This paper reports on the development of a metacarpal phalangeal prosthesis with an embedded NiTi pseudoelastic spring. The device was originally developed using a special steel spring. The use of a shape memory alloy was suggested by the observation of failures in the hinge structure of the prosthesis. In more detail it was frequently observed a permanent deformation of the stainles steel spring. In a large number of cases this was not due to the activation of the spring in the normal deformation range but by occasional overdeformation of the prosthesis.
A critical event in this respect is the installation surgical act itself.
Pseudoelastic NiTi withstands larger deformation without a permanent set.
Here we will not focus on the development of the prosthesis as a whole but on the characterization of the NiTi alloy used. One of the main difficulties encountered is related to the fatigue life of the springs which should ensure at least a 10⁷ cycles life. Even if a dependence from the test temperature was expected to some extend the results confirmed that this parameter is of foremost importance.
Results obtained by differential calorimetric measurement before the cycling procedure and on passed (i.e. cycled more than 10⁷ cycles) or broken components will be presented and discussed.