Interface fixation analysis of artificial joints

Implant/cement interface stresses, which affect the quality of implant fixation to the surrounding materials and therefore the long-term performance of the total joint replacement, are addressed. It is noted that previous modeling approaches result in stresses that are discontinuous at the interface between adjacent elements. Three distinct formulations that yield accurate continuous stresses as well as displacements are presented. These are a displacement-based interface element with an extra node on the interface, a penalty-modified compatible formulation, and a mixed stress-displacement formulation. Results for a tibial fixation model subjected to a nonaxisymmetric compression load are presented. The results are predicted using the penalty-based continuous stress formulation. The model has been analyzed for two types of loading conditions. One condition consists of two loaded areas, simulating a situation when the femur is exerting pressure evenly on the two tibial condyles. The second condition consists of a single loaded area simulating the loading of only one condyle. Results for the latter case are reported. The proposed formulations have performed satisfactorily in the test examples and are, therefore, considered as reliable tools to predict the interface mechanics in the total joint replacement.<>