Choosing Toric Peripheral Curves

BY THOMAS G. QUINN, O.D., M.S.,
JULY 1997

You've fit a two-diopter, with-the-rule astigmat with a spherical RGP lens. At dispensing, you observe significant thinning of peripheral curve lift in the horizontal meridian (Fig. 1). You know this bearing may result in mechanical chafing in this region, leading to 3 and 9 o'clock staining, poor comfort and in some cases, poor lens positioning. What do you do?

SPHERICAL BASE CURVE/ TORIC PERIPHERAL CURVE

One option is to reorder the spherical lens with a toric secondary or peripheral curve (Fig. 2). This allows for a flatter peripheral fitting relationship in the horizontal meridian, improving edge lift, comfort and associated corneal integrity.

This design works best for with-the-rule corneas with 2.00D to 2.50D of corneal toricity. On a highly toric cornea, problems such as lens flexure or warpage may haunt you, and spectacle blur can result from the uneven pressure exerted by the spherical base curve. On an against-the-rule cornea, the optic zone is horizontally oval, predisposing the patient to problems with flare as the lens translates vertically during blinking. On a with-the rule cornea, the optic zone of this lens is vertically oval.

When ordering the spherical base/toric secondary curve lens, remember that the specified optic zone size refers to the smallest diameter, which will always be in the flattest meridian. To verify that the secondary curve is in fact toric, look for an oval optic zone.

TORIC BASE CURVE/ TORIC PERIPHERAL CURVE

I choose toric base curve designs for patients with higher levels of corneal toricity (more than 2.00D). I order toric secondary curves and peripheral curves on all lens designs utilizing a toric base curve (i.e., back surface torics and bitorics). This avoids the flare from the oval optic zone that occurs when a spherical secondary curve is applied to a toric base curve lens. With such a design, the optic zone will be horizontally oval when fitting a with-the-rule cornea, causing problems with flare. This is the reverse of what occurs when fitting spherical base/toric peripheral curve lenses.

FIG. 1: RGP WITH PERIPHERAL CURVE SEAL-OFF TEMPORALLY (OD).

A toric peripheral curve system on a lens with a toric base curve also provides a more even edge lift around the circumference of the lens. This improves comfort and reduces the likelihood of adverse effects, such as 3 & 9 o'clock staining, on the peripheral cornea.

Some practitioners will avoid using a toric base curve design due to higher lab costs, especially if they think they'll need multiple lenses for a satisfactory result, but the economics are less of a factor now thanks to guaranteed and warranted fit programs for toric contact lenses.

Choosing the toric peripheral curves for a toric base curve lens is easy. Look at each base curve independently and assign the secondary and peripheral curves that you'd normally use with spherical lenses of the same base curve.

For example, if you're prescribing a 9.5mm diameter bitoric lens and you customarily use a secondary curve that is 0.8mm flatter than the base curve for this lens size, add this amount to each base curve to arrive at your desired secondary curves (e.g. base curves 7.5mm/ 8.0mm; secondary curves 8.3mm/ 8.8mm, peripheral curves 11.0mm/ 11.5mm).

FIG. 2: RGP WITH AN IDEAL PERIPHERAL TEAR POOL TEMPORALLY (OS)

Secondary and peripheral curve designs can improve contact lens positioning, optimize vision and enhance comfort -- all features that make patients happy and you successful. CLS

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