Abstract Nickel-titanium shape memory metal Nitinol (NiTi) is a new kind of implant material, which provides a possibility to prepare functional implants activated at body temperature and withstands kinking better than conventional metals. Applications utilizing these unique properties are a target of active research interest. Host reactions to NiTi and to the forces created by functional implants should also be studied. A functional NiTi intramedullary nail, which causes a bending force on the bone, was developed for correcting bone deformities. In the present studies, the action of the device was inverted to induce a bone deformity instead of correcting one, in order to test the hypothesis that bone modelling can be controlled using such functional nail. Implanting the nail into the medullary cavity of rat femur for twelve weeks caused bowing of the bone, retardation of its longitudinal growth, and thickening of the bone and the cortex. In another study the effects of functional and straight nails were compared. Bowing of the bone and significant overall thickening of the bone and the cortex were associated only with the functional nail, while the straight nail induced only minor thickening of the bone. Retardation of longitudinal growth was seen in both groups, and this may have been caused by perforating the distal epiphyseal plate during the nailing. Finite element model of the bone-nail combination was also created. Porous NiTi was studied as a bone graft substitute by filling a bone defect in the distal femoral metaphysis of a rat bone with porous NiTi implants of different porosities. After 30 weeks, porosity of 66.1% (mean pore size (MPS) 259μm) showed the best bone-implant contact (51%). However, porosity of 46.6% (MPS 505μm) with 39% bone-implant contact was not significantly inferior in this respect and showed a significantly lower incidence of fibrosis within the implant and thus seemed to be the best choice for a bone graft substitute, out of the porosities tested here. The porosity of 59.2% (MPS 272μm) showed lower contact values. NiTi tendon suture material was studied by implanting NiTi sutures into rabbit tendon and subcutaneous tissues for two, six, and twelve weeks. NiTi proved to be stronger than polyester, which served as control material. The encapsulating membrane was minimal with both materials, suggesting good biocompatibility in tendon tissue. The implantation did not affect the strength properties of either material. On the basis of the present studies, NiTi provides a possibility to develop new kinds of implants for correcting bone deformities, for filling bone defects in weight-bearing locations and a good candidate for a tendon suture material.

Abstract Whenever a foreign material is implanted into a human body an implant–tissue interface area forms between them. In this microenvironment, interactions take place between the implant and the surrounding tissue. The implantation of a biomaterial into tissue results in injury and initiation of the inflammatory response. This host response to biomaterials is an unavoidable series of events that occur when tissue homeostasis is disturbed by the implantation process. In bone tissue, biocompatible implants must initially be capable of strong bone implant contact and subsequently, allow the normal bone remodeling cycle around the implant. NiTi is a metal alloy composed of approximately a 50:50 ratio of nickel and titanium. It possesses shape memory and superelasticity properties, which make it an interesting biomaterial. NiTi has two phases: austenite and martensite. A decrease in temperature or applied stress induce the austenite-to-martensite transformation. Heating or removing the stress restores the parent austenite phase. The alloy in its martensite structure can be reshaped and strained several times more than a conventional metal alloy without irreversible deformation of the material. The alloy returns to its original shape as it changes from martensite-to-austenite. This transformation is seen as the macroscopic shape memory effect. This study further investigated the biocompatibility of NiTi, especially the bone cell response to both austenite and martensite. Different surface treatments were investigated in order to improve and possibly even control NiTi's bioactivity as a bone implant material. Osteoclasts grew and attached well on the austenite NiTi phase, but the results indicated that the biocompatibility of martensite NiTi was compromised. Oxidation of the NiTi surface improved osteoblast attachment and viability. This was due to the formation of a TiO2 surface layer of moderate thickness. Coating the NiTi surface with the extracellular matrix protein fibronectin was shown to enhance osteoblast proliferation and increase the number of cells in the G1 cell cycle stage. Austenite was more prone to show these effects than martensite. A sol-gel derived titania-silica surface treatment was observed to increase the bone implant contact of functional NiTi intramedullary nails. The surface treatment was most effective with the constant bending load provided by the NiTi nail.