Design rationale and dimensional considerations for a femoral neck prosthesis

Despite the high level of survivorship of cemented and uncemented stemmed components, there is a need for a joint prosthesis that does not invade the femoral canal. This type of prosthesis would be useful in treating younger and more active patients, in whom the use of such a design would usefully prolong the time before a conventional total joint replacement design was necessary. We present the rationale for a femoral neck prosthesis and show its feasibility as an off-the-shelf system based on a morphologic analysis of the proximal femur. The new design concept consists of a barrel with longitudinal flutes that enters the neck of the femur, loads on the calcar, and contacts the lateral cortex below the greater trochanter. Areas of porous ingrowth are placed so that the load transfer between the implant and the bone resembles that of the two main trabecular patterns in the proximal femur. A new method, cluster analysis, was used for defining a set of 12 sizes that could fit a population of femurs with acceptable dimensional discrepancies. We conclude that this type of device could be implanted reproducibly so that it was a close fit against the target areas of bone in the proximal neck area and against the lateral cortex.