What are the phases of bone repair?

Doctors typically divide the process of bone repair into four phases.

Whenever a bone breaks, the blood vessels running down the length of the bone are severed. Blood then leaks out of these veins and forms a clot called a fracture hematoma. This clot helps to stabilize the bone and keeps the broken pieces lined up for repair. The clots also stop blood flow to the jagged bone edges. Without this fresh blood, these bone cells quickly die.

Inflammation and swelling happen next, due to the work of cells removing damaged and dead tissue. Tiny blood vessels grow in the fracture hematoma, fueling the healing process.

In the second phase, several days later, the fracture hematoma develops tougher tissue and transforms into a soft callus. In this phase, cells called fibroblasts begin producing fibers of collagen, which is the major protein in bone and connective tissue. Then, chondroblasts begin producing a type of cartilage called fibrocartilage. This fibrocartilage turns the callus into a tougher fibrocartilaginous callus, which then bridges the gap between the two pieces of bone. The callus typically lasts for about three weeks.

In the third phase, osteoblasts start to produce bone cells, which transform the callus into a bone callus. This hard shell provides the protection and stability necessary for the bone to enter its final stage of healing. The callus lasts three to four months.

In phase four, the body establishes the position of the bone within the flesh and begins reabsorbing bits of dead bone. Then it creates a hard callus to bridge the gap between the two bone pieces. But this bulge of tissue needs substantial work before the bone can handle strain. Osteoblasts and osteoclasts work for months to remodel bone, replacing bone callus with harder compact bone. These cells also decrease the callus bulge, which gradually returns the bone to its original shape. Improved blood circulation and the influx of bone-strengthening nutrients, such as phosphorous and calcium, make the bone stronger.

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