prescribing for astigmatism

SPE is SPEcial

BY THOMAS G. QUINN, OD, MS
June 1999

Imagine a lens that fully corrects the refractive error of an astigmatic patient by utilizing a durable material that exceeds the cornea's oxygen requirements with a design that perfectly compliments the cornea's shape. Imagine that this lens offers the

compliments the cornea's shape. Imagine that this lens offers the unheard of benefit of permitting the lens to rotate without affecting vision. All of this adds up to a lens that provides superior vision to the astigmatic patient, while providing good comfort, easy care and optimal safety. Sound too good to be true? Well it is true and it's available through your local lab. It is a spherical power effect (SPE) designed RGP contact lens. The following will explain how it works and how you can put it to work for you and your patients.

Choosing an SPE Design

Fitting a patient with more than 2.00D of astigmatism can be frustrating. A toric soft lens with this degree of astigmatic correction will compromise vision if rotational stability is the least bit variable. A spherical RGP lens on such an astigmatic eye may not center well, which can lead to poor comfort, flare and physiological compromise.

An SPE design lens is easy to identify. If the difference in base curves (in diopters) is equal to the difference in lens power, then you have an SPE design. The beauty of an SPE design contact lens is that it provides the physical fitting benefits of a toric base curve lens (i.e. good centration, good tear exchange) and the vision benefits of a nonflexing spherical RGP lens (i.e. impervious to lens rotation).

Employing an SPE Design

Employ an SPE design whenever a spherical RGP design will fully correct the astigmatic error (i.e. the vertexed spectacle cylinder amount equals the corneal toricity), but is not an option because the degree of corneal toricity would lead to an unstable fit.

For example, a patient with a refractive error of -1.00 -3.00 x 180 and K readings of 41.00 @ 180; 44.00 @ 090 is an ideal candidate. Choose base curves that will mimic a spherical RGP on a slightly with-the-rule cornea. Fitting the horizontal meridian on alignment and the vertical meridian 0.75D flat will provide a good tear pump without introducing concerns about flexure. The patient's aperture size dictates that we fit him with a 9.0mm diameter lens. Since this is an intermediate size, choose a horizontal base curve that is equal to the horizontal K reading (41.00D or 8.23mm) and a vertical base curve 0.75D flatter than the vertical K reading (44.00D -0.75D = 43.25D or 7.80mm). A bigger lens requires flatter curves to avoid vaulting, and a smaller lens requires steeper curves to avoid excessive touch.

Since the horizontal meridian is fit on -K, no tear lens will be present. However, the vertical meridian is fit 0.75D flat, resulting in a -0.75D tear lens. Therefore, instead of needing -4.00D lens power in the vertical meridian, only -3.25D is necessary. We end up with the base curves 43.25D (7.80mm)/ 41.00 (8.23) and associated lens powers
-3.25D/ -1.00D. The difference in base curve powers (43.25D -41.00D = 2.25D) is equal to the difference in lens power (3.25D -1.00D = 2.25D), so we know that we have an SPE design. Now we can order the lenses knowing that even if they rotate slightly, we won't end up with blurred vision and a wild oblique overrefraction!

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