In vitro loading of the proximal femur has improved our understanding of stress shielding after total hip arthroplasty. However, previous load simulators often use simplified loading regimens that may not produce physiologic baseline strains. The purpose of this study was to compare the femoral strain levels produced when using simplified and more complex loading.

A mechanical load simulator was developed which could simultaneously apply a spinal load and nine of eleven available muscle loads to the proximal femur in heelstrike and stair climbing modes. Computer controlled electromechanical actuators were attached to a strain gauged fresh cadaver femur (donor body weight 39 6kg) with metal cables. A spinal load of 668 N (SPL) was applied alone and in combination with individual muscle loads of 267 N to determine the effect of each muscle on femoral strain. The magnitude and direction of the joint reaction force (JRF) was monitored in real time by a three-dimensional force transducer proximal to a metal acetabulum. Anterior, middle and posterior portions of the gluteus medius (ABD), iliotibial band (ITB), short external rotators (SER), vastus lateralis, adductors, rectus femoris, hamstrings, iliopsoas, and gluteus maximus were simulated.

SPL was applied and ABD and ITB were adjusted to produce a JRF magnitude of 2.0 BW. SPL was applied with two combinations of nine muscle loads adjusted in heelstrike mode to produce a JRF magnitude of 2.0 and 2.5 BW and JRF trajectory aligned within one degree of the radiographically determined compression trabecular stream axis.

Both nine-muscle combinations produced lower medial compression strains and substantially lower lateral tension strains than SPL+ABD+ITB in heelstrike and stair climbing. Simplified loading caused a bending moment in the proximal femur resulting in higher strains. Combined loading at 2.5 BW produced compression at 10 of 12 gauges in heelstrike mode and 9 of 12 gauges in stairclimbing.