Effects of hip posture on the frontal impact tolerance of the human hip joint

The pattern of left- and right-side hip injuries to front-seat occupants involved in offset and angled frontal crashes suggests that hip posture (i.e., the orientation of the femur relative to the pelvis) affects the fracture/dislocation tolerance of the hip joint to forces transmitted along the femur during knee-to-knee-bolster loading in frontal impacts. To investigate this hypothesis, dynamic hip tolerance tests were conducted on the left and right hips of 22 unembalmed cadavers. In these tests, the knee was dynamically loaded in the direction of the long axis of the femur and the pelvis was fixed to minimize inertial effects. Thirty-five successful hip tolerance tests were conducted. Twenty-five of these tests were performed with the hip oriented in a typical posture for a seated driver, or neutral posture, to provide a baseline measure of hip tolerance. The effects of hip posture on hip tolerance were quantified using a paired-comparison experimental design. In six pairs of tests, one side of each cadaver was tested with the hip joint oriented in the neutral posture and the contralateral hip from the same cadaver was tested with the hip joint adducted 10 degrees from the neutral posture. In four pairs of tests, the hip was tested in neutral and 30 degrees flexed postures. The average fracture tolerance of the hip in the neutral posture was 6.1-/+1.5 kN. Hip tolerance decreased by an average of 34-/+4% with 30 degrees of flexion from the neutral posture (p<0.0001) and by 18-/+8% with 10 degrees of adduction from the neutral posture (p=0.008).