Introduction: Impalement of the anterior compartment musculature remains a problem in the hybrid external fixation of distal tibial fractures. The purposes of this study were to develop a tensioned wire configuration which does not violate the anterior compartment and to analyze the biomechanical implications of new wire configuration.
Materials and methods: Thirty-seven adult volunteers without known pathology around either tibia were recruited. Axial computed tomography of the distal tibia was performed at 5-mm slices from the plafond to the upper margin of the syndesmosis. The wire convergence angle was measured at the 1-, 2-, and 2.5-cm levels using the following landmarks: tibialis anterior tendon (TA), tibialis posterior tendon (TP), peroneus brevis tendon (PB), anterolateral border of the lateral malleolus (LM). Two straight lines were drawn by connecting TA and PB and connecting TP and LM. The wire convergence angle was defined as an acute angle between these two lines. Then the orientation of the bisector axis of the wires was measured. As a second part of this study, a validated three-dimensional hybrid external fixator model was developed using finite elements modeling to analyze the stiffness of the frames constructed according to the measured wire convergence angle and orientation. Five simulated configurations were tested. The stiffness of each frame was analyzed under four load conditions: torsion, axial compression, side bending, and anteroposterior bending.
Results: The mean convergence angle was 30 degrees irrespective of the level. The bisector axis was oriented towards the anterolateral direction about 20 degrees from the coronal plane. The stiffness of the frame constructed with a wire convergence angle of 30 degrees and an anterolateral wire orientation of 20 degrees was 20-30% less than that of the frame constructed with 60 degrees wires oriented in a coronal plane. The addition of an anteromedial half-pin increased the stiffness significantly.
Conclusions: Two tensioned wires may be placed without violating the anterior compartment by using the above four clinically identifiable landmarks. Addition of a half-pin on the anteromedial surface of the distal articular fragment makes the frame markedly stiffer than is possible using the standard wire configuration.