RGP insights

Improving Centration With Toric Secondary Curves

BY TIMOTHY B. EDRINGTON, OD, MSED
JUNE 1998

Toric secondary curves -- what are they and when should you prescribe them? Consider them for patients who require a little more help with lens centration. It can provide you with a simple and cost-effective option for improving success with RGPs.

All Things Toric

When you order an RGP lens in a toric back surface design, such as a bitoric design, you typically receive a lens with a toric secondary curve. This combination of a toric base curve and equally toric peripheral curves results in a lens with a round, not oval, optic zone. The assumption is that the corneal surface is uniformly toric from the center to the periphery. While not an entirely valid theory (as revealed by corneal topography data), it's probably a good clinical assumption for most of your patients needing a toric back surface rigid lens design.

A Spherical Base Curve

It might be necessary to prescribe a toric secondary curve without a toric base curve to optimize lens centration. Spherical rigid lenses often position temporally on patients with low to moderate amounts (0.75D to 2.00D) of against-the-rule corneal toricity. Prescribing a spherical base curve with a secondary curve toric by the amount of the corneal toricity may improve lens centration and alleviate symptoms of glare. If you order a spherical base curve with a toric secondary curve, the lens you receive will have an oval optic zone. The more toric you order the secondary curve, the more oval the resulting optic zone will be. The flatter and wider portion of the secondary curve will correspond to the narrower optic zone width (Fig. 1). A spherical third curve is necessary to start the back surface edge.

Optimizing the Fit

If your patient has with-the-rule corneal toricity and a low amount of against-the-rule overrefraction cylinder, you may want to prescribe a thin lens to enhance lens flexure and reduce the overrefraction cylinder. A toric secondary curve will maximize the effect by improving lens centration.

A toric base curve or bitoric design is indicated if the corneal toricity exceeds 1.50D or if the overrefraction cylinder of your trial spherical rigid lens is 1.00D or greater. A toric secondary curve design will not be as effective in centering an RGP contact lens if the base curve-to-cornea fitting relationship is flat. You can improve lens centration with these designs by prescribing a toric secondary curve fitted on alignment or slightly steep. This enhances the amount of bearing under the secondary curve area.

Specify the minimal width of the optic zone to your laboratory. In the past, laboratories created toric base and peripheral curves by crimping (planned bending) the lens by the amount of the desired toricity and then cutting it with a spherical lathe or tool. Today, most laboratories use automated lathes to generate these toric surfaces and will either charge you a small add-on fee or no additional fee for this design.

Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry.