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Application of shape memory alloys for active loosening protection of implant structures

Presentation held at Indo-German Workshop "Strategies for improved bone replacement materials and orthopaedic implants: Design - manufacturing - technologies", 19-21 February 2014, Dresden
: Rotsch, Christian; Werner, Michael; Senf, Björn; Grunert, Ronny

presentation urn:nbn:de:0011-n-2817807 (2.1 MByte PDF)
MD5 Fingerprint: 61d191054345f0c7b1373762245ecf65
Created on: 11.3.2014

2014, 15 pp.
Workshop "Strategies for Improved Bone Replacement Materials and Orthopaedic Implants" <2014, Dresden>
Presentation, Electronic Publication
Fraunhofer IWU ()
shape memory alloys; Nitinol; Implantat; SMA

As an alternative to the fixation with cement or by increasing the screw diameter in injured osseous environment a new screw concept was tested. Firstly the topology of the shape memory alloy (SMA) actuators was optimized using a parameter study. The resulting concept represents a compromise between maximal resistance against translational and rotational movement and the complexity of the actuator. It consists of two along the screw axis running SMA sheets made of NiTi alloy which expand fully by body temperature. The actuators were integrated into a screw demonstrator (Figure 1). First tests according to standard ASTM F543 with bone substitutes and alcohol fixated cervical and thoracic vertebrae by tensile were promising. The pullout strength increased from 432 N to 542 N with bone substitutes and from 240 N up to 293 N by using a human vertebra. The feasibility study shows that SMA actuators are suitable to increase the pullout strength of a pedicle screw.
The actuator concept was also used for an adaptive hip stem implant. In non-cemented hip implants, stem loosening can be caused by changes in the applied force and insufficient load transmission between implant and osseous anchor bedding. The target is to achieve a homogeneous force distribution at the implant-bone interface by using SMA. The active components were integrated in hip stems with a new designed surface and inner structures (Figure 2). The results proved that the frictional connection between a stem prosthesis anchored without cement and the femur can be achieved using SMA elements. So there is a potential to stabilize the prosthesis and increase ist lifetime.