Calcification Potential of Intraocular Lenses (IOL). Predictive in vitro and in vivo models

Opacification of intraocular lenses (IOLs) due to the formation of calcium phosphate occurs because of their contact with the aqueous humor (AH), which is supersaturated with respect to calcium phosphate. Calcification of IOLs was simulated in vitro using a batch reactor in which the IOLs were exposed to solutions simulating AH. Based on the precise and reproducible measurement of the rates of formation of mineral deposits on the IOLs, it is possible to develop reliable screening tests of different types of IOL materials. Depending on their material composition, IOLs are expected to present different active sites for the growth of deposits. Measurements of the kinetics of formation of hydroxyapatite (Ca₅(PO₄)₃OH; HAP) were used to compare the relative tendency of IOL towards calcification. The trend the IOLs tested showed towards calcification was found to correlate with contact angle values. In the case of hydrophobic IOLs, the surface charge calculated from the corresponding zeta potential measurements was low in comparison with the respective high (absolute) values of the mineralizing hydrophilic IOLs, suggesting strong correlation between surface charge and tendency to calcify. The in vitro results align with the in vivo tests, suggesting that in vivo tests can be safely substituted or at least significantly reduced by the in vitro model.