Evaluation of the immediate post-operative bone–implant condition of a proximal interphalangeal joint prosthesis by a comparative FEA modeling

The development and the optimization of orthopaedic implants require precise studies involving geometrical, mechanical and material considerations. Moreover, the long term performance of the implant results from a compromise between the kinematic of the movement, the mechanical behavior and the bone tissue interaction. Obviously, the clinical evaluation is the best way to assess the implant performance. Nevertheless, the focus of this paper is to propose a methodology to estimate the immediate post-operative interactions between a proximal interphalangeal (PIP) joint prosthesis and the surrounding bone tissue. A finite element model of two PIP prostheses has been developed: an Ascension PIP pyrocarbon prosthesis and a new prosthesis which is currently under development (prototype prosthesis). Extreme loadings are applied to the bone models, which can be made either of young or old bone tissue: the strain state at the interface between the bone and the implant is compared to bone tissue remodeling curves. An iterative procedure is then adopted in order to improve the design of the prototype stem such that the final strain state is compatible with physiological conditions. This study shows that the strain level is always higher in the pyrocarbon implant than in the prototype prosthesis. Such high strain levels can induce bone necrosis and could explain the clinical observations of radiolucent lines around the implant resulting from a settling to a stable position.