prescribing for astigmatism
Achieving First Fit Success With Toric Lenses
BY PETER D. BERGENSKE, OD, FAAO
When it comes to fitting specialty lenses, the issue of "chair time" inevitably comes up. Getting a lens to fit the first time -- rather than after a series of trial lenses -- is obviously desirable. Toric soft lenses are a particularly good example of lenses that you want to get right on the first try.
We tend to disdain the concept of "empirical" fitting, yet every clinician must decide what lens to try first. We're always pleased when the first lens is, as Martha Stewart would say, "just perfect." Can we make that happen more often? Is there a "measure twice, cut once" equivalent in contact lens fitting?
Manufacturers have designed modern toric lenses to fit the majority of eyes, and many popular designs are available in only one base curve and diameter. This is reasonable because most patients' eyes fall within a fairly narrow range of sagittal depths. But what if they don't, and for that matter, how would you know?
Taking the Right Measurements
One simple observation that can save you from misfitting the first lens is to take into account cor-neal chord diameter. Clinically this is what we can see and measure, and it's called the horizontal visible iris diameter (HVID).
The common relationship of corneal radius to HVID means that most eyes fall within a narrow range of fitting characteristics for soft contact lenses. That is, the shorter the radius (or the steeper the K), then the smaller the HVID tends to be. In other words, steep corneas tend to be small, flat corneas tend to be large, and this greatly neutralizes the effect of the K reading alone.
How does this affect your initial lens choice? When the HVID is substantially greater or smaller than average, this overwhelms the significance of the corneal radius when determining sagittal depth. In these cases, relying on K reading alone will send you off in the wrong direction. I'll spare you the math, but the effect of 1mm of HVID is equivalent to 5.00 diop-ters of K. For example, if one eye has an HVID of 12mm and the other is 13mm and both have a K reading of 43.00 diopters, then the eye with the 13mm HVID has a sagittal depth equivalent to an eye of 12mm HVID and K reading of 48.00 diopters.
Applying the Data to the Fit
Using this same principle, it's often more effective to increase or decrease lens diameter than it is to choose a different radius when trying to optimize a fit, especially on eyes with an HVID that is not average. Most toric lenses have diameters of 14mm to 14.5mm, allowing at least 1mm of overlap beyond the limbus. Toric lenses are typically larger than spherical lenses because stability is more important. But a number of toric lenses exist that are made with smaller and larger diameters. These are often custom lenses, but when you need them, they're worth the cost and trouble.
Measuring for Success
Next time a toric soft lens won't center, rotates excessively or is excessively tight, look at the patient's HVID. You may find that you need a steeper (or larger) or flatter (or smaller) lens than the K readings indicate. Better yet, get in the habit of observing HVID and understanding how it may affect lens fit. It might just save you a step or two.
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 email@example.com.