COMPARISON OF CORNEAL POWER AND INTRAOCULAR LENS POWER CALCULATION METHODS AFTER LASIK FOR MYOPIA

Purpose: To evaluate the accuracy of the two methods; the clinical history method (CHM), and the contact lens method (CLM), in refractive corneal power determination and comparing the accuracy of SRK II, SRK T and Holladay II (H II) formulas for intraocular lens (IOL) power calculation in patients who had laser in situ keratomileusis (LASIK) surgery for myopia with or without astigmatism Methods: In this interventional prospective study, we evaluated 46 eyes of 32 patients who had undergone LASIK for myopia with or without astigmatism and were visited at Noor Eye Clinic during September 2001 to September 2006, for cataract surgery. The corneal power was determined with two methods; CHM and CLM. Then, IOL power was calculated with three formulas; SRK II, SRK T and H II. We used the results of CHM+H II formula for IOL power selection. The manifest refraction (MR) one month after cataract surgery was regarded as the postoperative refraction. According to the rule that 1.5 diopter (D) change in IOL power results in 1.0 D change in a patient’s refraction at the spectacle plane, we calculated the expected refractive results with IOL powers based on other corneal power calculation methods and formulas, and compared them to results with CHM+H II formula. Results: The mean difference between keratometry values by CLM with CHM methods was 2.53±2.68 D (Pearson correlation, 0.6; P<0.0001). The mean manifest spherical equivalent (SEQ) was -10.44±3.92 D, before refractive surgery, -0.84±1.33 D (range; 1.00 to -5.00 D) before cataract formation, and -0.46±1.04 D (range; 1.87 to -2.62 D) after phacoemulsification and IOL implantation with CHM+H II. The mean expected post-cataract surgery manifest SEQ using the CHM with SRK II and SRK T, were 1.39±1.15 D and 0.56±0.94 D, respectively. The mean expected post-cataract surgery manifest SEQ, using CLM with H II, SRK II and SRK T, were calculated as 1.91±2.37 D, 2.97±2.17 D, and 2.59±2.07 D, respectively. Conclusion: CHM appears to be more accurate than CLM for true corneal power determination after LASIK surgery for myopia with or without astigmatism. In this series of patients, the most accurate formulas for IOL power calculation were H II, SRK T and SRK II, respectively.