Theoretical and clinical effect of preoperative corneal curvature on excimer laser photorefractive keratectomy for myopia.

BACKGROUND: Individual clinical and optical variables may influence the effect of excimer laser photorefractive keratectomy. A theoretical model to describe the influence of initial corneal power, astigmatism, and topography on the expected results of photorefractive keratectomy would be useful in identifying those variables that may ultimately improve the predictability of the procedure. METHODS: Using a mathematical analysis based on the change in sagittal depth of the central ablation zone following photoablation, we predict the effect of initial corneal curvature on the ultimate outcome of a standardized photorefractive keratectomy. Refractive results from the Phase III US Food & Drug Administration clinical trials of photorefractive keratectomy were analyzed to confirm these mathematical predictions. RESULTS: We find that the initial corneal power, theoretically, is not expected to significantly affect the refractive change that results from a given ablation. Similarly, the corneal astigmatism present before photorefractive keratectomy is expected to be only minimally altered by a spherical excimer laser treatment. Clinically, there is no detectable difference in predictability of the procedure amongst groups stratified by initial mean keratometric power. CONCLUSIONS: Our analysis provides a methodology to predict the optical effects of photorefractive keratectomy upon the cornea and may be applied to a variety of hypothetical clinical settings. The predicted lack of clinical association between initial corneal curvature and predictability of photorefractive keratectomy is confirmed.

Matrix methods for the evaluation of lens systems with radial gradient-index elements.

Matrix methods have been used to examine the paraxial performance of homogeneous systems. This method has been extended to include the presence of gradient lens elements in the optical array. To accomplish this, an inhomogeneous rotation-translation matrix is developed to describe the path of a ray through the lens elements with quadratic radial index gradients. The system matrix elements are then used to predict the paraxial properties of the array. Three examples are examined: the Wood lens, two thin lenses coupled to a radial gradient index rod, and a model of the human eye which contains a radial gradient index crystalline lens.

Toward an adaptive model of the human eye.

A model of the human eye is presented with the crystalline lens treated as having a gradient-index structure. By defining an accommodation index I ranging from 0 (unaccommodated) to 1 (accommodated), the optical parameters of the eye in various states of accommodation may be found. The results are in agreement with experimental values.