Torsional fixation of a modular femoral hip component.

Standard range of motion (S-ROM) modular total hip femoral stem has been designed to achieve precise fit in the diaphysis and metaphysis. This study compares torsional fixation of the S-ROM stem implanted with tight fixation of both the metaphyseal and diaphyseal components, tight metaphyseal and loose diaphyseal fixation. The mean load to failure was significantly greater in those implanted with tight fixation proximally and distally. Rotational micromotion was significantly decreased by fixation proximally, but permanent rotational displacement was improved primarily by distal fixation. Tight proximal and distal fixation were necessary to control both micromotion and permanent rotational displacement caused by torsional loading. Clinical success of this implant will depend on achieving tight proximal and distal fixation.