Contact Lens Case Reports

Corneal Changes in Ortho-k

By Patrick J. Caroline, FAAO, & Mark P. André, FAAO

One of the more interesting features of orthokeratology/corneal reshaping is that the size of the central, foveal treatment zone is relatively small, approximately 3mm or less. Beyond that area the cornea begins to radically steepen, resulting in increasing plus power across the pupil.

Estimating the Ortho-k Effect

Currently, there are two techniques to estimate the amount of epithelial tissue compressed (flattened) in orthokeratology. One is a corneal topography technique that uses elevation maps to measure the change in corneal height pre- and post-fitting. The second technique is to use Munnerlyn's formula (a common refractive surgery formula) to estimate the change in corneal shape required to achieve a desired refractive change (Figure 1).

Figure 1. The Munnerlyn formula for a −5.00D correction.

We evaluated the pre- and post-central corneal height changes in four patients who had post-orthokeratology refractive changes of −1.00D, −2.75D, −5.00D, and −7.25D. The patients' corneal height changes were measured (across a 3mm chord) with both the topography and Munnerlyn formula techniques (Figures 2 through 5). Figure 6 summarizes these data.

Figure 2. Differences in pre- and post-corneal elevation with a −1.00D ortho-k correction.

Figure 3. Differences in pre- and post-corneal elevation with a −2.75D ortho-k correction.

Figure 4. Differences in pre- and post-corneal elevation with a −5.00D ortho-k correction.

Figure 5. Differences in pre- and post-corneal elevation with a −7.25D ortho-k correction.

Figure 6. Estimation of tissue compression in orthokeratology using the corneal elevation technique and the Munnerlyn formula.

Both Techniques Have Merit

The results demonstrate two important features of ortho-k: 1. very little epithelial tissue compression is needed for significant refractive error changes; 2. a strong correlation exists between the two techniques used to measure/estimate the amount of compression. The difference between them across a wide range of powers (−1.00D to −7.25D) was 0.0 microns to 3.0 microns. CLS