We conclude that electromagnetic induction heating of IM NiTi implants is feasible and safe in a rat femur model. These findings reflect a further step in the development of novel concepts for IM fracture fixation that might lead to better fracture healing, less patient discomfort and less need for surgical interventions.
Shape memory orthopaedic implants made from nickel-titanium (NiTi) might allow the modulation of fracture healing, changing their cross-sectional shape by employing the shape memory effect. We aimed to show the feasibility and safety of contact-free electromagnetic induction heating of NiTi implants in a rat model. A water-cooled generator-oscillator combination was used. Induction characteristics were determined by measuring the temperature increase of a test sample in correlation to generator power and time. In 53 rats, NiTi implants were introduced into the right hind leg. The animals were transferred to the inductor, and the implant was electromagnetically heated to temperatures between 40 and 60 • C. Blood samples were drawn before and 4 h after the procedure. IL-1, IL-4, IL-10, TNF-␣, and IFN-␥ were measured. Animals were euthanized at 3 weeks. Histological specimens from the hind leg and liver were retrieved and examined for inflammatory changes, necrosis, and corrosion pits. Cytokine measurements and histological specimens showed no significant differences among the groups. We concluded that electromagnetic induction heating of orthopedic NiTi implants is feasible and safe in a rat model. This is the first step in the development of new orthopedic implants in which stiffness or rigidity can be modified after implantation to optimize bone-healing. © Keywords: shape memory alloy; fracture healing; rat model; NiTi; electromagnetic induction Nickel-titanium (NiTi) shape memory alloys (SMAs) were described as early as 1963. 1 NiTi medical devices have shown great mechanical strength and biocompatibility in vitro and in vivo, and exhibit pseudoelasticity and shape memory effects. 2 Thus far, clinical applications have comprised stents in vascular surgery and gastroenterology, 3 wires and brackets in orthodontic surgery, 4,5 porous implants in intervertebral body fusion, 2 and staples in foot surgery. 6 The shape memory effect is based on a reversibly martensitic phase transformation. Cooling the parent phase austenite to a critical temperature M s (martensite start temperature) causes the monocrystalline structure to transform to twinned martensite. This transformation is finished by reaching the martensite finish temperature (M f ). In this state, the martensite can be mechanically deformed; the maximum reversible deformation (ε) is 8% for NiTi SMAs. If the temperature is raised above the austenite finish temperature (A f ), the martensite converts back to austenite (Fig. 1), 7 and the SMA returns to its initial predetermined state, exhibiting the so-called one-way memory effect.
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