gms | German Medical Science

Question: Fracture healing depends on both biological properties, such as blood supply, cellular immune response, availability of osteoinductive cytokines, and biomechanical stimuli. Shape memory orthopaedic implants made from nickel-titanium (NiTi) might allow modulating fracture healing, changing their cross-sectional shape by employing the shape memory effect, and thereby changing their stiffness. This study aims to show the feasibility and safety of contact-free electromagnetic induction heating of NiTi implants in a rat model.

Method: 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 and positioned between the M. quadriceps and the femur. 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 hours after the procedure. IL-1, IL-4, IL-10, TNF-α, and IFN-γ were measured. Animals were sacrificed at 3 weeks. Histological specimens from the hind leg and liver were retrieved and examined for inflammatory changes, necrosis, and corrosion pits.

Results and conclusions: A minimum output power (P) of 3 to 4 kW was needed to achieve a temperature increase of 30°K of the implant. Adequate loop control aiming for fast increase and little exaggeration of the temperature was achieved at an integral action coefficient (Ti) of 75 to 100 and a transfer coefficient of 750. Figure 1 [Fig. 1] shows a temperature protocol of an induction series carried out with a P of 4 kW and Ti of 100.