Association between long-term orthokeratology responses and corneal biomechanics.

Myopia is very prevalent worldwide, especially among Asian populations. Orthokeratology is a proven intervention to reduce myopia progression. The current study investigated association between baseline corneal biomechanics and orthokeratology responses, and changes of corneal biomechanics from long-term orthokeratology. We fitted 59 adult subjects having myopia between -4.00D to -5.00D with overnight orthokeratology. Corneal biomechanics was measured through dynamic bidirectional corneal applanation (in terms of corneal hysteresis, CH and corneal resistance factor, CRF) and corneal indentation (in terms of corneal stiffness, S and tangent modulus, E). Subjects with poor orthokeratology responses had lower E (mean 0.474 MPa) than subjects with good orthokeratology responses (mean 0.536 MPa). Successful orthokeratology for 6 months resulted in reducing CH (reduced by 5.8%) and CRF (reduced by 8.7%). Corneal stiffness was stable, but E showed an increasing trend. Among subjects with successful orthokeratology, a higher baseline S resulted in greater myopia reduction (Pearson correlation coefficient, r = 0.381, p = 0.02).

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

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.