Comprehensive evaluation of total corneal refractive power by ray tracing in predicting corneal power in eyes after small incision lenticule extraction

PURPOSE: To assess the prediction accuracy of four variations of total corneal refractive power (TCRP) by the ray tracing method in determining corneal power in eyes after myopic small incision lenticule extraction (SMILE). METHODS: Forty eyes of forty patients who had undergone myopic SMILE were enrolled in this prospective study. Manifest refraction and Pentacam HR were performed preoperatively and three months or more postoperatively. Mean keratometry (Km), true net power (TNP), equivalent keratometry readings (EKR) and 4 subtypes of TCRP (pupil centered or apex centered within a ring or a zone)—TCRP(pupil,ring), TCRP(pupil,zone), TCRP(apex,ring) and TCRP(apex,zone)—were recorded and compared to the theoretical postoperative keratometry value using the clinical history method (CHM). RESULTS: The only keratometric values that showed no statistically significant differences from the CHM were 4.0 mm and 4.5 mm EKR, 6.0 mm TCRP(pupil,zone) and TCRP(apex,zone). Pearson’s correlation test revealed that 4.0 mm TCRP(pupil,zone) exhibited the highest correlation coefficient (r = 0.974) followed by TCRP(apex,zone) 4.0 mm (0.972) and EKR 4.5 mm (0.970). The 95% limits of agreement (LOA) of the 4.0 mm EKR and CHM, the 4.5 mm EKR and CHM, the 6.0 mm TCRP(pupil,zone) and CHM, the 6.0 mm TCRP(apex,zone) and CHM were (-1.27 to 1.22 D), (-1.04 to 0.98 D), (-1.39 to 1.08 D) and (-1.38 to 0.96 D), respectively, while the modified 4.0 mm TCR(Ppupil,zone) (TCRP(puil,zone) + 0.70 D) and TCRP(apex,zone) (TCRP(apex,zone)+0.70 D) yielded the narrowest 95% LOA of (-0.96 to 0.95 D) and (-0.96D, 1.05 D). CONCLUSIONS: Total corneal refractive power using the ray tracing method could predict corrected corneal power derived from the CHM in eyes following SMILE surgery after simple modification.