|Title:||The changes of posterior corneal curvature, topographic corneal thickness and corneal biomechanical properties from overnight orthokeratology lens wear and recoveries of these parameters|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||School of Optometry|
|Pages:||xiii, 178 leaves : ill. ; 30 cm.|
|Abstract:||Orthokeratology (ortho-k) is a clinical technique to temporarily reduce refractive error (mainly for myopia correction) using specially designed rigid lenses. Anterior corneal flattening, central corneal thinning and mid-peripheral corneal thickening have been documented in myopic ortho-k treatment. Researchers assumed that the posterior corneal curvature was not affected and therefore adopted the Munnerlyn's formula to predict the ortho-k effect. In contrast, overall corneal bending involving the posterior corneal curvature has been reported to account for the ortho-k changes. To date, limited information regarding the posterior corneal changes after ortho-k treatment has been reported. Also, the recovery of topographical corneal thickness has not been fully studied after successful ortho-k treatment, even though changes of corneal thickness were previously demonstrated. On the other hand, measurement of corneal biomechanical properties required sophistical calculations. Clinical measurement of these properties (in terms of corneal hysteresis: CH, and corneal resistance factor: CRF) are now possible which may play a role to predict the efficacy of ortho-k treatment. The long-term corneal biomechanical changes from ortho-k and its recovery after the treatment have not been previously studied. This project aimed to evaluate the effect of ortho-k treatment on the posterior corneal changes, topographical corneal thickness, and corneal biomechanics variations so as to understand the mechanism of ortho-k treatment. The recoveries of these changes were also evaluated. In this project, three studies [two short-term studies (up to one overnight) and one long-term study (up to 6 months)] were undertaken. For the short-term studies, the corneal changes after 15, 30, 60 minutes, and one overnight of ortho-k lens wear were evaluated. Short-term study I involved wearing ortho-k lenses in both eyes whereas in short-term study II, one eye (treatment eye) wore ortho-k lens and the fellow eye (control eye) wore conventional alignment fitted lens. The long-term study consisted of three phases, including a 6-month treatment period (Phase I), monitoring of diurnal changes after immediate lens removal (up to 8 hours) on the day after 6-month treatment period (Phase II), and a 2-month recovery period after cessation of successful treatment (Phase III).|
In the short-term study I, significant posterior corneal steeping was found after 60 minutes and overnight ortho-k lens wear. There was significant reduction of CRF after overnight ortho-k lens wear. Significant central corneal thinning and mid-peripheral corneal thickening were found after 60 minutes and one overnight ortho-k lens wear respectively. The posterior corneal steepening has been confirmed from the short-term study II where similar posterior corneal steepening was found in the treatment eyes only. Significant corneal thickening was also found at mid-peripheral cornea after one overnight ortho-k lens wear with no significant change in the control eyes. In the long-term study (Phase I), significant posterior corneal steepening was seen only at the first overnight visit. A significant reduction of CRF was found after one week of lens wear and gradually reduced throughout the treatment period. Significant central corneal thinning was shown after one month of lens wear while mid-peripheral thickening was shown after overnight and one-week visits only. During the monitoring of diurnal changes (Phase II), the posterior cornea was found steepest after immediate lens removal but significantly flattened 2 hours after lens removal. Corneal biomechanical properties were stable throughout the day. The cornea was the thickest at mid-peripheral regions after immediate lens removal and significantly reduced 2 hours after lens removal. In the 2-month recovery period (Phase III), posterior corneal curvatures, topographic corneal thickness, and CRF returned to baseline level one week after cessation of lens wear. The present study clearly shows that ortho-k treatment has transient steepening effect on the posterior corneal curvatures, which disappeared 2 hours after lens removal. Its contribution to myopia reduction was not significant clinically. The topographic corneal thickness changes are associated with the transient posterior corneal steepening. Also, the ortho-k treatment has some short-term effects on the corneal biomechanical properties. On the other hand, all these changes from ortho-k treatment are reversible and are able to return to baseline level one month after cessation of successful ortho-k treatment. It could further evident that the ortho-k treatment has no permanent influence on these corneal parameters.
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