prescribing for astigmatism
Tips for Soft Toric Lens Fits
BY JULIE A. SCHORNACK, OD, MED
Sometimes soft toric lenses are delightfully easy to fit. The ideal patient presents with astigmatism that's easily correctable with off-the-shelf cylinder powers and a sphere power to match stock parameters. You select a diagnostic contact lens that exactly matches the patient's vertexed prescription, place it on the eye and allow time for stabilization. Examining the lens reveals a perfect fit and negligible or no rotation. The over-refraction is plano and provides excellent acuity for the patient. Aaah, a perfect scenario.
But what if it doesn't happen that way? What if your diagnostic contact lens is close but not perfect in cylinder power and axis compared to your refractive cylinder power and axis? You may observe the fit of the diagnostic lens and find more than 15 degrees of rotation. Your over-refraction may result in a power and axis that are unexpected, confusing and difficult to apply to make the LARS rule comprehensible. What then?
The Math of Cross Cylinders
Some key points will make these challenging soft toric fits more straightforward. A math calculation will solve the mystery of the strange over-refraction that is present with lens rotation. But before we talk about the specifics of the math, let's review the generalities. Snyder (1989) showed how a soft toric lens that misaligns with the refractive cylinder axis 30 degrees either nasally or temporally would result in the full amount of the lens' correcting cylinder appearing in the over-refraction at an axis oblique to the desired axis. Also, a soft toric lens that rotates 15 degrees would result in one-half the amount of correcting cylinder appearing in the over-refraction at an oblique axis. Finally, Snyder demonstrated that as little as 10 degrees of rotation results in one-third of the correcting cylinder appearing in the over-refraction at some axis.
So when a soft toric contact lens that has 3.00D of correcting cylinder rotates 30 degrees and you over-refract 3.00D of cylinder at an oblique axis, the outcome is logically confusing unless you remind yourself of the math of cross cylinders.
Make the Math Easy
Now for the really great news. Many downloadable and online tools are available for determining the complicated cross-cylinder formula calculations with ease. These tools allow you to plug in the measured and observed values for the diagnostic lens power, the lens misalignment and the over-refraction. The calculators take all the data into account and apply the cross-cylinder formula to determine the mathematical resultant power. Some examples of online cross-cylinder calculators are available at the following Web sites:
- Coopervision.com — you must register for the Web site with your account number to have access to the fitting tools.
- Eyedock.com — for an annual fee you can access many fitting tools and databases that are helpful in practice.
These tools are great to have on hand on a computer in the exam room or on a handheld device to determine the resultant power for an ordered lens in an instant.
The bottom line in fitting soft toric contact lenses is that many tools can help you determine the final lens sphere, cylinder and axis that will bring the best vision to each patient. Although we're grateful for the soft toric contact lens fittings that end by simply applying LARS and determining a final lens power, we all know it doesn't always end that way. When the findings of a soft toric lens evaluation lead to cross-cylinder results, don't hesitate to utilize these online tools to bring clarity to the final soft toric contact lens prescription. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #151.
Dr. Schornack is the associate dean of Clinical Education and serves in the Cornea and Contact Lens Service at the Southern California College of Optometry.