contact lens management
Tips for Soft Toric Success
Consider these guidelines to make your fitting experience worthwhile.
BY MILTON M. HOM, O.D., F.A.A.O.
How do you fit soft toric contact lenses -- empirically or diagnostically? Here's a look at both methods, with some tips for success that will help you save chair time.
Having a fit
When fitting contact lenses, we can choose from the empirical method or the diagnostic method. I think that most contact lens practitioners use a combination of these two methods. Below are a few guidelines to consider with each approach.
- The empirical method. Recent work led by Loretta Szczotka, O.D., M.S., F.A.A.O., and a team of researchers offer these new ideas for empirical fitting:
- Use the K readings for base curvature and vertexed spectacle refraction for power. You can follow tables to determine which lenses to order. In some more advanced forms, you can account for an automatic nasal rotation of lenses. For the right lens, adding 5 to 10 degrees to the ordered axis accounts for rotation. For the left lens, subtracting 5 to 10 degrees compensates for rotation.
- This variation is more sophisticated. One guideline is adding +0.25D to +0.50D for thicker prism-ballasted lenses. For powers over
±5.00, the adjustment should be +0.50D to +1.00D. The rationale is that the front surface of the thicker torics doesn't drape as well as the back surface. The front surface doesn't stretch or flex completely, inducing a minus tear layer. Plus power is needed to compensate.
- The diagnostic method. The other method for fitting toric lenses is diagnostic. Simply choose a lens from the diagnostic set and place it on the patient's eye. Then, perform sphero-cylindrical overrefraction and evaluate the fit. Based on these results, order another lens or fit the one you have on the eye.
Five types of astigmatism
Research shows that empirical fitting success can vary according to the type of astigmatism a patient has. In the past, our definition of astigmatism has been largely limited to the central 3 mm of the cornea as measured by keratometry. But corneal topography has identified five different types of astigmatism.
Corneal topography demonstrates that the astigmatism in the peripheral cornea (greater than 3.5 mm centrally) can differ from astigmatism in the central cornea. In some corneas, this difference has a significant effect on toric soft contact lens fitting. In Dr. Szczotka's study, fitting success dropped from about 75% to 50%, depending on the type of astigmatism the patient experienced.
Types of astigmatism
TYPE I SPHERICAL CORNEA: There's 0.75D or less central and peripheral astigmatism (6%).
TYPE II CENTRAL ASTIGMATISM: There's more than 0.75D centrally or at least 0.75D more corneal toricity centrally than peripherally (8.7%).
TYPE III LIMBUS: Limbus equal astigmatism: Central astigmatism within 0.75D of peripheral astigmatism (11%).
TYPE IV LIMBUS: Limbus greater peripheral astigmatism -- the difference between the peripheral astigmatism and the central astigmatism is 1.00D (27%) or greater.
TYPE V LIMBUS: Limbus irregular peripheral astigmatism -- the peripheral astigmatism is irregular with opposing hemi-meridians having a difference of greater than 3.00D (23%).
Data from Reddy T., Szczotka L.B., Roberts C. Peripheral corneal contour measured by topography influences soft toric contact lens fitting success. CLAO 2000;26(4): 180-185.
Astigmatism largely falls into two categories, including the five types:
- Category 1 (central astigmatism). Includes Types I, II and III.
- Category 2 (peripheral astigmatism). Includes Types IV and V.
When peripheral astigmatism is greater than central astigmatism, empirical success rates are around 50%. What makes this significant is the fact that two-thirds of corneas fitted for soft torics are Types IV and V.
Why does the fitting success drop with these types of corneas? Peripheral astigmatism changes how the lens fits on the eye. The thicker toric lenses drape and flex differently than predicted on corneas with peripheral astigmatism. When there's a difference from what is expected, the empirical fitting success rate drops.
Use a corneal topographer to identify the patient's type of astigmatism and to help troubleshoot your contact lens fittings. This extra step will result in less chair time and higher patient satisfaction.
Inventory or not?
As we know, keeping an inventory of contact lenses is a great convenience. In particular, a soft contact lens inventory carries the following benefits:
- Patients have "instant" access to their lenses.
- Costs per 6 pack are lower.
- Shipping costs are also lower.
Toric soft lens inventories enjoy the same benefits, but involve a much larger capital investment. Here are some helpful hints:
- When placing a toric lens inventory in your practice, make sure it's properly designed to enhance its return on investment. Most of the lenses (70% or more) should be with-the-rule (axis 180 plus or minus 10 to 15 degrees). A good part should also be devoted to against-the-rule (axis 90 plus or minus 10 to 15 degrees). If the rotation is greater than 15 degrees, choose another design for stability.
- One fitting rule is to undercorrect by cylinder. Therefore, emphasize lower cylinder powers of -1.00D or higher. There are uses for -0.75D cylinders, but sometimes an aberration-blocking soft lens will do fine for these patients.
First choice: empirical
Fitting lenses diagnostically may take a little longer than the empirical method, but it's a good tactic to use when all else fails. I fit toric soft lenses using empirical methods to choose my first lens out of the toric lens samples supplied by the manufacturer. Then I know what's wrong if my first choice doesn't work and I can find a solution more quickly.
References available upon request.
Milton M. Hom, O.D, F.A.A.O., authored Manual of Contact Lens Prescribing and Fitting with CD-ROM Second Edition and LASIK: Clinical Co-Management (www.bh.com). He practices in Azusa, Calif.