FORMATION OF BONE IN TIBIAL DEFECTS IN A CANINE MODEL - HISTOMORPHOMETRIC AND BIOMECHANICAL STUDIES

The histomorphometric, material, and structural properties of the reparative tissue and bone that formed in a two-millimeter tibial defect in dogs were determined at two, four, eight, and twelve weeks after the operation. At two weeks, the tibial defect was filled mainly with undifferentiated connective tissue. After two weeks, the relative proportion of undifferentiated connective tissue decreased and the amount of bone progressively increased throughout the twelve weeks. New bone formed primarily by intramembranous ossification, with a small degree of endochondral ossification. At twelve weeks, bone occupied 62 percent of the defect. The calcium content of the reparative tissue increased between four and eight weeks and then, at twelve weeks, reached a plateau of 77 percent that of normal cortical bone. Anisotropy of the new bone that formed in the defect increased from 13.8 per cent to 26 per cent, and the mean width of the trabecular bone increased 27 per cent between the eighth and twelfth weeks. Maximum torque and torsional stiffness increased between two and eight weeks and then, by twelve weeks, the values leveled off at 44 and 29 per cent of the values for intact bone. Indentation stiffness increased between the fourth and twelfth weeks; at twelve weeks, it was 22 per cent of the stiffness of normal cortical bone. Indentation stiffness increased 283 per cent between eight and twelve weeks, despite insignificant changes in calcium content, amount of new bone, non-osseous space, water content, or volume of trabecular bone during this time. This change in indentation stiffness did correlate with increases in anisotropy of the new bone between eight and twelve weeks. CLINICAL RELEVANCE: Knowledge of the remodeling processes that occur during fracture-healing may allow discrimination between normal and abnormal healing by non-invasive means, with techniques such as quantitative computed tomography or dual-energy x-ray absorptiometry. Additionally, if bone-induction agents, such as bone morphogenetic protein, decalcified bone matrix, or various growth factors, are to be used to enhance healing of fractures, the cellular and structural characteristics of union in skeletally mature animals must be understood. This study describes the structural, cellular, and material changes that occur during normal healing of an osseous defect in dogs and will serve as a foundation for comparison with canine models of delayed union and non-union.