A comparative study of 45 prostheses in vitro shows great variation in the strength of the femoral stems under controlled loading conditions. On the average the MP-35-N prostheses had a higher yield load than the Cr-Co prostheses, which in turn carried more than either the cast or wrought stainless steel devices before yielding occurred. The differences in average yield load for the first 2 types of prostheses were smaller than one would expect based on material strength alone. Thus, the MP-35-N prostheses derive most of their strength from material properties while the Cr-Co prostheses achieve their strength from stem design. The cast stainless steel devices tested possess less sectional rigidity compared to Cr-Co stems and less material strength compared to either the MP-35-N or wrought stainless steel prostheses and generally yield at the smallest loads. Several probable causes for deficient load-carrying ability of the femoral stems have been illustrated. These include areas of stress concentration due to geometry and, in the cast prostheses, grain size variation and internal inhomogeneity. The latter was the cause for the fracture of prosthesis 31 at a load of 2,800 N. This device would probably have suffered a brittle fracture in vivo if it were subjected to overloading beyond its elastic range. The test configuration, however, was not necessarily identical to the dynamic conditions encountered in vivo. Furthermore, since the relationship of the yield strength and the rigidity of the prosthesis to load-sharing, bone resorption and cement failure are not well understood, the strongest and most rigid stem may not be the most desirable for long-term clinical implantation. The exact nature of this relationship awaits further study.
ASJC Scopus subject areas
- Orthopedics and Sports Medicine