A stress analysis of acetabular reconstruction in protrusio acetabuli

We are reporting the results of a finite-element analysis of acetabular reconstruction for total hip replacement in the presence of protrusio acetabuli. In a protruded acetabulum, cortical bone stresses on the medial part of the pelvic wall increase with medial placement of the acetabular component, while normal placement of the component (more lateral placement) reduces these stresses. Metal backing of a polyethylene acetabular component causes a reduction in the peak cement and trabecular-bone stresses. A metal protrusio ring about only the periphery of the acetabular component increases stress levels within the lateral part of the pelvic cortex and has little effect on stresses in the medial part of the pelvic wall. A complete metal protrusio cup increases stresses in the lateral part of the pelvic cortex while decreasing substantially the stresses in the medial part of the cortex and the trabecular bone. Prosthetic reinforcement of the medial part of the acetabular wall has little effect on stress patterns in the acetabular region.

The major long-term problem with cemented total hip prostheses is loosening. Loosening is probably related in part to the stress state in the cement and surrounding bone. The protruded acetabulum is particularly difficult to reconstruct in a manner that ensures longevity of the total hip replacement. In patients with protrusio acetabuli, the prosthetic acetabulum should be placed in a normal and not in a protruded position. A metal-backed acetabular component or a complete metal cup incorporated within the cement reduces stress levels within the medial aspect of the pelvic bone and thus may reduce the incidence of loosening.