Review of in vitro studies on the biocompatibility of NiTi alloys

Nickel-Titanium (NiTi) alloys exhibit special mechanical properties known as shape memory effect and superelasticity. The first effect relates to the fact that after deformation at low temperature, the material regains its original shape after heating. The second effect denotes an unusual flexibility of the material that is far greater than the flexibility of common metallic materials. These properties can be put to excellent use in various biomedical applications, such as orthodontic wires, orthopaedic implants for osteosynthesis, stents for various applications, bone substitution materials and experimental systems for scoliosis correction. The purpose of the present review is to analyze in vitro NiTi biocompatibility compared to other titanium alloys. The problem of Ni's possible toxicity is discussed. Corrosion properties in an oral environment are investigated. The effect of surface chemical composition and surface roughness on cell adhesion, morphology and proliferation is shown as an example. Sterilization is taken into account and its influence on roughness and mechanical properties, chemical composition, wettability and bacterial adhesion inhibition is evaluated. The conclusion is that NiTi is at least as biocompatible as conventional titanium due to the TiO2 oxide layer, and that only in special chemical conditions can Ni diffuse out of the NiTi material.