Mechanical effect of posterior wire or half-pin configuration on stabilization utilizing a model of circular external fixation of the foot

Background: Numerous studies have addressed biomechanical characteristics of circular external fixation of long bones. The objective of the present study was to evaluate stabilization of a simulated foot model using external fixation with either calcaneal tensioned stopper wires or half-pins. Methods: Fixation configurations of the calcaneus included two parallel wires, two wires crossing at either 30° or 45° a 4-mm- and 5-mm-diameter single half-pin, or two half-pins inserted at a cross-angle of either 45° or 90°. All frames were tested in axial compression, anteroposterior (AP) bending, and mediolateral (ML) bending. Results: An increase in wire cross-angle improved the axial and AP bending stabilization but had no influence on ML bending. Utilization of a single calcaneal half-pin instead of two cross-wires resulted in a considerable reduction in ML bending stabilization. Frame configurations with two half-pins substantially improved axial and ML bending stabilization. Due to the medial location of the metatarsal wire stopper, an increase in half-pin cross-angle significantly improved ML bending stiffness under lateral foot loading. Under the medial foot loading, however, the half-pin cross-angle had no effect on ML bending stabilization. Replacement of cross-wires with two half-pins significantly improved the AP bending stiffness only when the half-pin cross-angle was reduced to 45°. In all modes of two half-pin frame loading, the half-pin diameter had a substantial effect on foot stabilization. Conclusions: Although the wire cross-angle, half-pin cross-angle, and half-pin diameter affect the stability of foot circular external fixation, the influence of these mechanical parameters on foot stabilization is dependent on the mode and location of loading. Clinical Relevance: The results of the present mechanical testing can be utilized as a useful guideline for the optimization of circular external fixation of the foot.