KERATOMETRIC INDEX, VIDEOKERATOGRAPHY, AND REFRACTIVE SURGERY

Purpose: To clarify the confusion resulting from the use of slightly d ifferent refractive indices in calculations related to optical modelin g of the cornea for refractive surgery, corneal diagnostics, and catar act surgery. Setting: Scientific Institute H.S. Raffaele, Milan, Italy . Methods: The cornea is represented as a centered optical system comp osed by 1, 2, or 3 spherical interfaces, in progression of modeling ac curacy. Optical analysis is performed with the usual formulas of parax ial geometrical optics as well as with ray tracing. Simple models are also provided for corneas having both incisional and photoablative ref ractive surgery. Values of geometrical parameters are taken from the G ullstrand eye model. Results: Using the keratometric index of refracti on of 1,3375 is validated for estimating optical power differences on untreated corneas or after incisional keratotomy. It is not as accurat e in assigning absolute values of dioptric power, where the value 1,33 15 is more appropriate. For photorefractive keratectomy (PRK), however , the proper stromal index of refraction, 1.376, must be used for abla tion calculations and dioptric change estimates. Conclusion: Videokera tographic instruments should include three distinct values of refracti on index (1.3375, 1.376, and 1.3315) for an accurate and complete char acterization of dioptric power maps. In cataract surgery, corrections must be introduced in the calculation of intraocular lens power for pa tients who have previously had PRK.