Does a trochanteric lag screw improve fixation of vertically oriented femoral neck fractures? A biomechanical analysis in cadaveric bone.

BACKGROUND: We assessed the biomechanical performances of a trochanteric lag screw construct and a traditional inverted triangle construct in the treatment of simulated Pauwels type 3 femoral neck fractures.

METHODS: An inverted triangle construct (three 7.3-mm cannulated screws placed in inverted triangle orientation) and a trochanteric lag screw construct (two 7.3-mm cannulated screws placed across the superior portion of the femoral neck and one 4.5-mm lag screw placed perpendicular to the fracture in superolateral to inferomedial orientation) were tested in nine matched pairs of non-osteoporotic human cadaveric femora. We used a previously described vertically oriented femoral neck fracture model and testing protocol that incrementally loaded the constructs along the mechanical axis of the femur to 1400 N. Specimens that survived incremental loading underwent cyclic loading. Apparent construct stiffness, force at 3mm of displacement, and survival of incremental loading were recorded.

FINDINGS: The trochanteric lag screw group had a 70% increase in stiffness (261 N/mm [29 standard deviation] versus 153 N/mm [16 standard deviation]; P=0.026) and a 43% increase in force required for displacement (620 N versus 435 N; P=0.018) compared with the inverted triangle group. One trochanteric lag screw and no inverted triangle specimen survived incremental loading.

INTERPRETATION: A trochanteric lag screw construct applied to vertically oriented femoral neck fractures provides marked improvement in mechanical performance compared with the inverted triangle construct.