Titanium particles generated from orthopedic and dental implants are suspected to play an important role in peri-implant osteolysis. Elucidation of the mechanisms involved in the effects of titanium particles on osteolysis may provide methods for preventing osteolysis. Therefore, in the present study, we investigated the effect of titanium particles on osteoclast activity in vitro. We evaluated bone resorption pits on dentin slices and osteoclast F-actin rings after treatment with titanium particles. The influence of titanium particles on the expression of TRAP, Cat K and CA II mRNA was also evaluated. We found that osteoclast bone resorption activity was enhanced at a lower concentration of titanium particles, but inhibited by a higher concentration of titanium particles. Titanium particles can be phagocytosed by osteoclasts, but the F-actin ring was not destroyed after phagocytosis. In addition, a lower concentration of titanium particles enhanced the mRNA expression of TRAP and Cat K, while higher concentrations of titanium particles decreased their expression. By contrast, the mRNA expression of CA II was decreased at all concentrations of titanium particles. These results suggest that direct exposure to titanium particles at a low concentration enhances osteoclast activity, while exposure to a high concentration inhibits their activity. This understanding of the direct effect of titanium particles on osteoclast activity suggests that aseptic loosening is caused not only by the generation of osteoclasts, but also by enhanced osteoclast activity.

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