prescribing for astigmatism
Designing a Fitting Set
for Toric GP Lenses
BY PETER D. BERGENSKE, OD, FAAO
With all the advances that we have in corneal topography, nothing quite beats using diagnostic lenses to determine an optimal GP lens fit. This statement is particularly true when fitting GP lenses that have a toric
contact lenses that have a toric base curve. Many practices have spherical trial lens sets, but more often than not, any toric trial lenses are a hodge-podge of earlier mistakes that aren't likely the best choice for accurate trial fitting. Having a set of standard toric GP trial lenses available can help improve your toric GP fitting success.
Fitting Set Considerations
To facilitate the fitting of toric GP lenses in our clinics, I devised a fitting set that has served us well. The design in each lens follows the principle of spherical power effect (SPE) to simplify interpretation of over-refraction data. The intent is to provide a tool for assessing the base curve-to-cornea relationship because, after all, the reason for using a toric base is to improve the fit. Optical considerations require further steps in some cases, but in reality, a majority of toric base fits are, or could be, SPE.
Table 1 shows the toric GP trial lens set that we use in our clinics. From a fitting standpoint, a common relationship for GP contact lenses is the steeper the lens, the smaller the diameter. For this reason, I chose to vary the diameter with the radius, rather than assigning to all lenses the same diameter. (This makes sense in spherical lens fitting sets as well.)
Another useful feature of our toric GP trial set is that I used three colors of plastic, thus minimizing the chance of confusing the trial lenses in the examination room. Valley Contax of Springfield, OR, manufactured our fitting set. The lenses are all made of Boston ES, as our lab suggested. We chose this material based on stability rather than permeability. You can, of course, order patient lenses in any suitable lens material. I wanted a stable trial set and had no real concerns about physiology because the trial lenses are on the eye only for a short time.
Size Isn't Everything
This small toric GP fitting set is surprisingly comprehensive and allows us to fit GP toric base lenses with confidence in a majority of cases.
Dr. Bergenske, a past chair of the American Academy of Optometry's Section on Cornea and Contact Lenses, has practiced for more than 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry. E-mail him at
Contact Lens Spectrum, Issue: February 2005