Bone-Healing Patterns Affected by Loading, Fracture Fragment Stability, Fracture Type, and Fracture Site Compression

The major factors determining the mechanical milieu of a healing fracture are the rigidity of the selected fixation device, the fracture configuration, the accuracy of fracture reduction, and the amount and type of stresses occurring at the bone ends dictated by functional activity and loading at the fracture gap. Of the effects of these factors on fracture healing and remodeling in the canine tibia under unilateral external fixation, the two most significant factors in promoting periosteal callus formation were the amount of physiologic loading as dictated by the body weight and the presence of a significant fracture gap. Uniform axial loading and motion, performed at two or four weeks, did not increase callus formation but did reduce the existing fracture gap. The time-related diminution of periosteal callus (external remodeling) was related to the progress of cortical healing. Cortical reconstruction was fast in stable transverse fractures and delayed in unstable oblique fractures. Motion with loading tended to promote external callus maturation in secondary bone healing. Intracortical new bone formation and porosity were related to the healing pattern and not to the loading magnitude. Endosteal new bone formation showed a strong correlation with the presence of a fracture gap. In terms of torsional strength and energy absorption at failure, the fractures healing through a combination of primary and secondary bone union mechanisms showed the mechanical strength of the healing bone closest to intact bone.