Cemented total hip arthroplasty (THA) has enjoyed widespread success since its introduction over thirty years ago. Despite the successes, there are threats to the long term stability of THA, chief among them aseptic loosening of the femoral stem. Femoral component loosening is a significant problem because it prevents the efficient transfer of hip joint forces through the cement to the proximal femur. This may cause pain in the patient severe enough to warrant surgery to replace the loose component.

The predominant approach to preventing loosening of cemented THA femoral components focuses on improving the bond between the prosthesis stem and the acrylic bone cement. This is done by using a stem with a rough surface or by pre-coating the stem with bone cement. An alternative approach is to incorporate stem-cement interface debonding into prosthesis design. A stem of this type is the subject of this study.

This study has two components. First, bulk specimens of Simplex P bone cement were subjected to compressive creep tests. Second, polished, tapered, cemented stems were implanted in cadaveric femora and subjected to physiologic loads. During loading, cement mantle hoop strains and axial displacement of the stem within the cement mantle were measured. Stem pull-out tests were performed before the specimens were loaded and after each load case. Finally, the cement mantles were examined for damage.

The compressive creep tests of Simplex P bone cement indicate that this cement has a relatively high resistance to creep. Measurement of bone cement hoop strains, stem subsidence and pull-out strength indicate that the stem debonds from the cement, that the cement mantle will experience creep under physiologic loading and boundary conditions and that the stem will undergo distally directed axial displacement within the cement mantle. The combined effects of this behavior lead to a mechanical taper lock between the stem and cement mantle that provides a means of stable fixation. Analysis of the cement mantles from the tested specimens indicates that cement mantle damage varies between formulations of bone cement. This indicates that choice of bone cement may be an overlooked variable in total hip arthroplasty.