The pattern of early healing of canine tibial osteotomies associated with two different types of external fixation devices was investigated. Two-plane fixation was significantly more rigid than one-plane fixation in three of the five loading conditions that were examined. The more rigid fixation allowed less lateral displacement at the site of the osteotomy, induced less formation of callus early in the healing process, permitted direct bridging by osteons more frequently, and provided healing that was more rigid early in the process of repair. In the later stages of healing there were no differences in the quantity of callus or in the strength of the healing bone. In this canine model, external fixation with higher rigidity resulted in rapid union that differed from the union that resulted after less rigid external fixation only with regard to porosity, torsional stiffness, and displacement at the site of the fracture. Clinical Relevance: The biological pathway to osseous union can be influenced by the rigidity of the fixation. Controversy has existed regarding the need for two-plane external fixation of higher or lower rigidity. In the present investigation, union of the fracture was achieved with both modes of treatment. Although higher-rigidity external fixation resulted in less callus and a stiffer repair of the osteotomy early in the process of healing, the later stages of healing and strength were similar. Therefore, clinical factors such as soft-tissue injuries may be a more important consideration in the selection of the configuration of the external fixator than are its characteristics with regard to mechanical rigidity.
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