Mechanical compatibility of noncemented hip prostheses with the human femur

It is generally accepted that more flexible implants are needed to reduce stress shielding and postoperative thigh pain. However, there is no detailed information on the stiffness of currently used implants relative to the human femur. The purpose of this study was to determine the stiffness characteristics (bending, torsional, and axial) of human femora relative to commercially available prostheses as a first step in assessing the mechanical compatibility of the implants. This was achieved by computerized tomography scanning of a collection of human femora from proximal to distal at 10 mm intervals, digitizing the cross-sectional contours, and calculating the stiffness characteristics of each section using standard beam theory. The results show that significant stiffness mismatches exist, especially for larger stem sizes and for stems fabricated from cobalt-chrome alloy. Interestingly, certain implant stiffness values are lower than those of the femur for stems up to 15 mm in diameter, substantially so if the implant is made from titanium alloy and incorporates design features that reduce area and moments of inertia. The data suggest that only larger implant sizes need to be adjusted for increased flexibility compared with current stands.