THE DESIGN OF A TINI ACTUATOR IN AN INTRAMEDULLARY LEG LENGTHENING DEVICE

Today's medical technology makes it possible to increase leg length fo r people with leg length discrepancies or excessively short limbs. Wit h the Ilizarov method bones can be gradually elongated (max. 1 mm/day) without implantation of bone grafts or multiple operations. Although the operative procedure is relatively simple, the negative side effect s for the patient are considerable. An external fixator is mounted to the bone. The fixation is made by pins through the skin. Amongst the d isadvantages of the external fixator are possible infection of the bon e or soft tissue, minimal weight bearing and restricted possibility fo r wearing clothes. A design for a fully implantable extractor is propo sed in order to eliminate these disadvantages for the patient. An actu ator of TiNi alloy is chosen for inducing the displacement of 1 mm/day . The advantages of such an actuator are the biocompatibility and the small physical dimensions needed, where the maximum diameter of the me dulla is only 13 mm. By using the Two Way Shape Memory Effect (TWSME) a very compact and simple actuator is designed. The most important fac tors in the design of the TWSME-actuator are the elongation, working f orce and transformation temperatures of the alloy. The TWSME-training method for the actuator used in our tests showed a sufficient generati on of force and elongation. Differential Scanning Calometry measuremen ts showed a shift in the transformation temperatures due to the traini ng. This is a result of induced internal stresses by the training. Nei ther the amount of temperature shift nor the efficiency of the trainin g were correlated to pre-training annealing temperatures (500 degrees C-700 degrees C).