prescribing for astigmatism

I Double (Base Curve) Dare You!

BY CHRIS SNYDER, OD, MS, FAAO
AUGUST 2001

A patient has greater than 2.25D of corneal toricity and an associated astigmatism in his prescription. The following is absolutely the simplest way to successfully fit (or begin fitting) a bitoric RGP lens. Before you read on, I acknowledge that what you read will raise the ire, perhaps reasonably so, of many of my esteemed, experienced colleagues.

Ready? Here goes!

Steps to Fit a Bitoric Lens

Step 1. Accurately measure central K values.

Step 2. Refract.

Step 3. Vertex the spectacle refraction back to the corneal plane.

Step 4. Order: base curves = K readings; contact lens powers = vertexed refraction; overall diameter = 9.2mm; optic zone diameter = 7.8mm.

Example:

Step 1. K = 42.00 @ 180, 45.00 @ 090

Step 2. Spectacle refraction = ­4.50 ­3.50 x 180 (vertex distance = 14mm)

Step 3. Vertexed refraction = ­4.25 ­3.00 x 180

Step 4. Order: base curves = 42.00/45.00; contact lens powers = -4.25/­7.25; diameter = 9.2mm; optic zone = 7.8mm

More often than not, this contact lens would be dispensable as a reasonable and successful lens for fit and vision. That's it. I double dare you to try it!

For those who say fitting a bitoric RGP contact lens in this way is too simple or not correct, read my next column in the December issue.

Figure 1. Optical cross of vertexed refraction.	Figure 2. Contact lens base curves and powers.45.00

Dr. Snyder is a professor of optometry and serves as chief of contact lens patient care at the School of Optometry at the University of Alabama at Birmingham.

The length (or lack of length) of my column shows you how easy fitting a bitoric RGP lens can be.