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The load-transfer mechanism and stress patterns in the acetabulum reconstructed with a threaded cup were investigated relative to the normal hip using the Finite Element Method (FEM). Several models were applied in the analysis to investigate the effects of axisymmetric 3-D versus 2-D approaches, inclusion of the femoral head, and local stress patterns around the threads in three types of implant/bone bonding modes. It was found that the threaded ring behaves as a relatively rigid implant shielding the trabecular bone and enhancing load transfer through the cortical shells. Load transfer from ring to bone is concentrated at the first and last threads where the subchondral bone layer is penetrated. Stress peaks of up to 24 MPa occur in these regions.

The load transmission and interface stresses in the Wagner resurfaced femoral head were evaluated for the purpose of studying possible failure mechanisms. We found that unnatural stress patterns occur in the head and at the implant-bone interfaces, in addition to regions of stress protection in the bone, possibly enhancing interface failure and bone remodelling. However, these stresses are not higher than those reported for other kinds of prostheses, e.g. acetabular cup, tibial plateau. From these findings, together with clinical observations, it is hypothesized that the femoral surface cup is more sensitive to local loosening than other prostheses. This hypothesis would indicate that prosthetic designs should be analysed relative to their potential to provoke failure propagation, rather than only initiation of mechanical failure and loosening.

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