A comparative study of the initial stability of cementless hip prostheses.

In the process of prosthetic loosening, the initial motion of the implant relative to the bone is a key element. In order to provide quantitative information, three straight and one curved cementless hip stems were submitted to in vitro tests in autopsy specimens of the femur under dynamic axial and torsional loads with the displacements measured in several directions. The results were compared to those of an additional group of cemented straight stem prostheses providing reference values. Subsidence of cementless implants on the average was significantly higher than the cemented reference, but single implants were comparable to the reference. Rotational motion was distinctly higher than the reference values, with considerable differences between designs. When comparing displacements perpendicular to the main load direction, the sagittal plane motion was found to be at least double the motion in the frontal plane. Micromotions at the prosthesis/bone interfaces under the load case selected were smallest for the curved and highest for one of the straight prostheses.