ORIGINAL ARTICLE Posterior Corneal Changes with Orthokeratology HELEN OWENS, PhD, LEON F. GARNER, PhD, FAAO, JENNIFER P. CRAIG, PhD, and GREG GAMBLE, MSc Department of Optometry & Vision Science (HO, LFG), Department of Ophthalmology (JPC), and School of Medicine (GG), The University of Auckland, Auckland, New Zealand ABSTRACT: Purpose. To investigate changes in corneal thickness and the radius of curvature of the posterior corneal surface after orthokeratology (OK) rigid lens wear. Methods. Nineteen young myopic subjects wore reverse-geometry OK lenses (BE/ABE, Ultravision Contact Lenses, Brisbane, Australia) every night for 1 month. Central and midperipheral corneal thickness (Allergan Humphrey ultrasound, Carl Zeiss Meditec, Dublin, CA), topography (EyeSys v.3.1, Houston, TX), subjective refraction, and posterior corneal radii (video photography of Purkinje images) were evaluated within 2 h of waking, prelens wear, and on four occasions postlens wear during a 1-month period. A mixed-models approach was used to analyze the data. We modeled the changes in posterior corneal radius of curvature and corneal thickness in terms of the sagittal height of the anterior and posterior cornea using an ellipsoidal model for the corneal surfaces. Results. Refractive error reduced from 2.28 to 0.01 DS within 1 month. A significant thinning of the cornea was evident between 1 (p  0.03) and 2 weeks (p  0.0048) postlens wear. A significant increase in the anterior corneal radius of curvature was present at all time periods beyond 1 night (p < 0.001), and a significant posterior corneal flattening occurred centrally and midperipherally after 1 week (p  0.04 and p  0.013, respectively). Conclusions. These findings suggest that in addition to the significant topographic flattening of the anterior corneal surfaces, there is also a significant flattening of the posterior surface during the early adaptive stages of OK lens wear. (Optom Vis Sci 2004;81:421–426) Key Words: orthokeratology, posterior cornea, myopia, corneal remodeling, rigid contact lenses, corneal thickness, corneal topography O rthokeratology (OK) was introduced more than 40 years ago as a method to reduce myopia on a temporary basis. During the first two decades since its inception, OK was demonstrated to be so ineffective and unpredictable that the method failed to gain credibility as a viable option for myopia correction. 1–3 However, recent improvements in lens design and technologic advances in the measurement of corneal topography have significantly enhanced the precision and degree of myopia reduction that may be afforded by the technique. 4–6 Lenses previ- ously were typically worn daily to reduce myopia to an acceptable level; thereafter, “retainer” lenses were worn intermittently to maintain the refractive effect. The advent of reverse-geometry de- sign rigid gas-permeable lenses with high oxygen transmissibility has improved the physiologic acceptability of such lenses. Contem- porary OK lens procedures more often include overnight wear in place of daytime wear, and within limits the reduction of myopia may be achieved rapidly 7 and retained throughout the day. 8 Although the processes involved at the cellular level are still uncertain, two alternative theories have been proposed to account for the refractive effect. The first theory involves the anterior cor- neal surface, in which a modification of central and paracentral corneal curvatures and thickness contribute to the overall change in refraction. The second theory predicts a modulation of the anterior and posterior corneal curvatures, causing an overall bend- ing of the cornea. Recent work has favored the former theory, with involvement of the epithelial and stromal layers alone. 9, 10 Uncertainties remain regarding the mechanism associated with OK. The response of posterior corneal surface to OK lens wear has not yet been quantified, and although most agree that the central cornea thins with OK lens wear, there is no consensus of the effect on the midperipheral cornea after this treatment for refractive er- ror. 8, 11 The aims of this work were to assess whether the posterior corneal surface is involved in the initial adaptation of the cornea to overnight OK lens wear using a Purkinje imaging technique, and to measure the changes in corneal thickness and radius of curvature subsequent to this lens wear modality during a 1-month period. 1040-5488/04/8106-0421/0 VOL. 81, NO. 6, PP. 421–426 OPTOMETRY AND VISION SCIENCE Copyright © 2004 American Academy of Optometry DOI: 10.1097/01.opx.0000135097.99877.5d Optometry and Vision Science, Vol. 81, No. 6, June 2004