The development of modern surgery is notably related to the development of biomaterials. Internal fracture fixation devices, articular prostheses, vascular prostheses and substitute heart valves are examples of breakthroughs in this area.
In the early 1960s, Buehler et al. discovered the shape memory effect in an equiatomic alloy of nickel and titanium (Nitinol, chemical symbol NiTi). Since that time, intensive metallurgical investigations have been made to explore the mechanics of its basic behavior. The use of NiTi for medical purposes was first reported in the early 1970s. In the early 1980s, the idea got more support and some orthodontic and experimental orthopedic applications were released. It was only in the mid-1990s, however, that the first widespread commercial stent applications made their breakthrough in medicine. Currently, NiTi seems to arouse notable interest in the medical and commercial sectors.
NiTi has unique properties that could be very useful in surgical applications. Thermal shape memory, superelasticity and good damping properties make it possible for such alloys to behave differently compared to ordinary implant metals (Buehler et al. 1967, Andreasen et al. 1987). Using its thermal shape memory property, a material sensing a change in external temperature is able to convert to a preprogrammed shape. While NiTi is soft and easily deformable in its lower temperature form (martensite), it resumes its original shape and rigidity when heated to its higher temperature form (austenite). The shape memory effect is based on this temperature-dependent austenite-to-martensite phase transformation on an atomic scale. Within a given temperature range, NiTi can also be strained several times more than conventional metal alloys without being plastically deformed. This superelastic property is also based on martensic transformation (Shimizu et al. 1987).
When applied in certain surgical implants, NiTi is expected to provide radically new functional capabilities, improved performance and a possibility of using minimally invasive techniques. It provides a possibility to make self-locking, self-expanding and self-compressing implants activating at body temperature (Drugacz et al. 1995, Blum et al. 1997, Ryhänen et al. 1998).
Because of the high nickel content of NiTi, it is theoretically possible that nickel may dissolve from the material due to corrosion and cause unfavorable effects. The biocompatibility of NiTi must be very well confirmed before it can be safely used as an implant material. At present, there are not enough conclusive biocompatibility data available on NiTi. Some previous studies revealed few or no biocompatibility problems, but unanswered questions are numerous. The consequences of surface conditions, the dissolution of nickel ions or compounds in vivo after a long period of implantation, the accumulation of trace ions, the responses in different tissues, carcinogenicity, the responses to fracture healing and bone formation, and the effects at the cellular and molecular levels are all issues which require further clarification.
The present studies approach the nickel-titanium shape memory alloy from the basic biocompatibility point of view. The primary acute cytotoxic effects of NiTi on human fibroblasts and osteoblasts, general soft tissue, and neural and perineural responses were evaluated. New bone formation and bone (re)modeling activity as well as the effect of NiTi on the osteogenesis and ossification of the osteotomy defect were studied. The surface corrosion properties and release of trace metals from NiTi were also examined. Common implant materials were used as control materials.