prescribing for astigmatism

Back Surface Toric RGPs

BY THOMAS G. QUINN, OD, MS
OCTOBER 1999

There are two types of toric RGPs: spherical power effect (SPE) designs and cylinder power effect (CPE) designs. With SPE designs, the difference between the two base curves (expressed in diopters) is equal to the difference in the power needed between the two major meridians. With CPE designs, these two values are not equal.

When you find that you have a CPE design, take a moment to determine if multiplying the difference in base curve powers by 1.4 equals the difference in lens power needed between the two major meridians. If they are equal, then you have a back surface toric contact lens.

The CPE Design

A back surface toric lens is simply a special CPE design in which the amount of toric power needed in the lens is equal to the power created by the toricity in the base curve. This power is always about one and a half times the difference in the base curves.

Why? Because when we express the power of the base curve in diopters, we use our conversion chart (e.g. 7.5mm = 45.00D), which is based on the index of the keratometer (n= 1.3375). The index of the keratometer is an artificial calculation which allows us to take into account the index of the tear (n=1.357), cornea (n=1.376) and aqueous (n=1.336) system as it affects the refraction of light entering the front of the eye.

Knowledge is Key

The index of a contact lens, which varies slightly depending on the material used, is higher than the index of the keratometer. Therefore, a contact lens will actually create more toric power (by about one and a half times) than what the stated toricity of the base curve suggests.

Do you need to know that a toric lens being ordered meets the special design requirements of a back surface toric lens? No, because it will work regardless. But being aware that you're working with this design is helpful because knowing that all of the toric power needed is being created by the back surface of the lens tells you that the front surface is spherical. This gives you the freedom to polish the front surface without being concerned about disturbing the toric optics, and also provides the ability to add spherical minus power to the lens if necessary.

Cutting Costs

If you discover that you have a back surface toric contact lens design, you only need to specify the power in the most plus meridian. Of course, when the lens arrives and the power is verified using the lensometer, two powers will be measured: the specified most plus power and a more minus power (more minus power = more plus power + [1.4 x difference in base curve power]). See Figure 1 for an example. By specifying a contact lens order with two base curves and only one power, you'll tip off the billing clerk at the lab that you're ordering a lens with only one toric surface, and you might get a break on your cost. Now that's a reason to do this simple calculation! 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

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