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

Objectives: This study was performed to determine whether four cancellous l ag screws provide significantly improved rigidity and fixation strength com pared with three screws for fixation of displaced femoral neck fractures wi th posterior comminution. Design: Biomechanical cadaver study. Intervention: Eight pairs of mildly osteopenic femurs were selected, and ea ch pair was fixed with three or four cancellous lag screws (randomly assign ed) after the creation of a simulated femoral neck fracture with posterior comminution. A separate comparison with an unmatched group of six similar f emurs 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 nondestructively tested to det ermine fixation rigidity in axial and anterior loading. Cyclic axial loadin g was then performed for 10,000 cycles; the femurs were retested for rigidi ty and finally were axially loaded until failure. Results: The femurs with a posterior defect stabilized with three screws ha d significantly less resistance to axial and anterior displacement and sust ained significantly lower axial loads to failure than those stabilized with four screws. The specimens instrumented with three screws without a poster ior defect exhibited greater resistance to displacement in anterior loading and sustained greater axial loads to failure than those with a posterior d efect stabilized with three screws. Conclusion: This study suggests that there are benefits to using four screw s for fixation of femoral neck fractures with posterior comminution.