Correction of Optical Aberration in Keratoconus with Custom
Wavefront-guided Soft Contact Lenses
Introduction to the wavefront-guided soft contact lens project: The Visual Optics
Institute (VOI) at The University of Houston, College of Optometry (UHCO) is currently
engaged in research to develop a novel contact lens technology for use in individuals with
highly aberrated optics of the eye. The subset of this population that is currently under
study at UHCO is comprised of individuals suffering from the eye disease keratoconus.
The contact lenses under development are known as wavefront-guided soft contact
lenses, or WGSLs, and these lenses share many of the same characteristics as
conventional soft contact lenses (SCLs) that are currently available in the clinic. Most
importantly, both WGSLs and SCLs have, as a goal, correction of optical error of the eye,
which improves the quality of the image formed on the retina. However, WGSLs differ
from SCLs in one important way: they give the researcher the ability to objectively
target a more complete set of optical defects for correction than is possible with SCLs.
This additional capability allows the researchers to customize the lens to the individual
needs of an individual patient, an important property when dealing with a progressive and
highly individual eye disease like keratoconus. As a first step in describing this research,
the clinical problem commonly encountered by keratoconus subjects is described. This is
followed by a description of the relevant technologies that this research brings to bear on
the problem, and is concluded with a summary of the current state of this research.
Why is it that glasses and SCLs don’t always work for patients with keratoconus?
When visiting the optometrist or ophthalmologist to get an optical prescription to correct
their vision, a patient will often encounter the process of refraction. This technique
involves the clinician asking the patient to make a judgment (which is better, one or
two?) about the clarity of their vision when looking through a series of lenses. The
choices that the patient makes guide the clinician in identifying an optical prescription.
This prescription is designed to compensate for two types of refractive errors: defocus
errors and astigmatic errors (or sphero-cylindrical errors). But what happens if the
optical errors experienced by the eye are not predominantly defocus or astigmatism?
Will this technique still work? In many instances, individuals with keratoconus do not
achieve excellent visual performance with spectacles or SCLs. The simulated retinal
image of a letter chart for a particular keratoconus subject wearing spectacles is shown in
Figure 1.
Figure 1: Simulation of the
retinal image of a letter chart
for an individual with
keratoconus with perfect
defocus and astigmatism
correction. As can be seen,
the acuity chart remains
blurred due to the presence
of non sphero-cylindrical
defects.
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This deficit in image quality and resulting visual performance occurs because the changes
in corneal shape that accompany keratoconus induce refractive errors (or aberrations)
which traditional spectacles and SCLs are simply not designed to correct. In other words,
keratoconus induces optical errors in addition to defocus and astigmatism. So, even
when defocus and astigmatism in the keratoconic eye are well corrected, these “other
aberrations” remain uncorrected and can lead to blurred vision.
What are these “other aberrations”: In order to quantitatively understand these other
aberrations, it is important to adopt a language to discuss them. The use of the Zernike
polynomial has become increasingly common in the ophthalmic community and has been
adopted as a standard for describing ocular aberration (ANSI Z80.28). Its utility can be
seen in the fact that the Zernike polynomial can describe not only defocus and
astigmatism, but these “other aberrations” as well. For the purposes of this discussion, it
is sufficient to know a few key concepts related to the Zernike polynomial. As seen in
Figure 2 the Zernike polynomial is often used to define aberration over the pupil.
Figure 2: The Zernike
polynomial is commonly
used to describe aberration
over the pupil (dark hole)
of the eye. In this figure,
the pupil of the eye is
identified by a yellow
circle inside the iris
(colored part) of the eye.
The fact that the pupil is
round gives wavefront
aberration maps their
characteristic round shape.
These aberration terms can be coarsely grouped into lower order aberrations and higher
order aberrations. Lower order aberrations include the defocus and astigmatism errors
while the “other aberrations” described above are classified as higher order aberrations.
Quantifying ocular aberrations with the Zernike polynomial allows one to talk about a
single aberration (defocus for example) a subset of aberrations (lower order aberrations,
higher order aberrations, etc) a large set of aberrations (66 Zernike terms are often
measured for a given eye) or aberrations for different pupil sizes (data describing 5mm
and 6mm pupils are commonly reported). Figure 3 is an example of how the Zernike
polynomial is used to report optical error. Here the first 15 Zernike terms for a
measurement of an individual with keratoconus are presented.
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Figure 3: A table reporting a subset
of the Zernike terms measured on an
eye with keratoconus. Here, the field
“N” is a unique identifier for the
individual Zernike terms being
reported. Term 4 reports the low
order aberration of defocus. Terms 614 report a subset of the higher order
aberration terms. The field “Coeff
(microns)” reports the amount of that
particular aberration term that is
present in the eye using the units of
microns.
How are higher order aberrations quantified clinically? The development of WGSLs
is, to a large degree, possible due to a relatively new clinical technology that allows for
the quick, objective quantification of the optical defects of the eye. This technology is
known as wavefront sensing. Wavefront sensing is currently used in a therapeutic
manner with state-of-the-art refractive surgery platforms. However, the wavefront sensor
is finding expanded clinical relevance as a diagnostic instrument in evaluating eyes that
suffer from elevated levels of optical distortion, such as keratoconus. Figure 4 shows a
wavefront sensor in the clinical environment. There are a variety of types of wavefront
sensors, with one common type being the Shack-Hartmann wavefront sensor (SHWS).
The wavefront sensor objectively collects information on the optical performance of the
eye that can be used to calculate the amount of individual Zernike terms that are present
(Figure 3). The aberration data can also be displayed in a graphical format (Figure 5
below).
Figure 4: A Shack Hartmann Wavefront
Sensor in a clinical setting. A wavefront
sensor is capable of quickly and
objectively quantifying the optical
properties of an eye. To record data with
this instrument, the patient places their
head in the forehead and chin rest (A).
The instrument operator aligns the eye
under study to the measurement head (B).
After acquiring an image, the data is
processed, stored on a computer (C) and
displayed on the screen (D).
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What kinds of aberrations are present in keratoconus? All eyes, including normal
eyes, have some level of both lower and higher order aberration. And every eye will
have a unique quantity of these individual lower and higher order aberrations. Depending
on the severity of the disease, eyes with keratoconus may be accompanied by elevated
levels of both lower and higher order aberration. The aberration data shown in Figure 3
can be mathematically combined and graphically represented as shown in Figure 5
below. Figure 5 shows two color-coded maps that describe the higher order aberration
measured in a normal eye (Figure 5A) and a keratoconic eye (Figure 5B). A majority of
the map in Figure 5A is green, representing a relatively flat wavefront. However, the
map in Figure 5B displays a much larger variation in color. This variation signifies the
presence of higher order aberration in this individual keratoconic eye in a greater quantity
than the normal eye shown in Figure 5B. The circular nature of the map denotes the
boundary of the measurement, which is defined by the pupil of the eye. The goal of a
WGSL is to correct these optical aberrations.
Figure 5: A wavefront aberration map of
the “other aberrations” or higher order
aberrations of two individual eyes. Figure
5A reports data for a normal eye and Figure
5B reports data for a keratoconic eye. The
keratoconic eye exhibits elevated levels of
higher order aberration (seen as increased
levels of red and blue in the map) as
compared to the normal eye in Figure 5A.
Note that the circular nature of the map
denotes the boundary of the measurement,
which is defined by the pupil of the eye.
How is this wavefront data used to design a wavefront-guided soft contact lens? The
coefficients of the Zernike polynomial, which are derived from measurement data
collected by the wavefront sensor, can be used to derive an optical prescription (or a
correction) for an eye. However, unlike the prescription provided by the process of
refraction described above, the Zernike prescription is more complete, providing both
lower order and higher order aberration data. In the case of keratoconus, the ability to
measure higher order aberrationsallows the wavefront sensor to quantify a more accurate
and complete representation of the optical performance of the eye. This gives the
researcher and contact lens designer a more complete description of the optical properties
of the eye. It is precisely this data that is needed to define an optimal correction for
keratoconus, and it is this data that is used in development of WGSLs.
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Figure 6a shows a simplified cartoon of a wavefront exiting the eye as it would during
wavefront sensor measurement. Light scatters from a point source at the retina (red dot)
and travels back through and out of the eye. If the eye were optically perfect, the tips of
the light rays (arrows) would form a straight line, as is shown by the green line.
However, aberration in the eye leads to distortion in the wavefront, as seen by the red and
blue lines. In this figure, the red portion of the wavefront is in front of, or leading, the
blue portion. The goal of the WGSL is to get all portions of the wavefront lined up, so
that no portion is leading or lagging behind. This is shown in Figure 6b. A threedimensional representation of wavefront maps in this process, defined over the pupil of
the eye, are shown in Figure 7 below.
wavefront error negated by
WGSL
wavefront error measured
by a wavefront sensor
direction of light
propagation
direction of light
propagation
A
Figure 6: Simple schematic demonstrating the concept of a WGSL. This
cartoon shows a picture of how the wavefront sensor would quantify
aberration in the eye. Light scatters from a point source at the retina (red dot)
and travels back through and out of the eye. The WGSL is designed to
account for both the lower order aberrations and higher order aberrations of
the eye. Here Figure 6A shows an uncorrected keratoconic eye suffering from
ocular aberration. The red portion of the wavefront is in front of, or leading,
the blue portion. Figure 6B shows the correction of that eye with a WGSL.
Unlike conventional SCLs, this WGSL is designed to correct both the lower
and higher order aberration present in this eye.
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B
Figure 7: The concept of a WGSL is shown. Figure 7A shows the wavefront
error of the keratoconic eye. Figure 7B shows the compensating wavefront
that is incorporated into the WGSL forming a unique correction for the
aberration structure in Figure 7A. Note that where the ocular wavefront is
red, the correcting wavefront is blue and vice versa. Figure 7A was captured
in a real eye and Figure 7B is a measurement of an actual WGSL. Figure 7C
shows the resulting on-eye performance of the lens, which has reduced the
higher order aberration experienced by the eye.
What is the process involved in designing a WGSL? The process for the design of a
WGSL is shown in Figure 8. It differs from a typical contact lens dispensation algorithm
in that the result is a contact lens tailored to the unique needs of an individual patient, a
WGSL. The algorithm also allows investigators at the VOI to define and ask research
questions regarding WGSLs.
1. Subject
identification
2. Clinical
examination/ev
aluation of
subject
10. Exit
criterion
achieved
9. Optically
profile contact
lens
n
3. Develop
correction
strategy
y
8. On-eye
evaluation of
contact lens
11. Report
results
4. Generate
contact lens
design
y
7. stability
satisfactory
n
6. Sterilize/
evaluate bulk
contact lens
5. Manufacture
contact lens
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Figure 8: Generalized
form of the algorithm
used to design,
manufacture and
evaluate WGSL at the
Visual Optics Institute,
University of Houston,
College of Optometry.
This algorithm
supports the active
study of WGSL design
for keratoconus
subjects.
Summing up: Wavefront sensing is an exciting technology allowing clinicians and
researchers to better understand the optical performance of the eye. Unlike refraction,
wavefront sensing allows the measurement of a full range of refractive errors, not simply
defocus and astigmatism. This becomes important in ocular conditions where higher
order aberrations play a visually significant role. The language of wavefront aberration
affords a way to describe both lower and higher order aberrations. While wavefront
sensors are not universally available in the clinical setting, their numbers are increasing
and will continue to increase as clinicians and scientists find more utility for the
information they produce. Such utility may come in the form of assistance in disease
detection, tracking disease progression, assistance in evaluating a conventional contact
lens correction, gaining better understanding about a patient’s vision and customization
of an optical correction to a patient’s individual needs.
The description above focused on this last application, which is an area of active research
at the VOI. To date, researchers are capable of designing, manufacturing and evaluating
these lenses in the clinical setting. However, challenges exist that currently limit their
effectiveness. First, these lenses are very sensitive to movement, and significant
movements reduce visual performance. In some instances of extreme movement, optical
performance falls to levels where the correction actually INDUCES new aberrations
instead of correcting aberrations. In this case, the correction is useless. Another
challenge exists in the highly custom nature of the lens. Each lens is uniquely
manufactured for a particular eye, making the process time consuming and expensive.
Methods are currently under investigation at The VOI to reduce the complexity of the
design and manufacture process and to make these lenses more successful more of the
time. This latter work is seen as a method to make the lenses more clinically relevant.
These lenses currently remain in the research domain and are not available for
dispensation. The VOI and other research groups believe they do hold future promise for
providing the clinician with an additional method to treat the optical defects of the highly
aberrated eye.
Please direct questions regarding this work to:
Jason D. Marsack
Research Assistant Professor
505 J Davis Armistead Bldg.
University of Houston, College of Optometry
Houston TX 77204
P: 713 743 0661
F: 713 743 2053
jmarsack@optometry.uh.edu
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