Tinting effect of ultraviolet radiation on intraocular lenses of polymethyl methacrylate.

Intraocular lenses (IOL) made of polymethyl methacrylate (PMMA) lack an important yellowish pigment useful as a filter in the visual process and in the protection of the retina from short wavelength light. It was found that the PMMA used for IOL manufacturing can be tinted by dry exposure to mercury near-UV emitting lamp (emission maximum, 365 nm, light bandwidth +/- 140 nm; irradiance of 50-100 W/sq m as measured at the PMMA surface; time of exposure 70-100 h, room temperature). The UV irradiated samples were stored in the atmosphere of N2 under the heating-protected and clean conditions. The IOL sample holder allowed to remove IOL loops and their fixation areas from the zone of the passing light. The absorptive properties of IOLs treated with UV light were similar to those of young human lenses. Raman vibration and fluorescence spectral analyses of IOLs have shown that the yellow colour and its intensity in the irradiated samples depends on the presence of conjugated C = C and C = O groups (pi-pi conjugation) in the chemical composition of PMMA. When the PMMA samples were exposed for 70 h to a high level of UV radiation (50-100 W/sq m) no damage was seen with scanning electron microscopy at both side surfaces of the IOLs. The PMMA water exhaustive extracts made by 70 h of UV radiation exposure did not show any cytological damage when injected into the cell cultures of fibroblasts. The threshold for optical breakdown in PMMA was detected by 100 h of UV radiation at the level of exposure equivalent to at least 20,000 times levels for expected ambient UV exposure to PMMA within the eye. A rigorous quality index defined as the ratio of the line C = C/C = O stretching mode intensities was introduced for the UV-absorbing PMMA photostability. The findings suggest an applicability of the photochemical tinting and further research to test the efficacy and safety of UV-absorbing chromophore induction in the PMMA IOLs.