Prescribing Bitorics With Spherical Ease

BY EDWARD S. BENNETT, O.D., M.S. ED.
MAY 1997

In the early 1980s, a well-known contact lens practitioner wrote that you could successfully fit spherical RGPs for corneal astigmatism as high as five diopters. I agree. Why prescribe a complicated lens design if you can achieve centration with a simpler one?

For high astigmatism, however, decentration with associated symptoms of dryness, fluctuating vision, and lens awareness, in addition to clinical signs such as flexure-induced residual astigmatism and 3 and 9 o'clock staining, result in patient dissatisfaction and failure.

And, even when centration is good, a poor lens-to-cornea alignment can result in radical topography changes and corneal warpage. Therefore, I recommend bitoric lenses when considering RGPs for highly astigmatic patients. These designs should be considered for 2.00D of corneal astigmatism and are essentially mandatory at 2.50D and greater.

BASE CURVE AND POWER

You can determine base curve radii and power empirically with an easy-to-use flowsheet such as the Mandell-Moore Guide. Or, you can use a bitoric diagnostic fitting set. Fitting sets are available with two, three and four diopters of back surface toricity.

Fitting bitorics is identical to fitting spherical designs. Select the diagnostic lens slightly flatter than K. Perform a spherical overrefraction, and add the overrefraction to the diagnostic lens power:

If the patient has significant residual astigmatism, and a spherical overrefraction results in reduced vision, perform a spherocylindrical overrefraction just as you would with a spherical design. In this case, add the overrefraction power to the diagnostic lens power for each meridian:

PERIPHERAL CURVES

For low to moderate cylinder bitoric lenses (2.00D to 3.75D), spherical peripheral curves typically will not compromise the fitting relationship. Simply add one millimeter to the mean value of the base curve radii (BCR) to obtain the secondary curve radius (SCR) and two millimeters to the SCR to obtain the peripheral curve radius (PCR):

For high cylinder bitoric lenses (4.00D or greater), I recommend toric peripheral curves. To obtain the toric SCR, add one millimeter to each base curve radius; for the toric PCR, add two millimeters to each SCR value.

CENTER THICKNESS AND MATERIAL

The center thickness of a bitoric lens is about equal to that of a spherical lens in the more plus power meridian. If the lens to be ordered is -3.00/-6.00D, and the recommended thickness for a spherical lens with a -3.00D power in that material is 0.15mm, then this would also be the bitoric value.

For stability and surface wettability, I recommend a low DK (25 to 50) fluorosilicone acrylate material.

HIGH ASTIGMATISM

When fitting a high astigmat, you may observe a dumbbell-shaped fluorescein pattern. If you have only a 3.00D cylinder bitoric lens, for example, simply steepen the steeper BCR and add a corresponding amount of minus power:

References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #24. Be sure to have a fax number ready.

Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis. He is executive director of the RGP Lens Institute.