Mechanical influences on tibial fracture healing.

Selected studies are summarized that measure interfragmentary fracture displacements in 6 degrees of freedom at intervals throughout healing in groups of patients with tibial diaphyseal fractures treated by external skeletal fixation. The results are compared with those obtained from experimental studies in which the ideal mechanical conditions for fracture healing were predicted. A finite element analysis model of the healing tibial fracture also was developed. Measured data were used for the analysis, and stress and strain patterns were defined for different stages of healing. Interfragmentary movement measured in the first 6 weeks after injury usually is a magnitude smaller in patients treated by external fixation than in patients treated with cast immobilization. This movement can be much smaller than that predicted to be optimal by experimental studies. A greater amplitude can be achieved, even in stable fractures, by ensuring patient activity. The interfragmentary movement is elastic during loading activity and is generally sinusoidal during steady walking. At the time of dynamization (the unlocking of the frame), a permanent set occurs at the fracture site in all planes. The cyclical movement range in each plane often decreases immediately after unlocking. The model analysis study of fracture healing predicts that tissue damage may occur in the later (hard callus) phase of healing, even while the fixation device is in place, because of abnormally high stresses and strains. This study indicates that fracture mechanics should be controlled more rigorously to provide amplitudes of movement in the first 4 to 6 weeks after fracture. The rigidity of fixation should be increased in the subsequent weeks until the fracture has healed and the frame is removed.