Open Access

These days, medical applications of shape memory alloys (SMAs) can be found in cardiovascular devices, gastroenterology and urology as well as in the area of orthopedic implants, orthodontic devices and clinical instrumentation. Their functional properties combined with excellent biocompatibility increase the possibility and the performance of minimally invasive surgeries. Overviews of existing applications can be found in [1-2]. Within the medical field, most of the applications with shape memory (SM) material take advantage of the superelasticity of NiTi SMAs. In contradiction to the superelastic or mechanical SM effect, the application described in this study uses the thermal SM effect for a new medical implant. Before explaining the SM driven intramedullary bone nail in detail, a short introduction to the bone elongation technique is given. The background of this application on based in the medical fact that normally any tissue reacts to an injury with repair and healing processes through multiplication of cells. If after a transverse osteotomy a strain stimulus is activated, for example by tensile stress, this multiplication of cells and new formation of tissue may be continued for any length of time. Due to this mechanism, even considerable loss of bone caused by fractures or congenital defective positions, may be compensated without bone grafts. The technique of callus distraction by means of external fixation or intramedullary nail stimulates the formation of callus in the bone gap. Callus is the repair tissue of the bone generated in the fracture gap in case of bone fracture or osteotomy. The gap to be bridged should not be wider than 1 mm per day [3]. The process starts with the exudation of callus around the ends of the broken bone. At first, callus is more like a fibrous tissue. Later it hardens due to deposition of calcium and eventually it is converted into true bone. Three weeks after severance, the vascular system is formed. An overview of current bone lengthening techniques, also called callus distraction, can be found in [3]. External systems are normally used for the extension of bones, the bone fragments being fixed on rings by wires. The decisive disadvantages of those external systems are primarily the considerable risk of infection due to protruding wires, noticeable discomfort for the patient because of the external rings, a coarse cosmetic result because of scarring, as well as rather long hospitalization. Therefore, internal bone extension systems are of great interest to orthopedic surgery.