A computer-linked magnetic motion transducer was used to monitor and record the six components of motion of the bone fragments in eight cadaveric tibiae in which a simulated, oblique fracture of the middle of the shaft had been stabilized with a functional brace. The limbs were mounted in a servo-hydraulic testing frame, and a cyclic load of 150 newtons was applied along the axis of the tibia. Motion sensors, attached to each side of the fracture, measured and displayed the values of the three translations (axial, anterior-posterior, and medial-lateral), the axial rotation, and the two angulations (anterior-posterior and varus-valgus) as they occurred. Although only an axial load was applied, the off-axis motions were comparable in magnitude with the motion along the axis. The elastic (recoverable) translations of the fragments ranged from 0.5 to 1.9 millimeters, about four to ten times larger than the corresponding motions that were recorded in an earlier study of such fractures that had been stabilized with two types of external fixators. The recoverable rotation and angulations of the fragments of the limbs in the functional brace ranged from 0.7 to 1.2 degrees, about ten times those recorded when the external fixators were used. CLINICAL RELEVANCE: There has been increasing recognition of the role of interfragmentary motions in the modulation of the mechanisms of healing of diaphyseal fractures. In the current study, in which a functional brace was used for stabilization of the fracture, these motions were found to be multidirectional and of an order of magnitude greater than those measured in a previous study in which external fixators were used. The results, particularly the observation that the off-axis displacements were sometimes as large as those in the direction of the applied axial load, provide greater insight into the nature of the motion of fracture fragments when a functional brace is used and contribute to the understanding of the role of motion of the fragments in osteogenesis.
