prescribing for astigmatism
Shapes of Things
BY PETER D. BERGENSKE, OD, FAAO

Like the people to whom they are attached, corneas come in a variety of shapes and sizes. Contact lenses that are expected to fit and perform well on an eye must therefore have a shape that is appropriate for  that eye. Kermit Kors, who taught ophthalmic optics at UC Berkeley for many years, used to put it this way when referring to the mundane task of adjusting spectacles: "If they don't fit, they're not comfortable and if they're not comfortable they won't wear them and if they want to see, they need to wear the damn things!" The wisdom in this, and its application to contact lenses, is obvious.

Figure 1. Uniform corneal toricity.

No Easy Task

Perhaps not so obvious are some of the considerations that come into play when fitting a toric cornea with a rigid contact lens. It would be easy to claim that you should fit every cornea with greater than, say, 2.25D of corneal astigmatism with a toric base curve lens, but it is unfortunately not that simple.

To determine which corneas need a toric base curve lens, look beyond the toricity determined by central keratometry readings and take into account the degree of toricity of the mid-peripheral cornea. This makes a lot of sense when you consider that this is where the lens actually "fits."

	Figure 2. Peripheral toricity.
	Figure 3. Central toricity.

Using the Tools

Fortunately we have tools available to guide us. Clearly fluorescein is the most precise and telling, but you can use topography as a faithful guide as well. Consider the three topographies shown here. Figure 1 indicates high astigmatism that is symmetrical, with an essentially even amount of astigmatism indicated from center to edge of the map. A lab can design a toric base curve lens using the K readings alone, as the periphery is simply a continuation of the central curvature.

Figures 2 and 3 illustrate corneas for which the central K reading only tells part of the story. Figure 2 is from an eye that, by K reading alone, might lead you to think a spherical base curve would suffice. But look closely and you'll see that a spherical lens fit close to the flat K will be loose in the vertical meridian. Figure 3 illustrates a cornea that, by K reading alone, seems a candidate for a toric base lens, yet a toric base lens designed around the central K readings would fit steep and tight in the mid-periphery.

To Fit a Toric or Not

Lid position, corneal diameter and overall sagittal depth also influence what it takes to fit a given cornea, but when deciding whether to "go toric," peripheral toricity is perhaps the most often overlooked aspect. Remember to look at the entire map, not just the simulated K readings.

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: berg1101@pacificu.edu.