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D.M. Grzybowski, C.J. Roberts, A. Mahmoud; Proposed Mechanism for Non-Ectatic Change in Posterior Surface Shape After an Ablative Procedure . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2556.
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Purpose: It has been shown that the posterior corneal curvature increases following laser refractive surgery, leading to concerns over ectasia. However, the mechanism driving this increase in curvature has yet to be elucidated. Methods: Three schematic models of possible posterior corneal response mechanisms to laser refractive surgery were developed. Increased curvature in the posterior surface could be a result of 1) isolated central forward bulging, 2) forward displacement over a large region of the posterior surface, or 3) backward peripheral movement into the anterior chamber. A retrospective study was performed to determine which of these models was consistent with posterior corneal behavior observed in 2380 eyes of 1255 patients after a myopic LASIK procedure (Technolas 217) at the Hong Kong Sanatorium and Hospital. Patients received pre-op and 6 months post-op Orbscan (version 2.0) exams. Pre-op posterior elevation maps were subtracted from post-op maps using various fitting protocols, and then regional differences were analyzed for consistency with proposed schematic models. Results: Significant (p<0.0001) increases in central posterior elevation were found for corrections greater than 4 diopters. However, these central increases corresponded with significant peripheral decreases in posterior elevation of similar magnitude. This pattern is consistent with schematic model #3, inward peripheral movement of the posterior cornea, possibly due to swelling. Conclusion: Increases in posterior corneal curvature after LASIK appear to be dominated by backward peripheral corneal swelling into the anterior chamber, rather than forward "bulging" of the central posterior cornea. This is consistent with stable remodeling of the corneal shape due to a surgically-induced change in structure, as opposed to an ectatic event.
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