The Biomechanics of Hybrid External Fixation

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Abstract
The objective of this study was to ascertain which factors determine the rigidity of a hybrid external fixation frame in a tibial periarticular fracture model. Laboratory investigation using a polyvinylchloride pipe periarticular tibia fracture model. Simulated periarticular tibia fractures were created in a tibia model. Instrumented specimens were tested in a biomaterials testing system. Groups of simulated periarticular tibia fractures were stabilized with one of nine different external fixation constructs with components from one manufacturer. Elastic stiffness was measured for each specimen in compression, torsion, flexion bending, extension bending, and varus and valgus bending. The four-ring Ilizarov fixator was the stiffest in all modes of testing. There was a trend toward increasing stiffness with an increasing number of rings. Fixators constructed with multiple levels of fixation in the periarticular fragment were stiffer in all modes of testing. The additional spacing between wires gained by the use of a single thick carbon ring or the use of a drop wire three centimeters from a single ring did not increase stiffness in this model. Frame modifications intended to augment the bar-to-ring connection did not increase stiffness. The use of rings mounted with half-pins instead of a unilateral bar mounted with half-pins for diaphyseal fixation increased the stiffness of the frame only in torsional testing. Although the ideal stiffness of an external fixator is unknown, our results show that the addition of more than one level of fixation in the periarticular fragment increases the stiffness of hybrid external fixators in this periarticular tibia fracture model. Augmentation of the ring-to-bar connection did not significantly affect the stiffness of the frame.