Comparison of spherical equivalent determined by Sanders-Retzlaff-Kraff-II (SRK-II) formula with target post-operative final refraction in patients undergoing cataract surgery

The most challenging subject in patient satisfaction after cataract surgery depends upon post-operative refractive outcomes. Accurately predicting post-operative refraction is necessary during the calculation of Intraocular lens (IOL) power. The Sanders-Retzlaff-Kraff (SRK) II formula is one of the few formulae being used for the calculation of IOL power. Our study is to compare the accuracy of the SRK-II formula with targeted final refraction of patients undergoing phacoemulsification. This study was performed on 102 eyes that underwent phacoemulsification. IOL power was calculated using the SRK-II formula. One and a half months after surgery, refraction was performed. Differences between actual post-operative refraction and predicted spherical equivalent was calculated and termed absolute refractive error. Paired t-test was used for comparing actual post-operative refraction and predicted spherical equivalent.The mean absolute error (MAE) of the SRK-II formula was 0.44 D (diopters) overall. The percentage of eyes within ±0.5 D of the predicted spherical equivalent was 81%. Patients having post-operative refractive error within ±1.0 D for short, average, and long axial length were 88% (P value 0.21), 92% (P value 0.0009), and 91% (P value 0.18) respectively.We found that the SRK-II formula was accurate and reliable for predicting the post-operative refractive error in eyes with normal axial lengths as the post-operative refraction is near emmetropia and is a good method for IOL power calculation with normal axial lengths.