A biomechanical study of 150° vs. 135° hip screws in femoral neck fractures

Femoral neck fractures have been difficult to treat with nonunion and avascular necrosis reported in 10% to 40% of cases. The hypothesis tested here was to determine whether a femoral neck fracture fixed with a 150° screw is more stable than that fixed with a 135° screw. The objectives were to measure axial stiffness, torsional stiffness, and failure strength of femoral neck fractures (Pauwels III). Also, CT derived bone mineral density (BMD in units of K₂KPO₄ mg/cc equivalent) was measured. The following results were obtained: 1) The 150° hip screw demonstrated greater axial stiffness, as well as a preferred mode of failure (impaction at the fracture site). This is less likely to lead to nonunion or avascular necrosis than is the shearing action at the fracture site with the 135° hip screw. 2) The 135° hip screw demonstrated greater resistance to rotation. 3) CT derived BMD did not correlate with either axial or torsional stiffness of intact or fixed bones