Toric RGP Base Curve Selection

By Thomas G. Quinn, O.D., M.S.
SEPT. 1997

Most highly astigmatic patients will have spectacle astigmatism nearly equal to their corneal toricity. For example, a patient with keratometry reading of 42.00 @ 180/45.00 @ 090 will likely have a refractive finding something like -2.00 -3.00 x 180. Three diopters of corneal toricity results in three diopters of spectacle astigmatism. In this familiar scenario, a spherical, non-flexing gas permeable lens would create a three-diopter tear lens in the steep meridian, correcting the patient's astigmatic refractive error perfectly.

What's wrong with this picture? It's likely a spherical lens on such a toric cornea will be unstable and not center well. A lens with a toric base curve will more nearly align with the toric corneal surface, providing comfort and stability. But how do you choose the back surface curves?

THE TWO-THIRDS PRINCIPLE

Many practitioners recommend the two-thirds principle, which entails prescribing a lens with back surface toricity that is two-thirds of the corneal toricity. For example, the patient above has 3.00D of corneal toricity. Using the two-thirds principle (3.00D of corneal toricity x 2/3 = 2.00D), you'd fit a lens with 2.00D of back surface toricity (42.00D/ 44.00D or 8.04mm/7.67mm).

SPHERICAL MIMIC APPROACH

Although the two-thirds principle works well in many cases, I prefer to design a lens with a back surface that provides a lens-to-cornea relationship that mimics the fit of a spherical lens on a slightly with-the-rule cornea (Fig. 1). This approach offers the most stable lens positioning and optimal tear exchange beneath the lens during the blink.

FIG. 1: A SPHERICAL RGP LENS ON A 0.75D WITH-THE-RULE CORNEA.FIG. 2: A SPHERICAL RGP LENS ON A 2.00D WITH-THE-RULE CORNEA.

Design a lens that closely aligns with the cornea in the horizontal meridian and is about 0.75D flatter than the cornea in the vertical meridian. This is flat enough to promote tear exchange but not so flat as to induce lens flexure or positional instability. The curves you select will depend on the lens diameter. If the patient's lid aperture size necessitates a lens diameter between 8.7mm and 9.3mm, fit the horizontal meridian to match the keratometry reading and the vertical meridian 0.75D flatter than the K reading. For a larger lens, flatten each meridian by another 0.25D to avoid excessive vaulting. For a smaller lens, steepen each meridian by a 0.25D. The base curve for a 9.0mm diameter lens would be 42.00D/44.25D or 8.04mm/7.63mm.

As you can see, this is nearly the same result as is obtained with the two-thirds principle, and this is often the case. So what's the big deal?

TWO-THIRDS CAN FALL SHORT

Let's look at a patient with a spectacle refraction of plano -6.50 x 180 and keratometry readings of 40.00 @ 180/46.00 @ 090. Vertexing the spectacle refraction to the cornea yields plano -6.00 x 180. As usual, the corneal toricity and refractive cylinder are identical.

Designing a lens for this patient using the two-thirds principle (6.00D of corneal toricity x 2/3 = 4.00D) would result in a base curve of 40.00D/44.00D or 8.44mm/ 7.66mm. This produces a near alignment fit horizontally and a 2.00D flat fit vertically (Fig. 2). This relationship should provide enough base curve toricity to prevent lens rotation. A lens with a base curve of at least two-thirds of the corneal toricity, as long as it is at least 2.00D of toricity, will minimize the chance of lens rotation. However, a 2.00D flat lens-to-cornea fitting relationship in the vertical meridian could result in lens flexure or warpage, and corneal molding will result in spectacle blur. The spherical mimic approach yields a base curve of 40.00D/45.25D. This lens more closely aligns the cornea, providing better vision, comfort and lens stability. 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.