[Assays of cytotoxicity of the Nickel-Titanium shape memory alloy]

The equiatomic Nickel-Titanium (NiTi) alloy has exceptional mechanical properties such as shape memory and superelasticity. It already has applications in orthodontics and is a promising orthopaedic biomaterial. Cytocompatibility studies must therefore be undertaken. The objective of this study is to determine the biological response that NiTi elicits compared to other orthopaedic metals currently used in orthopaedic surgery. Cytotoxicity tests constitute an efficient first step in a biocompatibility study and contribute to reduce animal use in laboratory. Direct contact and agar diffusion cytotoxicity assays were performed following ASTM standards #F813-83 and #F895-84 respectively. Confluent L-929 fibroblasts culture plates were incubated (directly or under an agar bed) in presence of NiTi, titanium (Ti), vitallium (Co-Cr-Mo) and 316L stainless steel discs. Following exposition to specimens, a vital dye was added to the plates. All cultures were evaluated for cytotoxic reactions, under light microscopy. Direct contact and agar diffusion assays indicated that all metals tested induced a mild biological reaction. Specimens were ranked according to an index of biological response, they are enumerated here in decreasing order of cytotoxicity: NiTi approximately Co-Cr-Mo << pure grade 4 Ti approximately pure grade 1 Ti approximately Ti 6A1 4V approximately 316L stainless steel. Furthermore, plasma surface modification increased the cytocompatibility of NiTi.