Download Applications of Wavefront Technologies in Ophthalmology

COVER STORY
Applications of
Wavefront Technologies
in Ophthalmology
Many potential uses lie beyond customized refractive correction.
BY NAOYUKI MAEDA, MD
T
he use of wavefront aberrometry is still in its
infancy; some surgeons rely on these measurements regularly to produce custom refractive
corrections, and others have never performed a
wavefront-guided treatment. We know that wavefront
analyses can uncover useful information regarding the
quality of vision when performed in normal and aged
eyes as well as those with various pathologic conditions.
However, does using this information during refractive
correction translate into optimal outcomes? I would
argue that wavefront-guided treatment is the most powerful clinical application of aberrometry, followed by
aberrometry’s ability to diagnose irregular astigmatism
and to assess optical quality.
BACKGROUND
Aberrometry uses wavefront sensing to measure the
complete refractive status of the eye, including imperfections in the optical system such as defocus, regular astigmatism, and higher-order aberrations (HOAs). A wavefront aberration is the deviation of a wavefront that originates from a measured optical system in comparison
with a reference wavefront from an ideal optical system.
Measuring these aberrations in an eye allows the surgeon
to evaluate its optical quality. In some cases, this information can be used to plan a refractive treatment.
The wavefront era began in 1999 with the development of wavefront-guided treatments.1 Using aberrometry, a customized ablation pattern is produced to correct spherical and cylindrical refractive errors and
HOAs. These customized patterns can also reduce surgically induced astigmatism. Seiler1 and McDonald2
were among the first to perform wavefront-guided
LASIK. From these and other reports, it has been con-
TAKE-HOME MESSAGE
• A wavefront aberration is the deviation of a wavefront that
originates from a measured optical system in comparison
with a reference wavefront from an ideal optical system.
• Customized ablations correct irregular astigmatism,
refractive errors, and HOAs as well as reduce surgically
induced astigmatism.
• The combination of corneal topography and wavefront
aberrometry helps to determine where the astigmatism
originates.
cluded that wavefront-guided LASIK is safe and effective for primary myopia or myopic astigmatism; its use
enhances refractive accuracy and postoperative UCVA
compared with conventional LASIK treatments.
HOAs are not completely eliminated, however. For
this reason, other factors must be introduced to
improve the results we have seen thus far in the era of
wavefront-guided treatments, including errors of wavefront registration between measurement and treatment, unpredictable changes of corneal shape due to
wound healing and/or biomechanics, fluctuation of
HOAs, and fluctuation of the beam profile. To date, the
acceptance of wavefront-guided treatments may have
been stunted compared with its potential applications
in refractive surgery.
UNDERSTAND THE CHARACTERISTICS
Surgeons must understand the characteristics of the
wavefront measurements they are taking. Because the
HOAs of the eye can be expressed as total root mean
square error, we can determine a set of coefficients for
FEBRUARY 2011 CATARACT & REFRACTIVE SURGERY TODAY EUROPE 45
COVER STORY
the Zernike terms, Strehl ratio, point spread functions, modulation transfer functions, and other metrics to estimate the deterioration in the quality of
vision.
Wavefront aberrometry can also create simulations of
retinal images, allowing us to identify some symptoms
associated with irregular astigmatism. Although we can
use corneal topography effectively to diagnose irregular
astigmatism in the cornea, when we combine this
modality with wavefront aberrometry we can determine whether the astigmatism originates in the cornea,
internally, or both.
QUESTIONS ANSWERED BY ABERROMETRY
Wavefront aberrometry has helped us to uncover the
answers to several questions regarding the effects of
aging, the characteristics of keratoconus and pellucid
marginal degeneration (PMD), and the outcomes of
refractive surgery.
Aging. Aberrometry has been used to show that the
degree of HOAs in a normal eye correlates with age and
is consistent with the age-related decrease of contrast
sensitivity.3 Additionally, research has shown that the lens
in older eyes does not compensate for corneal aberrations as it does in younger eyes.4-6
Keratoconus. The characteristic finding in eyes with
keratoconus is an increase in vertical coma due to
corneal inferior steepening.7,8 These eyes tend to have
a reverse coma pattern, in which vertical coma is
prominent.9 Other findings with aberrometry in keratoconus include heightened levels of trefoil, tetrafoil,
and secondary astigmatism. HOAs can be used to
detect keratoconus10,11 and to grade the severity of
keratoconus.12
PMD. Like keratoconus, PMD is categorized as a noninflammatory corneal thinning disorder; however, it has a
unique HOA pattern.13 The mean axes of coma are the
reverse of those in normal eyes, as with keratoconus, but
their magnitude is weaker, and the mean axis of trefoil is
the opposite of keratoconus. In eyes with PMD, wavefront aberrometry shows a gradual increase in coma-like
aberrations and a stable increase in spherical
aberrations.14
Refractive surgery. In multiple studies using wavefront aberrometry,15-20 refractive surgery increased
total HOAs in small and large pupils, with the most
noticeable increase occurring in large pupils. Another
finding was that refractive surgery tends to shift the
type of aberration from coma to spherical aberration;
this shift correlates with the amount of refractive correction.21 Because there is a strong correlation
between visual symptoms and ocular aberrations as
46 CATARACT & REFRACTIVE SURGERY TODAY EUROPE FEBRUARY 2011
confirmed with aberrometry, we can use aberrometry
to determine the cause of LASIK-induced
aberrations.22
CONCLUSION
Wavefront aberrometry has multiple applications in the
clinical ophthalmic setting, but in most cases it is used for
planning wavefront-guided refractive surgery. Although
there is room for improvement in the wavefront sensors
available today, wavefront-guided treatments produce reliable results. In the future, it is our hope that wavefront aberrometry will also be able to perform serial measurements
and to measure aberrations in eyes with severe astigmatism.
In the meantime, this technology has already contributed to
our ability to diagnose and treat many ocular diseases,
including refractive errors. ■
Naoyuki Maeda, MD, practices in the Department of
Ophthalmology, Osaka University Medical School, Japan.
Dr. Maeda states that he has received a research grant
from Topcon Corp. He may be reached at e-mail: [email protected].
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