Purpose: To investigate the changes in, and recovery of, posterior corneal curvature after 6 months of overnight orthokeratology (ortho-k). Methods: Twenty-eight healthy young adults were recruited for a 6-month period of ortho-k treatment and data from their right eyes were analyzed. The mean +/- standard deviation spherical equivalent refraction (SER) at baseline was -2.95 +/- 0.88 D. Posterior simulated keratometry (Sim K) readings were measured with a corneal topographer based on rotating Scheimpflug imaging. The three phases in the study were the 6-month treatment period (Phase I); diurnal changes over a period of 8 h immediately after lens removal at the completion of the treatment period (Phase II); and a 2-month recovery period after cessation of treatment (Phase III). Measurements were taken after lens wear overnight, and after 1 week, and 1, 2, 3 and 6 months of lens wear in Phase I. In Phase II, measurements were taken immediately, and then 2, 4 and 8 h after lens removal. In Phase III, corneal parameters were monitored 1 week, 2 weeks, 1 month and 2 months after cessation of ortho-k treatment. Results: In Phase I, the posterior Sim K readings were significantly steepened after the first overnight lens wear. These significant changes were not found at other visits. In Phase II, the posterior Sim K readings were the steepest immediately after lens removal and significantly flattened 2 h after lens removal. In Phase III, all the posterior Sim K readings were similar to the baseline results. Conclusions: Steepening of the posterior cornea was only observed immediately after lens removal. It returns to its original shape within 2 h after cessation of lens wear. These changes appear to be in line with recent reports of the diurnal variation in the posterior corneal shape in non-contact lens wearers. The reduction in myopia from ortho-k treatment is therefore mainly due to a flattening of the anterior cornea.
