Torsion and bending analysis of internal fixation techniques for femoral neck fractures: the role of implant design and bone density.

To evaluate pin/screw/plate fixation for management of femoral neck fractures, 39 proximal femora were tested in both torsion and flexion under physiological loading conditions. Three, four, or five implants of six commonly used multiple-fixation devices, and a sliding hip screw with and without an additional 6.5-mm cancellous screw were examined in paired femora. The intact and postfixation femora were initially subjected to a single applied moment, and the torsion and bending stiffness were determined from the load-deformation data. Postfixation femora were also subjected to cyclic loading in flexion at three load ranges, and fixation was judged successful if no failure occurred on or before 1,500 cycles of 667 to 2,000 N of a combined compressive force and moment. Anterior-posterior and lateral radiographs of each specimen were taken after fixation in order to evaluate Singh's index of bone density, fracture reduction, implant placement, and cross-sectional diameter of the femoral neck. Bone density was also evaluated by computed tomography (CT) and physical measurement of core samples obtained from the femoral head. The results indicate that there appears to be no justification for the use of more than three pin/screw implants for management of femoral neck fractures. Bone density was found to correlate with fracture stability and may be a useful predictor of fixation success.