Biomechanical analysis of in situ single versus double screw fixation in a nonreduced slipped capital femoral epiphysis model.

Slipped capital femoral epiphyses (SCFE) were created in 24 pairs of immature bovine femurs. In 17 pairs of femurs, the slip was left nonreduced (one-third diameter of physis), and in 7 pairs, the slip was reduced. Stabilization of the slips was with either 1 or 2 threaded 6.5-mm screws in a compression mode. The specimens were subjected to shear or torsional loading forces to failure, with the goal of trying to reproduce clinical conditions of in situ screw fixation for acute or unstable SCFE. In the nonreduced model, double-screw fixation was 312% stiffer than single-screw fixation under torsional loading. In the reduced model, double-screw fixation was 137% stiffer than single-screw fixation under torsional loading. The increased rotational stability of double-screw fixation under torsional loading conditions may justify its use in in situ stabilization of acute or unstable SCFE.