Anchoring and resulting primary stability of a kinked compared to a straight uncemented femoral stem.

INTRODUCTION: The number of revision hip arthroplasties being performed is growing and implantation of a cementless stem has become established as the gold standard. For producing a primary stability, the press-fit procedure is the method of choice, but also can be achieved by multiple-point impactions. Specific femoral stems should follow the anatomical shape and provide a more extensive anchorage. The objective of this study was to evaluate the type, localization of the impaction and resulting primary stability of two different femoral revision stem designs (kinked vs. straight) after implantation via an endofemoral approach in the case of more extensive defects of the proximal femur.

MATERIALS AND METHODS: Cementless stems of two different designs were implanted in synthetic femurs. The specimens were analyzed by CT and tested considering axial/torsional stiffness and migration resistance in a servohydraulic testing machine.

RESULTS: The present data do not show any significant differences between the two endofemorally implanted conical stems in contact area or in biomechanics with regard to migration and axial or torsional stiffness, despite having different designs.

CONCLUSIONS: The location, type and length of the stem anchorage are not only influenced by the kinked or straight design, but in particular also by the surgical approach. Also in the case of an extensive proximal bone defect, in the endofemoral approach, both a conical and a three-point anchorage occur. Here, the length of the conical anchorage determines the primary stability and should be at least 55 mm.