An RGP Surprise and
How to Avoid It

BY IRWIN M. SIEGEL, O.D., PH.D., JOEL SOLOMON, M.D.,
& QUIDO CAPPELLI, B.C.E., P.E.
JAN. 1996

With the emphasis now on thinner RGPs for improved patient comfort, practitioners must be alert to flexure that produces astigmatism.Most clinicians consider a rigid gas permeable lens ideal for correcting moderate to high astigmatism. With this in mind, we prescribed RGPs for a highly astigmatic patient and, with the high tech help of corneal topography, demonstrated flexure.

PATIENT FINDINGS

Our patient is a 50-year-old white male with a history of poor vision in his right eye. He was not wearing a DV spectacle prescription for his right eye, but was refracted to 20/40 with +1.25 = -4.50 x 25. Figure 1 shows the dramatic topography pattern of this eye. The vertical "bowtie" indicates that the meridian of greatest power is close to 90 degrees. His uncorrected visual acuity was 20/20 OS, refracted to +0.75 = -0.50 x 150. Keratometric readings were 45.00D @ 110/40.50D @ 20 OD, and 43.00D (spherical) OS.

FIG. 1: TOPOGRAPHIC MAP OF THE PATIENT'S CORNEA.

The first lens we fit to the right eye was a -1.50D Opticryl 60 lens with a radius of 7.8mm and diameter of 9.5mm. Other dimensions were:

• Center Thickness: 0.12mm
• OZ: 8.4mm
• Secondary Curve/Width: 9.7mm/0.35
• Peripheral Curve/Width: 11.5mm/0.2mm.

The lens seemed to fit well even though we did not attempt to compensate for the large amount of with-the-rule astigmatism (Fig. 2). Best acuity (20/30) required a sphero-cylindrical overcorrection of +1.00 = -4.00 x 20. Contrary to our expectations, the first RGP lens did not neutralize any of the astigmatism.

FIG. 2: AN RGP LENS, CENTER THICKNESS 0.12MM, ON THE PATIENT'S EYE.

CHECK FOR LENS FLEXURE

We took keratometric readings over the contact lens and noted some toricity, but the readings were inconsistent. However, corneal topography with the EyeSys CAS clearly showed the lens was flexing (Fig. 3) and producing at least 2.5 diopters of astigmatism. Readings were somewhat variable depending on how soon after a blink we took our measurements.

FIG. 3 TOPOGRAPHIC MAP FROM THE LENS SURFACE (C.T. = 0.12MM) WHILE WORN BY THE PATIENT; NOTE ASTIGMATIC "BOWTIE."

FIG. 4 TOPOGRAPHIC MAP FROM THE LENS SURFACE (C.T. = 0.17MM) WHILE WORN BY THE PATIENT; ASTIGMATIC ERROR IS NOW NEUTRALIZED.

VARY LENS THICKNESS

We increased the center thickness by 0.05mm to 0.17mm, adjusted the power of the lens to +0.50 and then rechecked our measurements. The patient's visual acuity with the new lens was now 20/25, and keratometry taken from the lens surface showed no toricity. This was more precisely confirmed by a nearly spherical topographic pattern from the lens surface (Fig. 4). However, the final lens was designed with toric peripheral curves (about a two-diopter spread) for better stability.

TOPOGRAPHIC CORRECTION FACTOR

Remember that the color-coded topographic dioptric scales and keratometry data do not represent valid power values. Like all keratometers, all topography instruments assume an average cornea-tear film index of refraction of 1.3375 for calculation purposes. However, when the reflections of the measuring rings are located not on the cornea, but on the surface of a contact lens (soft or rigid), you must use a correction factor.

In the present study, the dioptric correction would simply be the ratio of the indices of refraction of the RGP material and the cornea: 1.48/1.34 = 1.10. For example, a reading of 44.00D taken from the surface of a contact lens underestimates the power by 10 percent; the actual power is 48.40D. The astigmatic "bowties" induced by the flexing, shown in Figures 3 and 4, are actually 10 percent greater than they appear. However, since the purpose of this study is merely to demonstrate the relative astigmatic error induced by lens flexing, absolute dioptric values are not essential.

"RESIDUAL" ASTIGMATISM MAY NOT BE "NON-K"

The lesson in this case is clear. Before we search for sophisticated optical solutions to what appears to be a residual astigmatic error in a patient fit with a rigid lens, consider that the lens might simply be flexing. Using corneal topography, measure the front surface of the lens while it's on the eye. A "bowtie" indicates an induced astigmatic error and calls for a slightly thicker lens. Remember, small changes go a long way here. For increased patient comfort, modern RGP lens materials are now manufactured in thinner designs. But don't let this advantage turn into a disadvantage. CLS

The authors wish to thank Research to Prevent Blindness for an unrestricted grant to the Department of Ophthalmology, N.Y.U. Medical Center.

Dr. Siegel is professor of research ophthalmology at NYU Medical Center, and directs contact lens activities at the Manhattan V.A. and Bellevue hospitals; Dr. Solomon is associate professor of ophthalmology at NYU Medical Center, and chief of eye services at the Manhattan V.A. Medical Center; Quido Cappelli is president of the CLMA and director of Cappelli Ophthalmics, Tuckahoe, N.Y.