The biomechanics of hybrid external fixation

Objective: The objective of this study was to ascertain which factors deter mine the rigidity of a hybrid external fixation frame in a tibial periartic ular fracture model. Design: Laboratory investigation using a polyvinylchloride pipe periarticul ar tibia fracture model. Setting: Simulated periarticular tibia fractures were created in a tibia mo del. Instrumented specimens were tested in a biomaterials resting system. Intervention: Groups of simulated periarticular tibia fractures were stabil ized with one of nine different external fixation constructs with component s from one manufacturer. Main Outcome Measurements: Elastic stiffness was measured for each specimen in compression, torsion, flexion bending, extension bending, and varus and valgus bending. Results: The four-ring Ilizarov fixator was the stiffest in all modes of te sting. There was a trend toward increasing stiffness with an increasing num ber 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 t he 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-t o-ring connection did not increase stiffness. The use of rings mounted with half-pins instead of a unilateral bar mounted with half-pins for diaphysea l fixation increased the stiffness of the frame only in torsional testing. Conclusions: 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 fixat ors in this periarticular tibia fracture model. Augmentation of the ring-to -bar connection did not significantly affect the stiffness of the Frame.