Cytotoxic effects of cobalt and nickel ions on osteocytes in vitro

BACKGROUND: Metal-on-metal prostheses undergo wear and corrosion, releasing soluble ions and wear particles into the surrounding environment. Reports described early failures of the metal-on-metal prostheses, with histologic features similar to a Type IV immune response. Mechanisms by which metal wear products and metal ion causing this reaction are not completely understood, and the effects of metal ions on osteocytes, which represent more than 95% of all the bone cells, have not been also studied. We hypothesized that soluble metal ions released from the cobalt-chromium-molybdenum (Co-Cr-Mo) prosthesis may have cytotoxic effect on osteocytes. METHODS: MLO-Y4 osteocytes were treated with various metal ion solutions for 24 and 48 h. The effect of ion treatment on cytotoxicity was assessed by WST-1 reagents and cell death ELISA. Morphological changes were analyzed by a phase-contrast microscope or fluorescent microscope using Hoechst 33342 and propidium iodine staining. RESULTS: Cr and Mo ions did not cause cell death under 0.50 mM, highest concentration studied, whereas Co and Ni ions had significant cytotoxic effect on MLO-Y4 cells at concentrations grater than 0.10 mM and at 0.50 mM, respectively, in a dose-dependent manner. According to the ELISA data, osteocytes treated with Co ions were more susceptible to necrotic than apoptotic cell death, while Ni ions caused osteocyte apoptosis. The morphological assays show that cells treated with Co and Ni ions at high concentration were fewer in number and rounded. In addition, fluorescent images showed a marked reduction in live cells and an increase in dead osteocytes treated with Co and Ni ions at high concentration. CONCLUSIONS: Metal ions released from metal-on-metal bearing surfaces have potentially cytotoxic effects on MLO-Y4 osteocytes, in vitro.