Repair of Bone

The fracture of a bone is usually caused by direct violence or by a strong twisting strain. When a bone fractures the two fragments separate and in so doing tear the arteries in the Haversian systems that cross the fracture line. This damage results in the leakage of blood into the fracture, where it is trapped and soon clots. After a short time, the Haversian arteries go into spasm, causing the death of active bone cells not only at the fracture site, but also for some distance along the shaft. About two days after the break, the blood clot is invaded by capillaries and fibroblasts. The fibroblasts differentiate into bone-forming cells, or osteoblasts, and cells that form periosteal tissue on the outside of the bone. New tissue, called callus, surrounds the fracture and replaces the dead bone. The dead bone is absorbed and replaced by new bone, formed by the osteoblasts in the callus, which is remodeled by cells, called osteoclasts, to its original shape. This remodeling process is so effective that after a few months it is difficult to detect the fracture site.