Characterizing microscopic calcification deposits on acrylic intraocular lenses

Abstract

Intraocular lenses (IOLs) are implanted into the eyes during cataract surgery to improve vision. A rare complication following IOL implantation is the formation of crystallized deposits on the lenses, which significantly impair vision, necessitating subsequent surgical IOL exchange. Preventing these deposits from forming is critical, but this requires definitive molecular assignments of the crystals and knowledge of their mechanism of formation. Determining this information presents a significant analytical challenge due to the low numbers of crystals and curvature of the IOLs that limit use of conventional methods. Here, we report the development of multiple complementary analytical methods to characterize IOLs explanted from human patients and attain insight into the chemical identity and mechanism of the deposits. Initial elemental analyses using energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) revealed that IOL crystals contained calcium, phosphorus, and sodium and provided quantitative ratios between elements. Subsequent Raman spectroscopy identified carbonate in the crystals as well. An innovative single-crystal X-ray diffraction (XRD) method with integrated Rietveld analysis was then developed, which conclusively determined that IOL crystals were composed of substituted hydroxyapatite. Collectively, the data obtained from EDS, XPS, XRD, and Raman indicate that IOL deposits are composed of crystalline Ca₉Na(PO₄)₅(CO₃)(OH)₂ layered with amorphous calcium phosphate. This structure is similar to bone, suggesting that a similar ossification mechanism is followed. The robust analytical methods developed herein provide the most comprehensive characterization of IOL crystals to date and signify that the microenvironment of the eye is conducive to bone mineralization pathways that induce crystal formation on IOLs.