prescribing for astigmatism

Guarding Against RGP Toric Rotation

BY THOMAS G. QUINN, O.D., M.S.
AUGUST 1999

In my last column (June 1999), I explained that, with a toric RGP lens, if the difference in base curves (in diopters) is equal to the difference in lens power, then it is considered a spherical power effect (SPE) design toric lens. The advantage of this design is that the lens can rotate without adversely affecting vision.

On the other hand, if the difference in base curves does not equal the difference in lens power between the two meridians, then it is a cylinder power effect (CPE) design lens. It's important to be able to identify these designs so that you can take precautions to ensure that the lens does not rotate. Such rotation would lead to visual compromise and a spherocylinder overrefraction (SCOR) with an axis oblique to the major meridians of the cornea.

Tips to Avoid Rotation

In order to guard against lens rotation, there are two precautions you will want to take:

Tip #1 -- First, make sure your final design has at least 2.00D of toricity in the base curve of the lens. Anything less than this may be too spherical to permit the lens to "lock" into a rotationally stable position. This is generally only an issue when fitting a cornea with around 2.00D of toricity.

Fitting the horizontal meridian nearly aligned with the cornea while flattening the fit in the vertical meridian helps to promote good lens movement and an efficient tear pump. However, on a with-the-rule (WTR) cornea, flattening the lens in the vertical (steeper) meridian will result in a lower degree of toricity in the base curve of the lens.

If this fitting approach causes the toricity of a CPE lens to drop below 2.00D, steepen the fitting approach vertically so that the contact lens in this direction is fit near alignment, as in the horizontal meridian. This increased toricity should enhance rotational stability, but it may also inhibit tear exchange during the blink.

Observe lens movement on the patient's eye and carefully inspect their cornea after they remove the contact lens to ensure that the physiological needs of the cornea are being met.

Tip #2 -- The second precaution to take is to make sure that your CPE contact lens has at least two-thirds of the cornea's toricity in the base curve of the lens. For example, if a CPE toric lens is being fit to a cornea with 6.00D of toricity, then make sure that the toricity in the base curve of the lens is at least 4.00D. This approach allows the lens to position on the eye much like a saddle positions on a horse, thus limiting rotation.

As review, we have two types of toric RGPs -- SPE designs and CPE designs. SPEs can rotate, CPEs cannot. Simple enough, right?

Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.