Internal fixation of femoral neck fractures with posterior comminution: a biomechanical study.

OBJECTIVES: This study was performed to determine whether four cancellous lag screws provide significantly improved rigidity and fixation strength compared with three screws for fixation of displaced femoral neck fractures with posterior comminution.

DESIGN: Biomechanical cadaver study.

INTERVENTION: Eight pairs of mildly osteopenic femurs were selected, and each pair was fixed with three or four cancellous lag screws (randomly assigned) after the creation of a simulated femoral neck fracture with posterior comminution. A separate comparison with an unmatched group of six similar femurs with a simulated femoral neck fracture without posterior comminution and instrumented with three screws was performed to investigate the effect of posterior comminution.

MAIN OUTCOME MEASUREMENT: The specimens were non-destructively tested to determine fixation rigidity in axial and anterior loading. Cyclic axial loading was then performed for 10,000 cycles; the femurs were retested for rigidity and finally were axially loaded until failure.

RESULTS: The femurs with a posterior defect stabilized with three screws had significantly less resistance to axial and anterior displacement and sustained significantly lower axial loads to failure than those stabilized with four screws. The specimens instrumented with three screws without a posterior defect exhibited greater resistance to displacement in anterior loading and sustained greater axial loads to failure than those with a posterior defect stabilized with three screws.

CONCLUSION: This study suggests that there are benefits to using four screws for fixation of femoral neck fractures with posterior comminution.