FIGURE 8.23 • Photomicrograph of fractured long bone undergoing repair. a. This low-magnification photomicrograph of a 3-week-old bone fracture, stained with H&E, shows parts of the bone separated from each other by the fibrocartilaginous callus. At this stage, the cartilage undergoes endochondral ossification. In addition, the osteoblasts of the periosteum are involved in secretion of new bony matrix on the outer surface of the callus. On the right of the microphotograph, the fibrocartilaginous callus is covered by periosteum, which also serves as the attachment site for the skeletal muscle. 35. b. Higher magnification of the callus from the area indicated by the upper rectangle in panel a shows osteoblasts lining bone trabeculae. Most of the original fibrous and cartilaginous matrix at this site has been replaced by bone. The early bone is deposited as an immature bone, which is later replaced by mature compact bone. 300. c. Higher magnification of the callus from the area indicated by the lower rectangle in panel a. A fragment of old bone pulled away from the fracture site by the periosteum is now adjacent to the cartilage. It will be removed by osteoclast activity. The cartilage will calcify and be replaced by new bone spicules as seen in panel b. 300.
While the callus is forming, osteoprogenitor cells of the periosteum divide and differentiate into osteoblasts. The newly formed osteoblasts begin to deposit new bone on the outer surface of the bone at some distance from the fracture. This new formation of bone progresses toward the fracture site until new bone forms a bony sheath over the fibrocartilaginous callus. Osteogenic buds from the new bone invade the callus and begin to deposit new bone within the callus, gradually replacing the original fibrous and cartilaginous callus with a bony callus. The cartilage in the original callus calcifies and is replaced by bone as in endochondral ossification.
Endosteal proliferation and differentiation also occur in the marrow cavity, and medullary bone grows from both ends of the fracture toward the center. When this bone unites, the bony union of the fractured bone produced by the osteoblasts derived from both the periosteum and endosteum consists of spongy bone. As in normal bone formation, the spongy bone is gradually replaced by compact bone. While compact bone is being formed, the bony callus is removed by the action of osteoclasts, and gradual remodeling restores the bone to its original shape.
In healthy individuals, this process usually takes from 6 to 12 weeks, depending on the severity of the break and the particular bone that is broken. Setting the bone (i.e., reapproximating the normal structure) and holding the parts in place by internal fixation (by pins, screws, or plates) or by external fixation (by casts or by pins and screws) speeds the healing process and usually results in superior structural and functional restoration.
PLATE 11 Bone, Ground Section