FACTORS INFLUENCING THE ACCURACY OF THE SRK FORMULA IN THE INTRAOCULAR-LENS POWER CALCULATION

Several intraocular lens (IOL) power calculation formulas (either theo retical or empirical) are used to determine the emmetropic IOL power) The Sanders-Retzlaff-Kraff (SRK) linear regression formula is among th e most widely recognized empirical ones. In the present study intraocu lar lens power calculation aiming at emmetropia was performed, using S RK formula, in 145 cataractous eyes undergoing lens implantation. The final refraction was evaluated at 8 to 12 months after surgery. The pu rpose of this study was the identification and quantitative evaluation of the factors which influence significantly the accuracy of SRK in t he intraocular lens power calculation. The following factors were stud ied: (1) the error in preoperative biometry with regard to the differe nce between post and preoperative axial length measurements, (2) the p osition of the implantation of the intraocular lens (anterior versus p osterior chamber), (3) the intraocular lens style, (4) the intraocular lens power level, (5) the preoperative corneal astigmatism, (6) the s urgically induced corneal astigmatism, and (7) the postoperative astig matism. Multiple regression and stepwise regression analysis showed a strong correlation (R(2) = 0.65; p < 0.001) between postoperative refr active error (Rf) and error in preoperative biometry (Delta AL), surgi cally induced corneal astigmatism (SIA) and postoperative astigmatism (Ap) only. This correlation is expressed by the following equation: Rf = 0.07 - 2.55 Delta AL - 0.42 SIA + 0.34 Ap. This equation indicates the quantitative effect of each factor on the accuracy of the SRK form ula, by defining the pattern of the fluctuations of the amount or stat e (myopic or hyperopic) of refractive error induced by changes of vari ables Delta AL, SIA and Ap.