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. Relevance Analysis of different concepts of prosthetic stems with the presented standardized approach may contribute to future design improvements of hip replacement. With respect to the clinical outcome the role of primary stability for the long-term success will furthermore be elucidated. (C) 1997 Elsevier Science Ltd.