Predicting the refractive outcome and accuracy of IOL power calculation after phacoemulsification using the SRK/T formula with ultrasound biometry in medium axial lengths

Abstract

Purpose: To evaluate the accuracy of the SRK/T formula using ultrasound (US) biometry in predicting a target postoperative refraction of +/- 1.00D in eyes with medium axial length (AL) that underwent phacoemulsification. Methods: The present study was a retrospective analysis, which included 538 eyes with an AL from 22.0 to 24.60 mm that underwent phacoemulsification and foldable intraocular lens (IOL) implantation (six different IOLs) in the bag. Preoperative AL was measured by US biometry and IOL power (IOLp) was calculated with the SRK/T formula. Patients had a complete ophthalmic examination, preoperatively and 1, 7, and 30 days after surgery. The achieved spherical equivalent (SE) and the prediction error (PE) were calculated. The prediction error was defined as the difference between attempted predicted target refraction and the achieved postoperative SE refraction. Statistical analysis was performed with SPSS V21. Results: The mean age of the patients was 66.96 +/- 9.67 years, the mean AL was 23.29 +/- 0.62 mm, the mean K1 was 43.62 +/- 1.49D, the mean K2 was 43.69 +/- 1.53D, the mean IOL power was 21.066 +/- 1.464D, the mean attempted (predicted) SE was -0.178 +/- 0.266D, and the mean achieved SE was -0.252 +/- 0.562D. The mean PE (difference between predicted and achieved SE) showed a relatively hyperopic shift (mean +/- standard deviation: 0.074 +/- 0.542D, ranging from -1.855 to 2.170D, P=0.001). A total of 93.87% of eyes were within +/- 1.00D of the PE and 92.75% of eyes within +/- 1.00D of achieved postoperative refraction. A total of 39 eyes (7.25%) had a refractive surprise. A total of 32 of 39 eyes were more myopic than -1.00D and 7 of them were more hypermetropic than +1.00D. There was no correlation between the mean PE and IOL type, AL, K1, K2, and IOLp. There were a positive statistically significant correlation between PE and age (r=0.095; P=0.028) and a negative statistically significant correlation between achieved SE and AL (Spearmans r=-0.125; P=0.04), and age (r=-0.141; P=0.01). Conclusion: The IOLp calculation using the SRK/T formula with US biometry may demonstrate very good postoperative refractive outcomes in medium eyes with a few refractive surprises.