When 7-year-old Jacob fell from the tree limb he had been hanging from and held out his arm at an odd angle, it was obvious that he had broken a bone. An X ray at the hospital emergency room confirmed this, and Jacob spent the next six weeks with his broken arm immobilized in a cast.

Many of us have experienced fractured, or broken, bones. A fracture is classified by its cause and the nature of the break. For example, a break due to injury is a traumatic fracture, whereas one resulting from disease is a spontaneous, or pathologic, fracture.

A broken bone exposed to the outside by an opening in the skin is termed a compound (open) fracture. It has the added danger of infection, because microorganisms enter through the broken skin. On the other hand, a break protected by uninjured skin is a closed fracture. Figure 7A shows several types of traumatic fractures.

Repair of a Fracture

Whenever a bone breaks, blood vessels within it and its periosteum rupture, and the periosteum is likely to tear. Blood escaping from the broken vessels spreads through the damaged area and soon forms a blood clot, or hematoma. Vessels in surrounding tissues dilate, swelling and inflaming tissues.

Within days or weeks, developing blood vessels and large numbers of osteoblasts originating from the periosteum invade the hematoma. The osteoblasts multiply rapidly in the regions close to the new blood vessels, building spongy bone nearby. Granulation tissue develops, and in regions farther from a blood supply, fibroblasts produce masses of fibrocartilage.

Meanwhile phagocytic cells begin to remove the blood clot as well as any dead or damaged cells in the affected area. Osteoclasts also appear and resorb bone fragments, aiding in cleaning up debris.

In time, fibrocartilage fills the gap between the ends of the broken bone. This mass, termed a cartilaginous callus, is later replaced by bone tissue in much the same way that the hyaline cartilage of a developing endochondral bone is replaced. That is, the cartilaginous callus breaks down, blood vessels and osteoblasts invade the area, and a bony callus fills the space.

Typically, more bone is produced at the site of a healing fracture than is needed to replace the damaged tissues. Osteoclasts remove the excess, and the final result is a bone shaped very much like the original.

The rate of fracture repair depends upon several factors. For instance, if the ends of the broken bone are close together, healing is more rapid than if they are far apart. Setting fractured bones and using casts or metal pins to keep the broken ends together help speed healing, as well as aligning the fractured parts. Also, some bones, naturally heal more rapidly than others. The long bones of the upper limbs, for example, may heal in half the time required by the long bones of the lower limbs, as Jacob was happy to discover. He also healed quickly because of his young age.

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