Purpose. To investigate the time course of corneal thickness changes in overnight hyperopic orthokeratology (OK) lens wear for a 4-day lens-wearing period. Methods. Fourteen subjects (age range, 20-37 years) were fitted with hyperopic OK lenses in one eye only. The fellow eye acted as a non-lens wearing control. Lenses were worn overnight only for 4 nights, and changes from baseline in subjective refraction, corneal topography, and corneal thickness (Holden-Payor optical pachometer) at central and para-central locations were measured on days 1 and 4 after overnight lens wear, at lens removal (AM), and 8 hours after lens removal (PM). Results. There was a significant refractive and corneal topographic effect at all visits. The central total cornea thickened significantly at AM visits only because of significant stromal thickening consistent with the overnight lens wearing edema response, and returned to baseline at PM visits once edema resolved. The para-central epithelium significantly thinned at all AM and PM visits. This counteracted para-central stromal thickening at AM and resulted in significant thinning of the total para-central cornea at PM visits when stromal thickness had returned to baseline. Conclusions. Para-central corneal epithelial thinning explains corneal anterior surface steepening in hyperopic OK and is sufficient to account for the lens-induced refractive response. Whereas corneal thickening is an additional factor reported in myopic OK, this was not the case in hyperopic OK. Constraint of corneal surface change mechanisms to para-central corneal epithelial thinning alone in hyperopic OK may explain the reduced refractive effect compared with myopic OK.
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