Langenbucher A, Schrecker J, Eppig T, et al. - In this investigation, a Monte-Carlo simulation based on biometric data was used to analyse the influence of biometric values on the ratio between the refractive power of the phakic lens and the manifest spectacle refraction. A multidimensional probability density distribution was constructed using a data set of over sixteen thousand biometric measurements, including axial length, phakic anterior chamber depth, corneal equivalent, and astigmatic power. Fifty thousand artificial data sets were used to calculate the phakic lens spherical equivalent and astigmatism needed for emmetropization, and to determine the corresponding ratios for these two values. When selecting high refractive power phakic IOLs, fixed spherical and astigmatic ratios should not be used because their variation can result in refractive errors of up to ± 0.3 D for an 8 D lens. Using a linear regression based on the refractive spherical equivalent and the axial position, both ratios can be estimated with clinically acceptable precision.