A study of fracture callus material properties: Relationship to the torsional strength of bone

Abstract

This study was designed to quantitate the local material properties of fracture callus during gap healing and to relate these local properties to the torsional strength of bone in a canine model under external fixation. Bilateral tibial transverse osteotomies were performed in 32 dogs and stabilized using unilateral external skeletal fixators with a 2-mm gap. Dogs were divided into four equal groups and euthanized at either 2, 4, 8, or 12 weeks. The torsional properties of one bone of each pair were determined. In both bones of each pair, the indentation stiffness, calcium content, and histomorphometric properties of six sites of periosteal callus, six sites of endosteal callus, four sites of cortical bone, and two sites of gap tissue were determined. Each of the four types of tissue had a specific structural or material property change during the study period. The indentation stiffness of periosteal callus increased up to 8 weeks and then plateaued. Endosteal callus stiffness peaked at 8 weeks and then decreased by 12 weeks. Gap tissue stiffness increased linearly over time. Cortical bone stiffness decreased over time. Indentation stiffness was significantly associated with the calcium content of periosteal callus (R² = 0.50, p < 0.0001) and gap tissue (R² = 0.66, p < 0.0001). The local stiffnesses of gap tissue and periosteal callus were significantly associated with the maximum torque (gap, R² = 0.50, p < 0.0001; periosteal, R² = 0.34, p < 0.05) and the torsional stiffness (gap, R² = 0.44, p < 0.0001; periosteal, R² = 0.65, p < 0.0001) of the bone. Changes in local material properties corresponded to histologic changes in new bone formation and porosity of the four tissues.