Influence of Short-Term Orthokeratology to Corneal Tangent Modulus: A Randomized Study

Abstract

Purpose: Influence of orthokeratology on corneal biomechanics is equivocal using Ocular Response Analyzer, ORA. Implementing indentation method, corneal tangent modulus was measured and monitored in short-term orthokeratology.

Materials and methods: Sixteen young subjects with refractive errors between -4D to -5D sphere and astigmatism within -1.50D were recruited. One randomly selected eye wore orthokeratology lens (treatment), and the fellow eye wore conventional rigid gas permeable lens (control). Lenses were worn for 30 and 60 minutes and one night separately with a week of washout period in between. The first two visits were randomly scheduled and before the overnight visit. Eyes were kept closed during all the lens wearing periods. Corneal radius, thickness, and biomechanics (using both ORA and an indentation device) were compared between eyes prior to each visit, and then before and after lens wear. Associations between baseline corneal biomechanics and central cornea from overnight visit were investigated.

Results: Corneal parameters were similar in each visit before lens wear. Significant corneal flattening was observed in treatment eyes, and flattening increased with wearing time. Control eyes showed no significant corneal curvature changes. Corneal resistance factor (CRF) reduced by 0.42mmHg (± 0.68mmHg) after 30 minutes of orthokeratology treatment. Corneal hysteresis (CH) reduced by 0.42mmHg (+/- 0.63mmHg) in control eyes from overnight wear. Both eyes showed stable tangent modulus, E, throughout the study. A lower CH (r = 0.51, p = 0.046) and a higher E (r = 0.53, p = 0.037) at baseline was significantly associated with greater corneal flattening along the flattest meridian in treatment eyes.

Conclusions: Short-term orthokeratology had no significant effect on corneal tangent modulus. Changes in CH and CRF could be related to their intrinsic measurement variability. Corneal tangent modulus provided another measure of corneal biomechanics. Long-term study is required to investigate predictive role of corneal biomechanics in orthokeratology.

Keywords: Corneal tangent modulus; corneal hysteresis; corneal indentation; corneal resistance factor; orthokeratology.