Calcification of poly(2-hydroxyethyl methacrylate) hydrogel sponges implanted in the rabbit cornea: A 3-month study

Poly(3-hydroxyethyl methacrylate) (PHEMA) hydrogels have been used in the p ast as ocular implants. In a recent development, PHEMA sponges have shown s uitable properties as materials fur the peripheral component of an artifici al cornea (keratoprosthrsis). However, the propensity of PHEMA to calcify c ould threaten the long-term stability of the implanted devices. In an attem pt to improve the understanding of the calcification mechanism, the dynamic s, extent, and nature of calcified deposits within PHEMA sponges implanted in the cornea were investigated in this study, and the possible correlation between necrosis of cells and calcification was critically examined. Sampl es of a PHEMA sponge were implanted in rabbit corneas and explanted at pred etermined time points (2, 4, and 12 weeks). The samples were examined by mi croscopy (light, transmission, scanning) and energy dispersive analysis of X-rays. Histological assessment and semiquantitative analysis of the amount of calcium deposited was performed using image analysis. An in vitro exper iment was also performed by incubating sponge samples for 2 weeks in a solu tion of calcium and phosphate ions at a ratio similar to that in hydroxyapa tite, in the absence of cells. Calcification was not seen in the 2- and 4-w eek explants, however, small deposits were detected in two of the 12-week e xplants, both within and on the sponge's constituent polymer particles. The deposit volumes represented 0.094% and 0.21%, respectively, of the total s ponge volumes. Calcium deposits were present in large amounts both within t he constituent polymer particles and on the surface of the sponges incubate d in the abiotic calcifying solution. Cooperative mechanisms are suggested for the calcification of PHEMA sponges in vivo. The initial event may occur at a molecular level, when plasma proteins are adsorbed onto the polymer s urface and bound through chelation to the calcium ions present in the mediu m. After their natural degradation, these structures may act as nucleation sites for calcium phosphate crystallization. Concurrently, the calcium ions can diffuse into the hydrogel particles and then the spontaneous precipita tion of calcium phosphate may be caused by supersaturation due to the levie r content of water in polymer, an effect which is likely predominant in vit ro. The second event is the recruitment of phagocytic cells to clear calciu m debris. Degeneration of these cells may then form nucleation sites for se condary calcification.