Cellular responses to metal ions released from implants

In the process of calcified tissue formation, cells secrete a protein-rich matrix into which they add a metal ion that nucleates in the presence of phosphorus to form an inorganic salt (usually calcium hydroxyapatite). Cellular and tissue responses to metal ions-released from implants, for example-can therefore be considered from the perspective of how cells handle calcium ions. A critical factor in determining cellular toxicity will be free ion concentrations and the competitive interactions that occur in a physicochemical manner. Three of the parameters used to assess the biocompatibility of implant materials are (1) the ability to influence mitotic activity, (2) intercellular adhesion, and (3) promotion of cell death. A spectrum of responses to free intracellular calcium ions can be identified, ranging from presence of the ion being essential for cell division through to an excess of the free ion that results in cell death (apoptosis). In between these extremes, cells may become postmitotic and express phenotypic variations as they adapt to their environment and establish equilibrium to maintain intracellular calcium homeostasis. The response of cells to implants can be linked to ions released and interactions between these and other ions and/or molecules present in the tissues, similar to the manner in which cells handle calcium ions.