Contact Lens Practice Pearls
Not All Peripheral Curves are Created Equal
By John Mark Jackson, OD, MS, FAAO
Modern lathing technology can produce some pretty amazing lens designs for irregular corneas. I recently had to use a quadrant-specific peripheral curve system while fitting a patient who had keratoconus. This case illustrates a straightforward application of this technology.
My patient was recently diagnosed with keratoconus and was referred to me for fitting. Her best refraction OD was –0.50 –4.00 x 005 to 20/80. The topography OD (Figure 1) showed a pretty typical nipple cone pattern with a small, central steep area within the pupil. This type of cornea can be fit with a smaller-diameter, traditional keratoconus design, and we chose to use the Rose K2 (Blanchard) design. Figure 2 shows the best-fitting lens from our diagnostic set, which had a 6.0mm base curve radius and an 8.7mm overall diameter.
Figure 1. Right eye topography.
Figure 2. Lens without ACT.
Although the overall pattern looked good, with some central pooling and minimal clearance over the cone apex, the bottom edge of the lens exhibited significant edge stand-off. This can lead to lens discomfort, significant edge awareness, and possible lens dislodgement from the eye.
Tweaking the Peripheral Curves
Figure 3 shows the final lens for the patient. We used the same base curve and diameter as the diagnostic lens, but ordered it with a “Grade 3 ACT,” or Asymmetrical Corneal Technology, Blanchard's name for a quadrant-specific peripheral curve system. In our case, we steepened the periphery at the bottom of the lens, “tucking” the edge in toward the cornea, to eliminate the edge stand-off. The patient noticed a resulting significant increase in lens comfort.
Figure 3. Lens with ACT.
Why did we get so much edge stand-off with the standard periphery? The elevation map is the key. The blue area at the bottom of the map shows that the inferior corneal elevation is far below the rest of the cornea; this makes the edge of the lens much farther from the cornea in that quadrant than it is everywhere else. This is a typical finding in keratoconus due to the central steepening, but the severity can vary. In our case, it was enough to cause stand-off.
Customize Any Curve
With today's advanced lathes, most labs can make quadrant-specific designs. This technology can be applied to the base curve or to the periphery, customizing the fit to the cornea. CLS
|Dr. Jackson is an associate professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at email@example.com.|