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 ext ernal skeletal fixation. The results are compared,vith those obtained from experimental studies in which the ideal mechanical conditions for fracture healing were predicted. A finite element analysis model of t he healing tibial fracture also was developed. Measured data were used for the analysis, and stress and strain patterns were defined for dif ferent stages of healing. Interfragmentary movement measured in the fi rst 6 weeks after injury usually is a magnitude smaller in patients tr eated by external fixation than in patients treated with cast immobili zation. This movement can be much smaller than that predicted to be op timal by experimental studies. A greater amplitude can be achieved, ev en in stable fractures, by ensuring patient activity. The interfragmen tary movement is elastic during loading activity and is generally sinu soidal during steady walking. At the time of dynamization (the unlocki ng of the frame), a permanent set occurs at the fracture site in all p lanes, The cyclical movement range in each plane often decreases immed iately after unlocking. The model analysis study of fracture healing p redicts that tissue damage may occur in the later (hard callus) phase of healing, even while the fixation device is in place, because of abn ormally high stresses and strains. This study indicates that fracture mechanics should be controlled more rigorously to provide amplitudes o f movement in the first 4 to 6 weeks after fracture. The rigidity of f ixation should be increased in the subsequent weeks until the fracture has healed and the frame is removed.