prescribing for astigmatism
I Double (Base Curve) Dare You!
BY CHRIS SNYDER, OD, MS, FAAO
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.
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.