The purpose of this study was to derive a modified equation for contact lens method (CLM) in calculating post myopic laser refractive surgery corneal power. Methods: A total of 93 subjects who underwent myopic laser refractive surgery at IIUM Eye Specialist Clinic were recruited. The accuracy of postoperative corneal power using the standard CLM and newly-derived contact lens modified method (CLMmod) were compared to the standard comparison method ; the historical method (HM). The CLMmod equation was derived by adjusting postoperative corneal power of CLM according to amount of refractive change. Results: The mean postoperative corneal power using standard CLM was significantly higher than HM (mean difference: -0.24 D, p < 0.001). Fifty seven percent (n = 53 eyes) of the standard CLM results were within ±0.50 D of HM results. The difference between postoperative corneal power using standard CLM and HM increased significantly with the amount of refractive change (r = 0.835; p < 0.001). The mean postoperative corneal power of CLMmod showed that there was no statistical significant difference compared to the HM results (mean difference: 0.00 D, p= 0.964). Eighty eight percent (n = 82 eyes) of the CLMmod results were within ±0.50 D of HM results with improvement of 31% from the standard CLM results. Conclusion: The CLMmod equation provides more accurate calculation in determining post myopic laser refractive surgery corneal power. In near future, this modified equation can be used as an alternative equation to calculate postoperative corneal power when the preoperative data is unavailable.

The purpose of this study was to evaluate inter-session repeatability, inter-examiner reproducibility and inter-device agreement of corneal power measurements from manual keratometer, autokeratometer, topographer, Pentacam high resolution and IOLMaster. Methods: Two sets of mean corneal power measurements (n=40) were compared for inter-session repeatability and inter-examiner reproducibility in each instrument. Repeatability and reproducibility were evaluated by within-subject standard deviation (Sw), coefficient of variation (COV) and intraclass correlation coefficient (ICC). A oneway repeated measures analysis of variance was conducted to compare differences in the corneal power between each instrument pair. The Bland and Altman analysis and Pearson’s correlation were employed to assess agreement and determine strength of relationship between measurements. Results: There were no significant differences in mean corneal power measurements between 2 different visits (p > 0.05). The Sw and COV values between 2 visits were lower than 0.09 D and 0.20 % respectively. The ICCs were stronger than 0.99 in all instruments. For reproducibility of each instrument, differences of the measurements between 2 different examiners were also insignificant (p > 0.05). The Sw and COV values between 2 examiners were lower than 0.11 D and 0.23 % respectively. The ICCs were 0.99 and above in all instruments. The 95% limit of agreement between instruments ranged from -0.29 to 1.13 D and the r-values were stronger than 0.84. Conclusion: The corneal power measurements using these 5 instruments were repeatable and reproducible. These instruments can also be used interchangeably, however the topographer should be used with caution.