title: Posterior corneal curvature change and recovery after 6 months of overnight orthokeratology treatment.
abstract: OBJECTIVE
  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.
authors.affiliation: School of Optometry, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China. 05901208r@polyu.edu.hk
authors.forename: Davie
authors.initials: D
authors.lastname: Chen
authors.forename: Andrew K C
authors.initials: AK
authors.lastname: Lam
authors.forename: Pauline
authors.initials: P
authors.lastname: Cho
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D003261:Contact Lenses / Q000592:standards*; D003315:Cornea / Q000473:pathology*; D019781:Corneal Topography; D005260:Female; D006801:Humans; D008297:Male; D009216:Myopia / Q000503:physiopathology / Q000628:therapy*; D054158:Orthokeratologic Procedures / Q000379:methods*; D013997:Time Factors; D014792:Visual Acuity; D055815:Young Adult