Electromagnetic induction heating of an orthopaedic nickel–titanium shape memory device

Abstract

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.

A Preliminary Study of Bending Stiffness Alteration in Shape Changing Nitinol Plates for Fracture Fixation

“…For instance, in the first few weeks in vivo after initial stabilization, 5 this mechanical change would have to depend on the type of fracture, its location and co-morbidities. 14 It will be the goal of future studies to specify reliable recommendations of the variance of these parameters on plate stiffness. The results to be determined would provide the recommendations in a clinical setting to best know when and how much the stiffness of fixation should be altered to promote the best healing outcome.…”

“…11 Changing the construct rigidity of such devices over the course of fracture healing could promote accelerated healing as well as avoiding additional invasive procedures. 14 The goal of this study was to measure the stiffness changes of Nitinol implants of different design before and after transformation, i.e. after initiation of the one way shape memory effect (SME).…”

Noninvasive induction implant heating: An approach for contactless altering of mechanical properties of shape memory implants

“…Similar experiments were shown by Mohr et al as they used the inductive heating (f = 258 kHz) of magnetic nanoparticles in thermoplastic polymers [10]. Short time heating of cylindrically NiTi-SMAs in biological tissue (f = 250 kHz), has been shown by Müller et al [30]. General problems of this method can be seen in the direct exposition of living tissue to high-frequency fields resulting in an unintended heating of the tissue (e.g.…”

Biomechanik und Implantate (2)

“…It was shown that the temperature-controlled in-vivo heating of cylindrically NiTi test specimens causes only marginal effects on biological tissue [3]. It was shown that the temperature-controlled in-vivo heating of cylindrically NiTi test specimens causes only marginal effects on biological tissue [3].…”