The effect of transfixion wire crossing angle on the stiffness of fine wire external fixation: A biomechanical study

To analyse the effect of transfixion wire-crossing angle on the stiffness of fine wire external fixation, a laboratory investigation using a fibreglass tibia fixed into an idealised fixator was performed with a servohydraulic test frame. Load-deformation behaviour was compared at the different wire-crossing angles (30°-90°) under identical conditions of central axial compression, medial compression-bending, posterior compression-bending, posteromedial compression-bending, and torsion. Stiffness values were calculated from the load-deformation and torque-angle curves. The increase in wire-crossing angle led to an overall increase in the stiffness, except medial bending stiffness. The wire-crossing angle of 90° provided significantly greater stiffness than all other angles in all load configurations (p < 0.05) except medial bending. In medial bending, the wire-crossing angle of 30° provided significantly greater stiffness than all the other angles (p < 0.05). Increasing wire-crossing angle from 30° to 90° contributed to an overall increase of 75% in external fixation stiffness, which included axial, torsional, and bending stiffness, but bending stiffness was a function of the wire positioning with respect to the loading axis. Therefore, using the widest possible wire-crossing angle and placing wires as close to the loading plane as possible can increase the stiffness of external fixation.