topography topics

Enhancing Your RGP Fitting Efficiency

BY LORETTA B. SZCZOTKA, OD, MS
May 2000

Contemporary RGP contact lens fitting modules on new generation corneal topography systems allow you to perform more fittings in less time,
with the same quality and accuracy you achieve now. The goal of fitting RGP lenses with computer-driven artificial intelligence software is to achieve an appropriate base curve-to-cornea fitting relationship, with little to no reliance on diagnostic lenses.

Many practitioners still use simple central keratometry readings to fit RGP lenses. In fact, first-generation topography RGP fitting programs utilized this "keratometric fitting technique," and were limited to applying established manufacturer design recommendations based solely on simulated keratometry values and toricity.

Lens-to-Cornea Relationships

Current contact lens modules utilize a "topographically-driven" fitting method which analyzes the entire corneal shape during initial base curve selection, and calculates suggested posterior lens curves based on a pre-selected tear layer thickness and/or cor-neal eccentricity. Most second-generation programs incorporate peripheral corneal flattening into their lens design algorithms.

With a measure of peripheral corneal flattening or eccentricity in addition to central curvature, you achieve the desired lens-to-cornea relationship much sooner. Corneal topography is the best method of determining the rate of peripheral corneal flattening, which significantly affects the RGP fitting relationship.

Consider two corneas with the same central radius of curvature but different rates of peripheral eccentricity. With a standard keratometric fitting nomogram, the same spherical base curve would have been chosen for both eyes. If fitting on flat K, the eye with rapid peripheral flattening would exhibit a central clearance lens-to-cornea fitting relationship because the sagittal depth of the spherical lens would be greater than that of the aspheric eye.

Conversely, on an eye with minimal peripheral flattening (low eccentricity), a flatter lens-to-cornea relationship would occur because the corneal peripheral shape approaches that of a sphere, and peripheral lens impingement would be minimized. Topography-based fitting nomograms predicated on corneal eccentricity would select different RGP base curves using a pre-selected tear clearance or fitting relationship.

In one study performed on the EyeSys 2000 Pro-Fit software, I fit 22 patients with RGP lenses in a randomized contralateral eye trial. One eye of each patient was fit manually with diagnostic lenses, and the other eye was fit topographically with no trial lens application, although fluorescein pattern manipulation on the computer screen was allowed. Each group had an identical success rate (77 percent), although the topographically fit eyes were completed in less than half the amount of time than the manually fit eyes. This translates to greater efficiency without sacrificing accuracy for experienced RGP fitters, and possibly an enhanced first-time success rate for novice fitters.

Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.