Hybrid External Fixation of the Proximal Tibia: Strategies to Improve Frame Stability

Abstract

OBJECTIVE: To determine the specific frame construction strategies that can increase the stability of hybrid (ring with tensioned wires proximally connected by bars to half-pins distally) external fixation of proximal tibia fractures. DESIGN Repeated measures biomechanical testing. SETTING: Laboratory. SPECIMENS: Composite fiberglass tibias. METHODS: Using the Heidelberg and Ilizarov systems, external fixators were tested on composite fiberglass tibias with a 1-cm proximal osteotomy (OTA fracture classification 41-A3.3) in seven frame configurations: unilateral frames with 5-mm diameter half-pins and 6-mm diameter half-pins; hybrid (as described above), with and without a 6-mm anterior proximal half-pin; a "box" hybrid (additional ring group distal to the fracture connected by symmetrically spaced bars to the proximal rings) with and without an anterior, proximal half-pin; and a full, four-ring configuration. Each configuration was loaded in four positions (central, medial, posterior, and posteromedial). MAIN OUTCOME MEASUREMENTS: Displacement at point of loading of proximal fragment. RESULTS: The "box" hybrid was stiffer than the standard hybrid for all loading positions. The addition of an anterior half-pin stiffened the standard hybrid and the "box" hybrid. CONCLUSIONS: The most dramatic improvements in the stability of hybrid frames used for proximal tibial fractures result from addition of an anterior, proximal half-pin.