Adjustable bone fixation of an artificial hip stem by using an intergrated sensor actuator system

The implantation of total hip endoprostheses is a frequent orthopaedic surgery. Therefore, it is necessary to improve the long-term fixation between bone and total hip implant to reduce the number of revision operations. Due to osteolysis the fixation strength between bone and implant can decrease and aseptic loosening can occur. It would be beneficial to adjust the resulting force between bone and implant after surgery, even after several years in order to guarantee primary and secondary implant stability. Our present research approach is predicated on the shape memory effect (SME) and superelasticity (SE) of NiTi. Superelastic sheets are integrated in the geometry of the total hip stem and prestressed by deformed actuating shape memory alloy (SMA) elements. The actuating elements could be activated by thermal energy, which can be provided wireless by a by electro-magnetic coupling using a transponder system. As a consequence of heat input, the actuating SMA elements return to their initial shape and release the superelastic sheets. By differing the sheet thickness it is possible to adjust the contact force/pressure between implant and bone stock. The outer surface of the superelastic sheets is coated and micro structured to improve the friction between bone and implant and to influence the bone ingrowth behaviour. The anchorage of the hip stem has to be monitored after the surgery with a sensor system which is integrated in the femoral head of the implant. Measurements has to be done in regular time intervalls after the implantation. The history of the measurements gives a picture about the progress of the anchorage process. Additive manufacturing by laser beam melting was used to manufacture the femoral stem with complex shaped cavities for later integration of the sensor and actuator components. Static and dynamic mechanical tests and bioinvestigations of the instrumented implant system provided promising results.