contact lens case reports
Correcting Residual Refractive Error Post-Hyperopic LASIK
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
For some patients, myopic LASIK surgery is a reasonable alternative to eyeglasses and contact lenses. For patients with hyperopic refractive errors, there are two ways to make the central cornea steeper to correct the refractive error. One way is to create a bulge or ectasia in the central cornea. The other is to remove tissue from the adjacent midperipheral cornea, resulting in a gully around the central cornea. In hyperopic LASIK the latter is used. The resulting topography is one in which the cornea is steep at its apex and rapidly flattens in the midperiphery.
Our 30-year old male patient underwent bilateral hyperopic LASIK in 2005. His pre-operative refractive error was OD +4.00D and OS +4.50D. Over the years he has experienced a gradual loss of his hyperopic correction and currently his refractive error is OD +3.25D and OS +3.50D.
Figure 1 (top image) shows the corneal profile of his right eye with a high postoperative corneal eccentricity of 0.98. The bottom image shows a normal, unoperated eye with an eccentricity of 0.45. It's clear that the topographical profiles of these two patients' corneas are different from one another and that the post-hyperopic LASIK eye will require a lens design with a significantly higher (flatter) eccentricity.
Figure 1. Corneal profile of the patient's right eye post-hyperopic LASIK (top). Lower image, a normal, unoperated cornea.
Our lens of choice in these situations is the custom KBA design from Precision Technology Services, Ltd. Fitting KBA lenses begins by obtaining topography with the Medmont E300 Corneal Topographer (Precision Technology). Then activate the Medmont lens design software and select a base curve radius to provide 30 microns of apical clearance. The peripheral asphericity is then adjusted to align the midperipheral and peripheral cornea.
The major advantage of the KBA software is the ability to custom design every lens parameter. This is especially true in corneas with excessively high eccentricities. You can adjust andvisualize the various lens parameters in a cross sectional tear profile as well as a simulated fluorescein pattern (Figure 2).
Figure 2. The Medmont lens design software with the simulated fluorescein pattern (left) and the actual fluorescein pattern (right).
Our patient's final lens specifications were OD 7.00mm base curve, 10.2mm diameter, +1.25D, 1.00 eccentricity and OS 6.90mm base curve, 10.2mm diameter, +1.00D, 1.20 eccentricity (Figure 3). CLS
Figure 3. The final right and left custom aspheric KBA lenses.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.