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This research aimed to develop and validate a new method for calculating anterior and posterior corneal topography. The novel calculation method was then used to evaluate the influence of orthokeratology (OK) lens wear and overnight edema on posterior corneal shape, and to develop a physical model for anterior and posterior corneal shape changes during myopic overnight OK. The new method uses anterior corneal topographic data derived from the Medmont E300 corneal topographer, and total corneal thickness data measured along the horizontal corneal meridian using the Holden-Payor optical pachometer. The data are then entered into an Interactive Data Language computer program, which calculates the anterior and posterior corneal apical radii of curvature and Q (asphericity) based on the calculated anterior ellipsoid curve, the locations of corneal pachometry measurements, direction of thickness measurements, and the local radius of curvature. Pachometry data were optimised based on the local radius of anterior corneal curvature and an exact solution for the relationship between real and apparent thickness. The newly developed method was validated using a range of test surfaces and repeatability was also investigated with five adult subjects. Eighteen young adult subjects wore BE OK lenses overnight only in both eyes for 14 days. Ten young adult subjects participated as a control group, wearing J-Contour RGP lenses in one eye only for one night. OK subjects were assessed on days 1, 4, 7, and 14, and control group subjects on day 1, in the morning and evening. Subjective refraction, visual acuity, corneal topography and corneal thickness were measured. Most change in refractive error occurred within the first 7 nights of overnight OK. This rapid reduction in myopia was associated with improvement in unaided VA, and flattening of anterior corneal curvature. There were no changes in posterior corneal apical radius but an increase towards an oblate posterior corneal Q was found during 14 days of overnight OK, at both morning and evening measurements. Fixed anterior corneal points were determined at 8.13mm and 5.30mm chords at morning and evening measurements in the OK group. The points of maximum increase in anterior corneal elevation at evening and morning measurements were at 6.90mm and 7.00mm chords respectively. In the OK group, significant backwards movement of anterior and posterior corneal apices was found relative to the fixed anterior corneal points at the morning measurement. Backwards movement of the anterior corneal apex remained during the day whereas the posterior corneal apex recovered to baseline. In conclusion, our results support the theory that the OK refractive effect is achieved through remodelling of anterior corneal layers, without overall corneal bending. Central corneal thinning in the evening after overnight OK lens wear is primarily due to to posterior movement in space of the anterior corneal apex. The posterior direction of overnight corneal edema across the cornea in OK lens wear is consistent with previous models that predict that the cornea swells in a posterior direction during edema. |
