Biomechanics of olive wire positioning and tensioning characteristics

The effect of the olive wire on the stiffness of external fixation has been studied previously, but there are limited data on the effect of the olive wire positioning and tensioning characteristics on the stiffness of external fixation. This study evaluated the influence of olive wire positioning (medially, laterally, posteriorly, and anteriorly) and tensioning characteristics (tensioning one end or both ends) on the stiffness of ring external fixation. A fiberglass composite tibia fixed into an idealized ring external frame was tested. All mechanical testing was performed with a servohydraulic test frame (MTS Bionix 858, Minneapolis, MN). Load-deformation behavior was compared among different wire design and olive wire positioning and tensioning characteristics under central axial compression, medial compression-bending, posterior compression-bending, posteromedial compression-bending, and torsion. The differences between tension values on opposite ends of the wire during different modalities of olive wire tensioning were also investigated. Olive wires tensioned on both ends provided significantly greater bending stiffness compared with smooth wires and olive wires tensioned only on the end opposite to the olive (P < 0.05). There was a significant difference between the tension values on opposite ends of the olive wire during tensioning (P < 0.05). Tensioning both ends of the olive wires and positioning the olives on the side of bending significantly increase the bending stiffness of ring external fixation. This work provides a rationale for clinical decision-making about the use of tensioned olive wires opposed to smooth wires in ring external fixation.