We have found that IOL power is underestimated if corneal power is overestimated, and vice versa. In our simulations, the use of the classical keratometric approach with a keratometric index of 1.3375 led to overestimations of IOL power up to -5.6 D and -6.2 D using Le Grand and Gullstrand eye models, respectively.
An adjusted keratometric approach was defined by our research group consisting of the use of a variable nk, which was dependent on the radius of curvature of the first corneal surface and the eye model used, as summarised in Table 1. With this adjusted keratometric approach, maximal error was within ±1.1 D, with most values ≤ ±0.6 D.
Considering that 1 D of variation of IOL power induces about 0.9 D of change in subjects’ refraction at the corneal vertex, this error can be considered as clinically acceptable, with most of cases not exceeding ± 0.60 D for most r1c-r2c combinations. Only the error was maximal for extreme values.