Porth's Essentials of Pathophysiology, 4e

Disorders of the Skeletal System: Trauma, Infections, Neoplasms, and Childhood Disorders 1087

C h a p t e r 4 3

Bony Callus Formation. Ossifi­ cation represents the conversion of the fibrocartilaginous cartilage to bony callus. In areas close to well- vascularized bone tissue, osteogenic cells develop into osteoblasts, or bone-building cells, which produce spongy bone trabeculae. The newly formed osteoblasts first deposit bone on the outer surface of the bone some distance from the fracture site. The formation of bone progresses toward the fracture site until a new bony sheath covers the fibrocartilag- inous callus. In time, the fibrocar- tilage is converted to spongy bone, and the callus is then referred to as bony callus. Gradually, the bony callus calcifies and is replaced by mature bone. Bony callus formation begins 3 to 4 weeks after injury and continues until a firm bony union is formed months later. 3 Remodeling. During remodeling of the bony callus, dead portions of the bone are gradually removed by osteoclasts. Compact bone replaces spongy bone around the periphery of the fracture, and there is reor- ganization of mineralized bone along the lines of mechanical stress. During this period, the excess mate- rial on the outside of the bone shaft and within the medullary cavity is removed and compact bone is laid down to reconstruct the shaft. The final structure of the remodeled area resembles that of the original unbroken bone; however, a thick- ened area on the surface of the bone may remain as evidence of a healed fracture. 4

Proliferating osteogenic cells

Trabeculae of forming bone

Replacement of callus with mature bone

Reorganization of newly formed bone along lines of stress

Osteoclasts