Comparative analysis of treatment zone diameter, location and amount of maximum defocus and pupil size with orthokeratology.

PURPOSE: The purpose of this study was to retrospectively examine data from young patients who had undergone orthokeratology (OK) for myopia control to quantify the size and amount of central (treatment zone) and mid-perpheral (reverse zone) total refractive power (TRP) changes and compare where these changes occur with respect to the pupil size (PS) and location. METHODS: Participants' charts were reviewed, and data from the Pentacam instrument were collected prior to OK and after the final visit four years later. Variables included the initial Rx (sphere), pupil size, total mid-peripheral corneal refractive power (TRP) at the location of the maximum change and the resulting treatment zone size (TZD), measured centrally with the Pentacam. All parameters are reported in nasal, temporal, superior and inferior locations, but only the horizontal location was analyzed. All participants were fitted with Paragon CRT lenses with a BOZD=6mm. Data analysis was conducted to analyze the effects of lenses on TZD and the location and amount of TRP with pupil size. RESULTS: A total of 31 patients (62 eyes) age 13.93±5.05 years, 13 M and 18 F, were successfully fitted with CRT lenses. The mean sphere was -3.11±1.41 D, range -0.75 to -6.00 D). The final treatment zone size (diameter of minimal power change) was 2.45±1.11mm in the horizontal dimension. The location of maximum change in TRP occurred at 2.16±0.23mm temporally and 1.65±0.29mm nasally (P<0.0001). The maximum power change in the reverse zone area was 3.03±1.97 D temporally and 2.99±2.28 D nasally (P=0.056). The mean pupil size was 3.37±0.64mm, which was significantly greater than the horizontal TZD (P<0.0001). There was significant negative correlation between the temporal pupil size and the temporal TZD (P=0.0079). CONCLUSION: The results of this study provide valuable insights into the size of the treament area and refractive changes occurring with orthokeratology when used for myopia control. The pupil size and horizontal treatment diameter appear to be equivalent, but the area of maximum power (myopic defocus) that is necessary to slow the progression of myopia may be outside of the pupil area in high illumination conditions.