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The aim of this study was to investigate the effect of the rigidity of external fixation on osteotomy healing based on radiographic, morphological, histological, and biomechanical analyses of canine tibial osteotomies treated with more rigid (six half-pins) and less rigid (four half-pins) unilateral external fixators. Successful healing was obtained in all tibiae, but the bone-healing mechanisms appeared to be different depending on the rigidity of the device used. On the side with more rigid fixation, early clinical union was observed thirty days after osteotomy. Periosteal callus formation was significantly increased on the less rigidly fixed side at both ninety and 120 days (p less than 0.05). Fixation stiffness was found to affect the incidence of pin loosening (p less than 0.01) and of bone porosity (p less than 0.05) at the osteotomy site. However, new-bone formation and the structural strength of the bone were not significantly different between the four-pin and six-pin sides. These results seem to indicate that a longer period is required for fracture repair and remodeling when external fixation with less rigidity is used.