Article Date: 7/1/2001

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.

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:

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:

Toric soft lens inventories enjoy the same benefits, but involve a much larger capital investment. Here are some helpful hints:

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.


Optometric Management, Issue: July 2001