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BACKGROUND: Corneal refractive surgical procedures alter the shape and structure of the tissue, possibly compromising its mechanical stability. METHODS: One or both eyes of 87 subjects were studied: 26 of these 87 had been treated for myopia by excimer laser ablation and 36 by radial keratotomy; 25 without previous corneal surgery functioned as controls. Corneal topography was evaluated by TMS-I videophotokeratography before and after 180° tilting of the patient, and the pressureinduced ring-wise changes in spherical equivalent power (axial power) were calculated. Changes in corneal shape also were evaluated by computing the instantaneous radius of curvature. The intraocular pressure was measured before and after tilting by a hand-held applanation tonometer. RESULTS: The mean intraocular pressure increased from 13.9 + 2.3 nun Hg before tilting to 30.0 ± 3.8 mm Hg during tilting. In the control eyes, the mean power of the central cornea during tilting decreased 0.187 + 0.045 diopters (D) (p<.05); in the excimer-laser-ablated eyes, 0.038 ± 0.056 D (not statistically significant); and in those treated with radial keratotomy, 0.523 ± 0.054 D (p<.01). After radial keratotomy, the cornea steepened outside the clear zone in response to pressure loading, whereas it did not change significantly in the laserablated or control eyes. CONCLUSIONS: Pressure-induced deformation of normal and excimer laser-ablated human cornea is small, whereas radially incised corneas have significantly decreased mechanical stability. [J Refract Surg. 1996;12:459-466.]

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