Effects of zinc and strontium substitution in tricalcium phosphate on osteoclast differentiation and resorption

Bone replacement materials must be able to regulate both osteoblastic synthesis of new bone and osteoclastic resorption process in order to maintain the balance of bone remodeling. Osteoclasts generate from differentiation of mononuclear cells. In the present study, we have studied osteoclast-like-cell responses (differentiation from mononuclear cells and resorption) to beta tricalcium phosphate (beta-TCP) doped with zinc (Zn) and strontium (Sr). Osteoclast-like-cell differentiation and resorption was studied in vitro using an osteoclast-like-cell precursor RAW 264.7 cell, supplemented with receptor activator of nuclear factor kappa beta ligand (RANKL). Morphological and immunohistochemical analysis confirmed successful differentiation of the osteoclast-like-cells on the doped and undoped beta-TCP substrates after 8 days of culture. Cells on the substrate surface expressed specific osteoclast markers such as; actin rings, multiple nuclei, tartrate-resistant acid phosphatase (TRAP) synthesis, and vitronectin receptors. However, quantitative TRAP assay indicated the inhibitory effect of Zn on osteoclast differentiation. Although, Zn doped beta-TCP restricted osteoclast-like-cell differentiation, the samples were resorbed much faster. An increased resorption pit volume was noticed on Zn doped beta-TCP samples after 28 days of culture compared to pure and Sr doped beta-TCP. In this work, we demonstrated that beta-TCP bone substitute materials can be successfully resorbed by osteoclast-like-cells, where both osteoclast-like-cell differentiation and resorption were modulated by Zn and/or Sr doping-a much needed property for successful bone remodeling.