Intraocular Lens Calculation Concept Based on Aphakic Refraction—Considerations on a Cornea Model With Two Refracting Surfaces

PURPOSE: To develop a concept for predicting the power of an intraocular lens (IOL) and the spectacle refraction after cataract surgery based on aphakic refraction (REFa) using vergence transforms with a two-surface cornea model. METHODS: This simulation study is based on a large dataset originally used in IOLCon ( www.IOLCon.org ) for optimising lens constants. Proxy values for REFa calculated from the available information in this dataset were used to develop a strategy for predicting the effective lens position (ELP). A variety of models, including linear, quadratic, sigmoidal and two stepwise linear models, were evaluated. Using this ELP, vergence transform techniques were applied to derive the IOL power for a given target refraction or the spectacle refraction for a given IOL power. This concept was applied to subsets of the IOLCon dataset with the Hoya Vivinex and the Bausch & Lomb enVista lenses to show the efficiency of the calculation. RESULTS: After evaluating the ELP prediction models on the 22,577 IOLCon data, it was decided to use the sigmoidal model for calculating the ELP corrected by a linear term for the A constant. Compared with the fully disclosed Castrop formula, the predicted ELP showed a root-mean-squared deviation of 0.221/0.221 mm with the Vivinex/enVista lens and the predicted IOL power showed a root-mean-squared deviation of 0.369/0.375 D. CONCLUSIONS: Where biometric data for a classical IOL power calculation are unavailable, this vergence-based calculation concept may help to identify the appropriate IOL power and to predict the spectacle refraction after cataract surgery. A clinical study with 'real' REFa measurements is required to validate these results.