Topography-Guided Laser Vision Correction
A preview of an ongoing US clinical trial for topography-guided customized ablations.
Topography-guided customized ablation treatments (T-CAT) are a new treatment modality currently available internationally and under investigation in the United States.1 A phase 3 FDA clinical trial has recently commenced to evaluate the new technology on the Allegretto Wave Eye-Q excimer laser (Alcon Laboratories, Inc.; Fort Worth, TX). What follows is general information about the procedure and preliminary details about the study.
WHAT IT IS
Conventional and wavefront-optimized excimer laser
treatments place a spherocylindrical correction on the
cornea. Wavefront-guided treatments measure aberrations
in the optical system of the eye and change the
corneal curvature to correct them. T-CAT differs from
traditional procedures available in the US because they
use information about corneal topography along with
refractive data to design a treatment plan.
The topography-guided procedure may offer several advantages over wavefront-guided and wavefront-optimized ablations. First, because higher-order aberrations in the young eye typically arise from the cornea, it makes sense to measure and correct the cornea rather than measuring the optical system of the entire eye and then correcting aberrations on cornea. Second, T-CAT may be more accurate and reproducible than wavefront-guided measurements, especially in eyes that are highly aberrated and in most need of correction.2 Third, T-CAT may provide a more accurate correction of peripheral corneal curvature compared to wavefront-guided treatments, which are limited in coverage by the diameter of the pupil (topography-guided corrections are independent of pupil size). For these reasons, topography-guided treatments may provide a better quality of vision in both bright and dim lighting conditions than wavefront-guided treatments. T-CAT may also offer better outcomes than wavefront-guided treatments for individuals with highly aberrated corneas.3
HOW IT MAY BE USED
Topography-guided ablations may be used as a primary
treatment for patients who want to eliminate their need for glasses or contact lenses. This treatment
may also be used as a secondary procedure to correct
eyes that are symptomatic after previous LASIK or
PRK–especially those that have decentered ablations
and small optical zones (see Dr. Lin’s article on page 11).
Topography-guided ablations may also be effective for
the treatment of patients who have undergone previous
radial keratotomy or corneal transplants.
CLINICAL TRIAL
So far, about 20 patients with myopia or hyperopia have
been treated as part of the ongoing FDA T-CAT clinical trial,
and the early postoperative observations have been extremely
encouraging. The sponsor also submitted a plan to
treat patients who had visual symptoms after previous refractive
surgery, but the FDA has so far not approved a protocol
to do so. It is our hope that the FDA will take this
opportunity to offer treatment to this small but important
segment of the population that might benefit from T-CAT.
SUMMARY
The FDA clinical trial will evaluate whether topographyguided
laser vision correction provides a strategy to
improve objective and subjective visual performance in
patients who otherwise would receive conventional, wavefront-
optimized or wavefront-guided treatments. The
approval range is expected to be similar to that in existence
for excimer lasers that are currently marketed in the
US. Many hope that future clinical trials will include
patients who are symptomatic after previous corneal
refractive surgery.
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