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 fract ure are the rigidity of the selected fixation device, the fracture con figuration, the accuracy of fracture reduction, and the amount and typ e of stresses occurring at the bone ends dictated by functional activi ty 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 perio steal callus formation were the amount of physiologic loading as dicta ted 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 ga p. The time-related diminution of periosteal callus (external remodeli ng) was related to the progress of cortical healing. Cortical reconstr uction was fast in stable transverse fractures and delayed in unstable oblique fractures. Motion with loading tended to promote external cal lus maturation in secondary bone healing. Intracortical new bone forma tion and porosity were related to the healing pattern and not to the l oading magnitude. Endosteal new bone formation showed a strong correla tion with the presence of a fracture gap. In terms of torsional streng th and energy absorption at failure, the fractures healing through a c ombination of primary and secondary bone union mechanisms showed the m echanical strength of the healing bone closest to intact bone.