Correlation of shear to compression for progressive fracture obliquity.

The accompanying shear force at the point of bone opposition is an important factor inherent to tibial fracture stability during axial loading. We determined at which angle of fracture obliquity shearing becomes a dominant force after stabilization with circular external fixation, and how modifications to the external fixator can reduce this effect. We constructed tibial fracture models with a successively increasing fracture angle obliquely (from 0 degrees to 60 degrees) to determine the stability in the classic Ilizarov frame and subsequent frame modifications during axial loading (maximum, 1000 N). Stability was determined by measuring the fracture line displacement for each fracture obliquity model after an applied axial load. Fracture line displacement was recorded as coordinate component changes as measured by an ultrasonic digitizer. We defined construct stability as less than 2 mm of fracture line migration with loading. More than 3500 data points were collected for this study. The classic Ilizarov construct successfully stabilized fractures with up to 30 degrees of fracture obliquity, after which divergent instability occurs. The addition of proximal and distal perpendicular half-pins provides little benefit. Arced wires provided stability up to 40 degrees fracture obliquity. A formal steerage pin construct provided stability for all fracture models (up to 60 degrees of fracture obliquity) with all applied loads (up to 1000-N axial load).