Adaptable orthopedic shape memory implants

Abstract

Apart from biological factors, the bone healing process strongly depends on the mechanical properties of the implant applied. The stiffness of the implant influences the micro-motion, one of the most important stimuli of the healing process within the fracture gap. Both, the "ideal" stiffness of an implant as well as the alteration of the mechanical properties over the course of healing are still a topic of research. Apart from biodegradable implants or "fixateur externe", it is not possible to alter the mechanical properties of the implants without second surgery. A "stiffness alterable" implant might have a positive impact on the healing process, e.g. a shorter healing time, an early ability to bear weight or the avoidance of a follow-on operation. In order to adapt the implant to the actual healing situation, nickel-titanium (NiTi) shape memory alloy (SMA) based implants that exhibit a variable middle piece were developed. Due to contactless induction heating, the transition temperature of the SMA is reached and triggers the one-way shape memory effect (SME). This leads to a slight modification of the second moment of inertia and to an adaption of the bending stiffness of the implant, respectively. Based on laser cutting and welding, NiTi-SMA based implants were manufactured by using thin, commercially available NiTi (thickness: 0.5 and 1.0 mm, transition temperature Af between 55-65°C) sheets. Depending on the implant design, 4-Point-Bending measurements showed feasible bending stiffness alterations around a range of 50 %, with regard to the original stiffness. Thereby, two approaches, increasing the bending stiffness and decreasing the bending stiffness, have been realized. Currently, the implant design is being adapted in order to meet the requirements for human applications. Custom made NiTi-SMA with a transition temperature around 45-50°C could continue to reduce the impact of the heating process and expand this concept to further medical applications, e.g. compression, anchoring or expansion of implants or devices.