bone fractures

A fracture hematoma forms.: A bone fracture tears blood vessels inside the bone and within the periosteum, causing bleeding, and then a fracture hematoma forms from the clotted blood.

A fibrocartilaginous (soft) callus forms: Regenerated blood capillaries infiltrate the fracture hematoma due to an increase in osteoblasts in both the periosteum and the endosteum near the fracture site. First, the fracture hematoma is reorganized into an actively growing connective tissue called a procallus. Fibroblasts within the procallus produce collagen fibers that help connect the broken ends of the bones. Chondroblasts in the newly growing connective tissue form a dense regular connective tissue associated with the cartilage. Eventually, the procallus becomes a fibrocartilaginous (soft) callus (kal's; hard skin). The fibrocartilaginous callus stage lasts at least 3 weeks.

A hard (bony) callus forms: Within a week, osteoprogenitor cells in areas adjacent to the fibrocartilaginous callus become osteoblasts and produce trabeculae of primary bone. The fibrocartilaginous callus is then replaced by this bone, which forms a hard (bony) callus. The trabeculae of the hard callus continue to grow and thicken for several months.

The bone is remodeled: Remodeling is the final phase of fracture repair. The hard callus persists for at least 3 to 4 months as osteoclasts remove excess bony material from both exterior and interior surfaces. Compact bone replaces primary bone. The fracture usually leaves a slight thickening of the bone (as detected by x-ray); however, in many instances healing occurs with no obvious thickening.