prescribing for astigmatism

BY JOHN MARK JACKSON, OD, MS, FAAO

Using an SCOR to Improve Soft Toric Fitting

When I was in optometry school, a well-meaning clinician told me not to bother doing a sphero-cylinder over-refraction (SCOR) when fitting soft toric lenses. "They just don’t work," she told me. "You get very strange, meaningless numbers that don’t tell you anything about how the lens is fitting." She was very wrong, but it took me a while to realize this and to learn to make sense of the data from an SCOR.

What the SCOR Indicates

Over-refractions are important and useful for just about any type of contact lens correction. The data provides you with two pieces of information: the quality of vision attainable with that lens brand or design, and how to change the power for optimal vision. This is especially true for soft torics.

For example, if you can’t get better acuity than 20/30 with the SCOR, but the patient has 20/20 best-corrected acuity, then something is really wrong about the lens. It could be that it doesn’t fit well and the axis doesn’t maintain a stable position. Some lenses just don’t work well with some eyes and you may never know why; the bottom line is that if you can get a good, stable acuity with the SCOR, then no matter what the numbers are, it’s possible to refine the power so the patient can see well. If you can’t get stable acuity with an SCOR, try another base curve or another lens brand.

How to Use SCOR Data

What about the numbers you get from an SCOR? They’re strange because they can result from cross-cylinders. If the term "cross-cylinders" gives you night sweats and you don’t want to know what the numbers actually mean, you can simply use one of the many calculators available to find the best lens power. You simply input the refraction, toric lens power, the SCOR and, sometimes, the observed amount of rotation.

My favorite calculator is called OphthalmiCalc, available online from the University of Melbourne. (Visit http://www.optometry.unimelb.edu.au/misc/ophthalmicalc/OphthalmiCalc.html or just Google "OphthalmiCalc.") I like it because it tells you how much rotation you should see when you look at the lens, given the SCOR you obtained. This is a nice feature, because if the lens doesn’t appear to be rotated this amount, then something is probably wrong with the lens power or it’s not maintaining a stable axis position. Other useful calculators include Coopervision’s ToriTrack and one available through http://www.eyedock.com.

What’s most important, though, is knowing when to use a calculator in the first place. For lenses that come in 10-degree axis steps and cylinder powers below -2.00D, I usually just use LARS to adjust for any observed rotation. In such cases, I use the SCOR only to make sure I can get good acuity with the lens.

But for higher cylinder powers and smaller axis steps, LARS isn’t as useful because it’s harder to measure the rotation precisely enough. For these lenses, I usually use a calculator to determine the final lens power.

Stay Tuned

In my next column, I’ll discuss why you get such strange numbers with soft toric SCORs and how to interpret them. CLS