Simulated Keratometry Versus Total Corneal Power by Ray Tracing: A Comparison in Prediction Accuracy of Intraocular Lens Power.

PURPOSE: To investigate the relationship between corneal powers measured by simulated keratometry (Sim-K) and ray tracing and understand whether their difference influences intraocular lens (IOL) power calculation.
METHODS: In a first sample of healthy eyes, corneal curvature was measured using a rotating Scheimpflug camera (Sirius, CSO). Sim-K was obtained from anterior corneal curvature using the 1.3375 keratometric index. Ray tracing was performed through both corneal surfaces to calculate the total corneal power (TCP). The difference between Sim-K and TCP was correlated with various parameters. In a second sample of patients undergoing cataract surgery, IOL power was subsequently calculated using both Sim-K and TCP.
RESULTS: In the first sample (114 eyes), Sim-K (43.64 ± 1.44 D) was higher than TCP (43.07 ± 1.41 D, P < 0.0001); the difference ranged between 0.07 and 1.95 D and correlated with the anterior/posterior (A/P) ratio (r = 0.7292, P < 0.0001), which ranged between 1.10 and 1.30. In the second sample (107 eyes), the A/P ratio influenced the outcomes of the Holladay 1 and SRK/T, but not Haigis and Hoffer Q formulas. However, using TCP, which takes the A/P ratio into account, did not improve the prediction error of any formula.
CONCLUSIONS: Sim-K provides a higher corneal power compared with TCP. This difference is not constant but depends on the A/P ratio and can influence the refractive outcome of IOL power calculation by theoretical formulas. However, TCP values do not improve the results of these formulas, as they were developed for keratometric values such as Sim-K.