Temporal changes in dynamic inter fragmentary motion and callus formation in fractures.

This study examined whether callus proliferation at long bone fractures is triggered by cyclical inter-fragmentary displacement which arises from routine activity. It also examined whether a growing callus increases the stability of a fracture, thereby reducing displacement amplitude during relative motion. Seven tibial fractures stabilised with external fixators were monitored up to and beyond fixator removal. An instrumented spatial linkage was developed which was attached to the bone screws to measure inter-fragmentary displacement at the fracture in all six degrees of freedom during routine walking. Callus index (final bone width/initial bone width) was measured at the posterior and lateral cortical surfaces from orthogonal radiographs. In all seven subjects, callus growth was initiated subsequent to a peak in displacement which occurred within the first 42 days; at nine of the 14 surfaces occurred callus initiation occurred within 14 days of the peak displacement. With the exception of two lateral surfaces, maximum callus size, subsequent to fixator removal (at up to 119 days after removal). Displacement reduced during callus growth in five out of six subjects. Since the reduction in displacement did not arise from reduced weight-bearing, increasing callus size must correlate with progressive mechanical union. This was confirmed by end point stiffness tests. Therefore, peak cyclical displacement appears to be the stimulus for callus growth, the effect of which is to reduce displacement and strain which allows the following stages of bone formation and remodelling to unite the fracture.