Failure of an uncemented acetabular prosthesis - a case study

Severe wear and aseptic loosening in an uncemented acetabular prosthesis have been observed in a revision surgery carried out at King Edwards VII hospital by L. Taylor and P. Heaton-Adegbile, twelve years following the primary total hip replacement operation. The superior-lateral wall of the polyethylene liner and part of the titanium cup were found to be completely worn out, such that the ceramic head was in direct articulation with the titanium cup.A three-dimensional finite element model was developed. The polyethylene liner was modelled with the outer surface of the liner fully constrained to represent the much stiffer metal cup. Contact analyses were performed between the articulating surfaces under physiological loading conditions, including normal walking, climbing upstairs and downstairs, using the finite element software ANSYS. The results show high initial contact pressure along the periphery of the liner due to the oversize of the femoral head. The maximum contact pressure was found in the superior-posterior quadrant, which correlates well with the location and the direction of the wear. Both wear particles and stress shielding may have contributed to the periprosthetic bone loss and ultimately the late loosening. Reduction of the interference between the liner and the femoral head seems to be effective in the reduction of the initial contact pressure.