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

Soft torics relate to one another much the same way contact lens care products relate to one another. They all perform basically the same function, but have varying features that alter their performance. The features you choose depend on the patient's specific needs.

To select a soft toric lens, look at the patient's refractive error. Is a high cylinder power, hyperopic and/or high sphere power, or oblique cylinder axis needed? Your answer to this question, will help you narrow down your toric lens options considerably. Next decide whether to opt for a prism-ballasted or thin zone stabilization design.

PRISM-BALLASTED DESIGN

For patients with a borderline dry eye or for those whom you suspect may have difficulty with lens handling, I suggest a prism-ballasted design. The added thickness at the vulnerable inferior corneal region can provide a heavier "wet blanket" to protect this region from desiccation. The thickness of this design may also assist lens manipulation and increase durability, especially for low sphere power lenses. I've had success with the Hydrasoft Toric, Optima Toric and Sunsoft Toric, among others.

THIN-ZONE DESIGNS

For patients with against-the-rule astigmatism (minus cylinder axis 090), my first choice is a thin-zone design such as the Torisoft lens. The thinned superior and inferior aspects complement the thickness distribution in an axis 090 power correction.

A prism-ballasted lens is a viable option in these cases. Many of today's prism-ballasted lenses use thin-zone design strategies as many have a thinner aspect of the lens, known as eccentric lenticulation. This thinning improves comfort and reduces the potential for rotational instability from the lower lid interacting with the lower lens edge. In fact, I find many prism-ballasted lenses equal in comfort to thin-zone lenses.

DURABILITY

For patient's who are likely to tear lenses, a lower water content lens such as the Durasoft 2 Optifit Toric or CSI Clarity Toric may be ideal. Durability can also be enhanced with these handling tips:

• Suggest patients instill a rewetting drop in the eye prior to lens removal. This will make the lens more forgiving during handling.
• Suggest patients rotate the lens on the eye immediately before removal. This will prevent patients from stressing the same part of the lens during removal.

An ideal handling design may be less comfortable and may not transmit adequate oxygen, particularly in patients with high refractive errors and long wearing schedules. These patients may be better off with thinner, high-water lenses.

FRONT VS. BACK TORICS

Conventional wisdom suggests that a hydrogel lens with the toricity manufactured on the front surface is preferable when fitting nearly spherical corneal surfaces. Conversely, back surface toric designs are thought to perform better on highly toric corneas. I have found irregular flexure to be an issue when fitting hyperopes above +2.00 diopters. The added thickness appears to reduce the draping action of a soft lens so that it does not conform regularly to the corneal shape. If the patient complains of variable vision and the overrefraction is inconsistent and doesn't yield a crisp endpoint, observe the red reflex, keratometry mires or topographical map of the lens surface while on the eye to properly identify irregular flexure. Thinner designs such as the Hydrasoft Toric XW may be helpful.

Consider frequent replacement toric soft lenses for patients who have shown a propensity for lens coating or when the patient has shown poor lens care habits. CLS

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.