prescribing for astigmatism
I'll "See You" and Raise Your (Bitoric) Peak
BY CHRIS SNYDER, OD
In my August column, I resisted the temptation to complicate bitoric lens fitting. As a reminder, we designed an RGP lens to match the corneal K readings with powers matching the spectacle refraction that was vertexed back to the corneal plane. Lens OAD = 9.2 mm and OZD = 7.8 mm.) Essentially,
we used a fitting philosophy of matching base curves to K values.
Alas, it can be that easy for a workable and acceptable result. At the risk of losing the clarity of the former column, I offer a few simple additional steps. If I lose you, go back to the former column, confident that you still have a simple cookbook for basic bitoric fitting.
Applying a Fitting Philosophy
If you don't have a favored fitting philosophy of your own, let me suggest:
- For a with-the-rule (WTR) cornea:
After beginning with a design of "matched base curves to K values" (suggested above), flatten the steep base curve 0.75D and add 0.75D more plus to the power meridian of the contact lens. This will provide the correct lens power and mimic the corneal bearing relationship seen with a spherical base curve RGP on a 0.75D WTR toric cornea an arrangement that gives the right amount of lens rock for optimal tear exchange under the lens.
Our measurements from the example eye in the August column:
K = 42.00 @ 180, 45.00 @ 090
Spectacle refraction =4.50 3.50 x 180 (vertex distance=14 mm)
Vertexed refraction = 4.25 3.00 x 180 (Figure 1)
Applying the recommended fitting philosophy would yield: base curves = 42.00/44.25; contact lens powers = 4.25/6.50; diameter = 9.2 mm; optic zone =7.8 mm.
- For an against-the-rule (ATR) cornea:
After beginning with a design of "matched base curves to K values," fit the flatter (vertical) corneal meridian with a base curve 0.75D flatter than that meridian's K value and add 0.75D more plus to the power meridian of the contact lens. The power will be correct and this again will attempt to mimic that "ideal" corneal bearing for optimal tear exchange.
Lens Diameter/Optic Zone
The dimensions suggested are an average that will work in most cases. You may end up with a larger or smaller lens, depending on the individual's aperture size, lid position and pupil size. Using the intermediate dimensions suggested here is a great way to start.
Why an Empirical Approach?
Even when you have a bitoric fitting set, you will often have toricity and refractive error that are well away from the parameters of your diagnostic lenses. Because lens mass and thickness profile interacting with the eyelid dynamics are so important to lens performance on the eye, it is reasonable to simply order that customized first pair of lenses, as described in this article, to evaluate on the eye. Your patient will be pleased and your practice will grow because of it!
|Figure 1. Optical cross of vertexed
||Figure 2. Contact lens base curves and powers.
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
Contact Lens Spectrum, Issue: December 2001