The prediction of urgically induced refractive change from corneal topography

Purpoe: To develop a method to predict the refractive power of the cornea from corneal topography. Method: We reviewed preoperative and pot-operative cycloplegic refraction, keratometry, and corneal topography in 40 eye of 40 patient who had undergone photorefractive keratectomy, radial keratotomy, myopic keratomileui in itu, or hyperopic lamellar keratoplaty. For each axial dioptric power map, we calculated the apheric ellipoid that bet fit that map. Central corneal point were weighted more heavily than peripheral point, baed on the tile-Crawford effect. The equation of the bet-fit ellipoid yielded the pherical and atigmatic power and axi for each cornea preoperatively and potoperatively. Reult: The preoperative corneal pherical and atigmatic power meaured by the bet-fit method were conitent with the pherical and atigmatic power meaured by keratometry and imulated keratometry. The change in corneal pherical power predicted by the bet-fit method wa ignificantly (P< .05) more accurate at predicting the change in pherical equivalent refraction than change either in pherical equivalent keratometry or in pherical equivalent imulated keratometry. The prediction of the atigmatic change wa le precie than that of the pherical, but the bet-fit method wa the mot accurate. Concluion: The bet-fit method i more accurate than imulated keratometry and tandard keratometry are in evaluating corneal refractive power after refractive urgery. An improved method of calculating corneal refractive power may facilitate ubjective refraction after refractive urgery, improve the accuracy of intraocular len power calculation for eye that have had previou refractive urgery, and improve ablation profile for excimer laer refractive urgery.