Two-dimensional finite element analysis in revision arthroplasty of the hip, using allografts and cement

In revision hip arthroplasty when there is only a limited proximal bone stock available, the surgeon may use bone grafts to create a neo-medullary cavity to obtain a better fit of the stem. Our experimental model was the Exeter-type hip prosthesis, used in revision surgery. A thin cement layer (± 2 mm) is used to fill the gaps between bone grafts and stem. Clinical experience has shown that a well-defined trabecular structure with specific orientations develops within the initially amorphous bone grafts. We have investigated the correlation between the trabecular orientations in the bone grafts and the principal stress trajectories. A two-dimensional equivalent thickness finite element model was made of a proximal femur with bone grafts, a thin cement layer and a metal stem. The interfaces between cortex and bone grafts and between bone grafts and cement were assumed to be bonded. The interface between cement and stem was either simulated by non-linear gap elements (once with and once without friction), either assumed to be bonded. The results show good correlation between trabecular orientations (observed on a frontal X-ray picture) and the stress trajectories in the bone grafts. The agreement was most pronounced proximo- medially.