Screw placement in subtalar arthrodesis: a biomechanical study.

BACKGROUND: Clinical outcomes of subtalar arthrodesis using screw fixation have been reported but biomechanical support for selection of screw trajectories and patterns has been lacking. This investigation was designed to assess the compressive and stabilizing abilities of different common screw insertion patterns for subtalar arthrodesis.

MATERIALS AND METHODS: Forty-two cadaveric subtalar joints underwent arthrodesis with a single talar neck screw, a single talar dome screw, double parallel screws, or double diverging screws. Single talar dome screw fixation was subdivided by screw tip placement in the anteromedial, posteromedial, anterolateral, or posterolateral dome. Joint compression, construct torsional stiffness, and joint angulation under torsional load were quantified.

RESULTS: Higher compressive force, torsional stiffness, and joint rotation resistance were achieved by double screw fixation compared to single screws (p < 0.001). Torsional stiffness with double diverging screws exceeded that of double parallel screws in internal rotation (p < 0.05). There was greater internal rotation with a single talar neck screw (p < 0.001) and greater external rotation with an anterolateral talar dome screw (p < 0.01) compared to the other single-screw orientations. Greater rotation in both directions was noted with the single screw tip located in the posterolateral talar dome (p < 0.01).

CONCLUSION: Under the described testing conditions, double diverging screws confer the highest compression, the greatest torsional stiffness, and the least joint rotation. When a single screw is used, placement of the screw tip in the talar neck or lateral talar dome should be avoided.

CLINICAL RELEVANCE: The results provide an objective basis for selection of optimal screw placement by the surgeon.