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T Nguyen, S Soni, D Carter, T Biehl; Corneal Changes Associated With Overnight Orthokeratology . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3092.
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Purpose: To examine the changes in corneal curvature and thickness associated with reverse geometry orthokeratology lenses over a 3-month treatment period. Methods: 16 eyes from 8 subjects (21 to 43 yrs) with myopia less than 3.00D and astigmatism less than 1.00D were fitted with reverse geometry OK lenses (OK B & D Series, Contex Inc., Sherman Oaks, CA). The lenses were worn overnight and removed during the day. Data was collected once before treatment (baseline) and then at 0, 4, 8, and 12 hrs following the removal of OK lenses after overnight wear at 1 day, 1 wk, 1 mo, and 3 mos. Corneal topography and corneal thickness were measured using Orbscan. Epithelial, stromal and total corneal thickness of the right eye was determined using confocal microscopy. Keratometry measurements were made with a standard keratometer. Results: The corneal curvature in the horizontal meridian showed a statistically significant flattening of 1.22D on Day 1, 0 hr (p<0.05) . At 4 hr, the curvature change was reduced to 0.68D and remained relatively the same throughout the day. These changes were not statistically significant at p<0.05. By 1 wk, the corneal curvature flattened significantly by 1.91D at 0 hr and 1.47D at 12 hrs. These changes were stable for the remainder of the study. For all test visits, the mean central corneal thickness measured by Orbscan increased by approximately 20 microns at 0 hr. and returned to baseline at 4 hrs and was not significantly altered thereafter. At 0 hr, the epithelial thickness decrease by 17.77 microns, and by 12 hrs, the difference was reduced to 14.70 microns. These reductions are statistically significant at p<0.05. At 0 hr, the stromal thickness increase by 40.1 microns, and by 12 hrs, the difference was reduced to 23.60 microns. These changes were not statistically significant at p<0.05. At 0 hr, the total corneal thickness increased by 23.56 microns (not statistically significant at p<0.05), thereafter, there was very minimal increase in total corneal thickness. Conclusion: The change in horizontal keratometry measurements at all visits showed statistically significant correlation (r=.61, p<0.05) with the change in refractive error. The small or absence of change in total corneal thickness shown with Orbscan and confocal microscopy can be explained by a decrease in central corneal epithelial thickness accompanied by increase in stromal thickness. We believe that a portion of the increase in total central corneal thickness at 0 hr is the result of edema following sleep.
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