Front Surface Toric RGPs

BY THOMAS G. QUINN, O.D., M.S.
MAY 1996

Sometimes there's just no way around it. Fitting a front-surface toric, prism-ballasted gas permeable lens is the right thing to do. How do you know when?

When your patient:

• is a highly hyperopic astigmat for whom a soft lens may not provide adequate oxygen to the cornea;
• is a successful RGP wearer;
• is refractively suited to gas permeable correction in the other eye;
• is a candidate for prism-ballasted bifocal gas permeable lenses;
• is sensitive to soft lens solution systems.

When fitting these lenses, the first and perhaps most important step is to apply a standard spherical, non-flexing gas permeable lens to the eye. Sometimes the K readings don't give the true corneal shape and you may be surprised to find no significant residual cylinder with overrefraction. A simple, spherical design serves the patient well. If you find some residual astigmatism, it may be different than you expected based purely on calculation from K readings and refraction. This approach provides a more precise measurement of how much toricity must be incorporated into the lens.

I always start with a fairly large lens diameter (i.e., 9.2mm to 9.8mm depending on the patient's aperture size). All prism-ballasted RGPs tend to ride low. A larger overall diameter and optic zone will reduce the likelihood of flare as the optic zone positions low along the line of sight.

With any large gas permeable lens, the base curve must be flattened slightly to achieve a near alignment fitting relationship with the underlying cornea. A front-surface toric, prism-ballasted lens should align with the cornea or fit slightly flat. This allows unrestricted lens movement and promotes consistent lens rotation. As a starting point, select a base curve a half diopter flatter than the flat K reading. Observe lens performance on the eye to select the final base curve.

The ideal amount of prism allows for the thinnest design that is rotationally stable on the eye. The required amount of prism increases with the higher minus powers to counteract the increased thickness of the superior aspect of the lens. Prism powers vary from 0.75 to 2.50 prism diopters; the most common are 1.00, 1.25 and 1.50 prism diopters.

Determine the spherical component of lens power as you normally do with a spherical gas permeable lens. The cylindrical power is equal to that found in the overrefraction, but the axis should be adjusted for an anticipated 10-degree nasal rotation of the prism base. For example, if refraction over -2.00 spherical lens on the right eye is plano -1.00 x 090, order lens power -2.00 -1.00 x 080.

When checking the power of a front-surface toric, prism-ballasted lens, don't be alarmed if you have difficulty finding a crisp lensometer mire image. This is expected with this lens design.

Ask the lab to dot the base of each lens so you can observe lens rotation on the eye. (Also, dotting the right lens and the left lens with different colors is helpful to patients.)

If the lens rotation is variable, you may need to change the base curve or increase the prism. If only a slight adjustment in stability is needed, consider truncation. The flat edge of the truncation will align with the lower lid and improve rotational stability. A well-polished truncation has virtually no adverse effect on comfort.

With practice, you can truncate lenses in your own lab using a fine, flat file, then polish the lenses. It only takes a few minutes and you'll avoid the inconvenience of returning the lens to the lab for modification.

I should add a word of caution with truncation, especially when dealing with higher minus lenses. A large truncation will remove considerable thickness from the prism base, possibly counteracting any stabilizing benefits of truncation.

A FRONT-SURFACE TORIC, PRISM-BALLASTED RGP WILL RIDE LOW SO THE DIAMETER MUST BE BIASED LARGE

Dr. Quinn has served as a faculty member and research associate at The OSU College of Optometry. He is in group practice in Athens, Ohio.