Three-dimensional primary stability of cementless femoral stems

Objective. This study investigates by means of a new bone-prosthesis interface motion detector whether conceptual design differences of femoral stems are reflected in their primary stability pattern. Design. An in vitro experiment using a biaxial materials testing machine in combination with three-dimensional motion measurement devices was performed. Background. Primary stability of uncemented total hip replacements is considered to be a prerequisite for the quality of bony ongrowth to the femoral stem. Dynamic motion as a response to loading as well as total motion of the prosthesis have to be considered under quasi-physiological cyclic loading conditions. Methods. Seven paired fresh cadaveric femora were used for the testing of two types of uncemented femoral stems with different anchoring concepts: CLS stem (Spotorno) and Cone Prosthesis (Wagner). Under sinusoidal cyclic loading mimicking in vivo hip joint forces a new measurement technique was applied allowing for the analysis of the three-dimensional interface motion. Results. Considerable differences between the two prostheses could be detected both in their dynamic motion and total motion behaviour. Whereas the CLS stem, due to the wedge-shaped concept, provides smaller total motions, the longitudinal ribs of the Cone prostheses result in a substantially smaller dynamic motion. Conclusions. The measuring technique provided reliable and accurate data illustrating the three-dimensional interface motion of uncemented femoral stems.