scale whose
fixed range (30.0–65.5 D) is broad enough to encompass most varia-
tions in corneal curvature and whose standard contour interval (1.5 D)
will highlight only topographic features of clinical significance.4
Factors that affect the accuracy and reproducibility of corneal topog-
raphy maps include quality of the tear film and ocular surface, the
ability of the patient to maintain fixation, and operator experience in
focusing the images. For instance, a patient with a dry eye will often
have a placido disc image with multiple areas of absent or distorted
rings, making it impossible to accurately interpret the resultant map.
Corneal topography powers displayed are best viewed as estimates
and should not be routinely used in planning cataract surgery for
intraocular lens calculations.5
Peripheral corneal power estimates are
less precise than central measurements.
Corneal topography is used primarily as a screening tool to evaluate
prospective refractive and cataract surgical candidates and a diagnostic
aid in evaluating patients with poor vision following refractive surgery.6–10
Irregular corneas are poor candidates for refractive surgery. Keratoconus
and contact lens warpage are the most common causes of irregular cor-
neas in the screening population. Steep (i.e., red) areas isolated to the
inferior cornea suggest keratoconus. Many topographers come equipped
with programs to alert the clinician when a diagnosis of keratoconus is
likely (Fig. 4-2-2A,B). Postoperative patients with poor vision should
have topography, e.g., irregular ablation profiles, and decentered laser
ablations can be assessed with these devices (Fig. 4-2-3).11
Another area where videokeratography plays an important role is the
evaluation of patients with significant astigmatism.12,13
Topography
images can be helpful in planning interventions such as astigmatic
keratotomy, limbal relaxing incisions, or removal of tight sutures post
keratoplasty (Fig. 4-2-4).
The sensitivity of screening for keratoconus can be enhanced by com-
bining topography and tomography (thickness and elevation) data. The
abnormal shape characteristic of keratoconus or post-LASIK ectasia is
often associated with marked corneal thinning and elevation of the pos-
terior surface of the cornea (Fig. 4-2-5, 4-2-6).14,15
Several commercial
units are available that combine this data, such as the ORBSCAN® and
Visante Omni ®. The Orbscan® makes use of a scanning slit beam to
obtain elevation and corneal thickness data while the Visante Omni
combines topographic data from a placido disc image (Humphrey
Atlas®) with corneal shape and thickness measurements obtained from
the Visante OCT® (Ocular Coherence Tomography) unit.
WAVEFRONT ANALYSIS
This chapter covers some concepts useful in understanding wavefront
analysis. The current technology used in refractive surgery examines
what happens as light interacts with the optical system of the eye.16,17
A wavefront represents a locus of points that connects all the rays of
light emanating from a point source that have the same temporal phase
and optical path length. The optical path length specifies the number
of times a light wave must oscillate in traveling from one point to
another point. If the optical system of the eye is perfect, a point source
of light emanating from the back of the eye will create a locus of points
with the same optical path length that will exit the pupillary plane in
the form of a flat sheet. This represents an unaberrated wavefront.
When the cornea or lens has imperfections, optical aberrations are
Michael J. Taravella, Richard S. Davidson
4.2Corneal Topography and
Wavefront Imaging
SECTION 1 Basic Principles
PART 4 CORNEA AND OCULAR SURFACE DISEASES
Definition: Tools for imaging the cornea.
Key features
■ Critical in the evaluation and management of refractive surgery,
corneal transplants, and cataract surgery patients
■ Useful in the management of astigmatism and corneal ectasia
Fig. 4-2-1 Placido disc image. This image is captured and then digitized for analysis.

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169
Fig. 4-2-2 (A) Keratoconus suspect. Note the bright orange–red area of the map. This represents increased corneal curvature and steepness in the inferior part of the cornea.
(B) Contact lens induced inferior corneal changes mimicking keratoconus. Once contact lens wear is stopped, this type of pattern may“normalize”in weeks to months.
A B
Fig. 4-2-3 Decentered laser ablation pattern. The circular area of blue represents
corneal flattening induced by the laser. The pupil is outlined with a black circle.
Patients with decentered ablations may have poor vision and be bothered by night
vision problems such as glare or ghosting of images.
Fig. 4-2-4 Symmetric bowtie pattern typical of high astigmatism.

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CORNEAANDOCULARSURFACEDISEASES
Fig. 4-2-5 Orbscan quad map of a
patient following LASIK. The postop
maps are typical for ectasia. The posterior
float shows very abnormal elevation of
the posterior corneal surface compared
to a best-fit sphere. The pachymetry map
is notable for corneal thinning in the
same area as seen in the posterior float.
Fig. 4-2-6 Visante Omni® display of a keratoconus
suspect. The top three maps represent the axial power
solution for topography, pachymetry and anterior
corneal elevation. The bottom three maps represent
instantaneous curvature, relative pachymetry, and
posterior curvature elevation. Note how the
instantaneous curvature map highlights steepening
of the peripheral cornea better than the axial solution.
This area of the cornea is also thinner than normal
(relative pachymetry) and is associated with a
corresponding elevation of the posterior cornea.
created, causing the wavefront to exit the eye as curved or bent sheets
of light. The wavefront aberration of the eye is the difference between
the actual wavefront and the ideal flat wavefront.
The surface of the wavefront can be characterized by measuring the
slope of light rays running perpendicular to it from selected points.
This is the basic principle behind the Hartmann–Shack devices com-
monly used to measure the wavefront. Wavefronts exiting the pupil
plane are allowed to interact with a microlenslet array. If the wavefront
is a perfect flat sheet, it will form a perfect lattice of point images cor-
responding to the optical axis of each lenslet (Fig. 4-2-7). If the wave-
front is aberrated, the local slope of the wavefront will be different for
each lenslet and result in a displaced spot on the grid as compared to
the ideal. The displacement in location from the actual spot versus the
ideal represents a measure of the shape of the wavefront.
Once the wavefront image is captured, it can be analyzed. One
method of wavefront analysis and classification is to consider each
wavefront map to be the weighted sum of fundamental shapes. Zernike
and Fourier transforms are polynomial equations that have been adapt-
ed for this purpose. Zernike polynomials have proven especially useful
since they contain radial components and the shape of the wavefront
follows that of the pupil, which is circular. Fourier transforms, however,
may prove to be more robust and allow mathematical description of the
wavefront with less smoothing effect and greater fidelity.18–20
Illustra-
tions of the basic Zernike shapes are shown in Fig. 4-2-8.

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Fig. 4-2-7 Schematic of Hartman–Shack device used
to capture wavefront data. (Courtesy of AMOVISX.)HARTMANN SHACK
outgoing wave
CCD
camera
CCD
image
lenslet
array
Fig. 4-2-8 The fundamental Zernike
shapes. Complex wavefront maps can
be deconstructed to determine the
individual contribution of these shapes.
Spherical aberration is a fourth-order
term and is represented by the central
figure in row 4. Coma is a third-order
term and is represented by the two
central figures in row 3.
Fig. 4-2-9 Spherical abberration.
SPHERICAL ABBERRATION
Fm= marginal focal point
FP= paraxial focal point
Fm FP
The term higher-order optical aberration is replacing the older
term irregular astigmatism as wavefront analysis has become more
accepted. Lower-order aberrations, such as sphere and cylinder, require
lower-order mathematical terms within the polynomial expansion to
characterize them and are commonly referred to as second-order
aberrations. The most important higher-order terms are spherical
aberration (a fourth-order term) and coma (a third-order term).
Fortunately, spherical aberration is relatively easy to understand.
Light rays entering the central area of a lens are bent less and come to
a sharp focus at the focal point of a lens system. Peripheral light rays,
however, tend to be bent more by the edge of a given lens system so that
in a plus lens, the light rays are focused in front of the normal focal
point of the lens and secondary images are created. Many lens systems
incorporate an aspheric grind, so that the periphery of the lens system
gradually tapers and refracts or bends light to a lesser degree than if this
optical adaptation was not included (Fig. 4-2-9).
Unlike corneal topography, which measures the shape of the cornea,
wavefront technology represents a snapshot of the entire optical system
of the eye. It can be affected by pupil size, vitreous or lens opacities, and
the quality of the tear film.

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Fig. 4-2-10 Normal wavefront map. The left side of the map shows total aberrations
including nearsightedness and astigmatism. The right side represents higher-order
optical aberrations only.
Fig. 4-2-11 Wavefront map of a patient with post-LASIK night vision problems.
The patient complaints are most likely due to spherical aberration.
Wavefront maps are commonly displayed as two-dimensional maps.
The color green indicates minimal wavefront distortion from the ideal,
while blue is characteristic of myopic wavefronts and red is character-
istic of hyperopic wavefront errors. Like topography, wavefront maps
are a two-dimensional attempt to display a complex three-dimensional
shape. A normal wavefront is displayed in Fig. 4-2-10.
The Root Mean Square (or RMS) value is a useful way of quantifying
the wavefront error and comparing it to normal. This number can be
calculated for the wavefront as a whole or as individual components of
the wavefront when it is deconstructed into individual Zernicke terms.
The RMS value is calculated in microns. Although normal values have
not been published for all aberrations, RMS values approaching 1.0 for
individual aberrations are considered abnormal.
Wavefront technology has come to the forefront in laser vision cor-
rection for refractive errors. Traditional, or non-custom laser ablation
patterns tend to induce spherical aberration; the higher the degree of
correction, the greater the induction of this optical error (Fig. 4-2-11).
During the day, the pupil size tends to limit the effect of spherical
aberration, since peripheral light rays are blocked. At night, as the pupil
enlarges in dark or scotopic conditions, these light rays enter the eye
and can create a blurred focal point and secondary images. Such aber-
rations are thought to be one of the leading causes of night vision
symptoms and poor quality vision in low light, when the pupil is
dilated.21–24
Custom laser treatments incorporate a specific wavefront-designed
algorithm to help limit the induction of spherical aberration (see
Chapter 3.6).25,26
SUMMARY
In summary, corneal topography and wavefront analysis are important
tools for the ophthalmologist involved in cornea and lens-based refrac-
tive surgery. A basic understanding of both modalities is essential for
screening patients, avoiding serious complications, and maximizing
outcomes.27,28
KEY REFERENCES
Cairns G, McGhee, Charles NJ. Orbscan computerized topography: Attributes, applications,
and limitations. J Cataract Refract Surg 2005;31:206–20.
Klyce SD. Wavefront analysis. Focal Points 2005;23(10).
Maguire, LJ. Computerized corneal analysis. Focal Points 1996;14(5).
McCormick GJ, Porter J, Cox IG, et al. Higher-order aberrations in eyes with irregular corneas after
laser refractive surgery. Ophthalmology 2005;112:1699–709.
Petrauskas J. Reinventing refraction with wavefront technology. EyeNet 2000;4:33–5.
Pop M, Payette Y. Risk factors for night vision complaints after LASIK for myopia. Ophthalmology
2004;111:3–10.
Rabinowitz YS, McDonnell PJ. Computer-assisted corneal topography in keratoconus.
Refract Corneal Surg 1989;5:400.
Smolek MK, Klyce SD, Hovis JK. The Universal Standard Scale: proposed improvements to the
American National Standards (ANSI) scale for corneal topography.
Ophthalmology 2002;109:361–9.
Wilson SE, Klyce SD. Screening for corneal topographic abnormalities before refractive surgery.
Ophthalmology 1994;101:147–52.
Wilson SE, Lin DT, Klyce SD, et al. Topographic changes in contact lens-induced corneal warpage.
Ophthalmology 1990;97:734.
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REFERENCES
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