The design of a TiNi actuator in an intramedullary leg lengthening device

Abstract

Today's medical technology makes it possible to increase leg length for people with leg length discrepancies or excessively short limbs. With the Ilizarov method bones can be gradually elongated (max. 1mm/day) without implantation of bone grafts or multiple operations. Although the operative procedure is relatively simple, the negative side effects for the patient are considerable. An external fixator is mounted to the bone. The fixation is made by pins through the skin. Amongst the disadvantages of the external fixator are possible infection of the bone or soft tissue, minimal weight bearing and restricted possibility for wearing clothes. A design for a fully implantable extractor is proposed in order to eliminate these disadvantages for the patient. An actuator 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 medulla is only 13 mm. By using the Two Way Shape Memory Effect (TWSME) a very compact and simple actuator is designed. The most important factors in the design of the TWSME-actuator are the elongation, working force and transformation temperatures of the alloy. The TWSME-training method for the actuator used in our tests showed a sufficient generation of force and elongation. Differential Scanning Calometry measurements showed a shift in the transformation temperatures due to the training. This is a result of induced internal stresses by the training. Neither the amount of temperature shift nor the efficiency of the training were correlated to pre-training annealing temperatures (500°C-700°C).