Designing the Ideal Toric Soft Lens

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

If you could design the ideal toric soft lens, which traits would be on your list of essentials? If I were a contact lens manufacturer, my toric soft lens design would include the following:

TWO-WEEK REPLACEMENT

Most soft lenses prescribed today are daily wear lenses that are replaced every two weeks. Torics should be no exception. Patients are accustomed to this schedule and the benefits have been well documented. Two-week replacement is particularly helpful for patients who require a sphere for one eye, most of which are replaced every two weeks, and a toric for the other eye.

TWO BASE CURVES

Two base curves, providing they have significantly different on-eye performances, yield a successful fit for most patients without overburdening production of a planned replacement lens.

OVERALL DIAMETER

A single overall diameter will also reduce inventory and production demands. A large diameter (14.5mm to 15.5mm) will minimize lid destabilization upon lens rotation.

CYLINDER POWER AND AXIS

I'd offer lens cylinder powers of -0.75D, -1.50D and -2.25D in 10-degree steps around the clock. A toric lens that rotates 10 degrees off-axis will induce one-third of its cylinder power in residual astigmatism. Thus, a lens with up to -2.25D of cylinder power could mislocate without inducing residual astigmatism greater than 0.75D. Most patients can tolerate residual astigmatism under this amount.

I'd manufacture higher cylinder powers in half-diopter steps and five-degree axis increments. Axis mislocation induces more significant residual astigmatism in these higher cylinder powers, making proper axis selection more important.

SPHERICAL POWER

Patients who are nearly emmetropic or who are low ametropes with astigmatism are the most difficult to fit with toric soft lenses, so I'd omit sphere powers between +1.00D, at least initially. Success with toric soft lenses has been shown to be best if the sphere power of the lens is greater than or equal to two times the cylinder power.

FRONT SURFACE VS. BACK SURFACE TORIC DESIGN

Studies suggest that in low cylinder powers (under 2.50D), lens performance may not be affected by which surface is toric. In higher cylinder powers, it seems irregular flexure is more significant, especially when fitting astigmatic patients with more than two diopters of hyperopia. I attribute this to the added lens thickness of these powers. In these cases, a back surface toric may avoid possible irregular flexure.

LENS THICKNESS

To minimize the potential for irregular flexure, to improve comfort and to maximize oxygen transmissibility, I'd make my lenses as thin as possible. Of course, material characteristics must permit easy handling. I'd have to develop a material that allows density to be changed across the surface of the lens, providing ballast without adding to thickness.

HANDLING/VISIBILITY TINT

Patients, particularly hyperopes, have shown a strong preference for handling or visibility tint.

So, there it is, the 'Quinn Toric Soft Lens.' What would your design be? CLS

Dr. Quinn has served as an instructor and research associate at The Ohio State University College of Optometry. He is in group practice in Athens, Ohio.

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