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.