by Nan C Avery

Created on: December 20, 2011

The bones in the human body are absolutely amazing. The discovery of how bones are constantly renewed is awe-inspiring.

Bones are made up of living tissues and non-living substances. Living bones are blood vessels, nerves, collagen and two living cells: osteoblasts (cells that form bone); and osteoclasts (cells that help eat away old bone). Within the bone are cells called osteocytes. Osteocytes are mature osteoblasts that no longer form bone.Osteocytes provide metabolic exchange with the living and non-living substances (minerals, salts) within the bone.

The bones are classified as long (arm and leg bones); short (bones in wrists and ankles); flat (ribs, bones of the skull); irregular (back vertebrae along the spine).

Bone is not static. It continually replaces itself. Old bone is replaced by new bone all the time. The old osteoclasts are reabsorbed by the body. The cycle of forming new bone is the following:

1. Osteoclasts begin to develop on the outside of the bone, and eats away at the bone for three weeks. A tunnel forms as the osteoclasts eat the bone. The osteoclasts disappear for the osteoblast cells to do their job.

2. Osteoblasts invade the tunnel and new bone begins to develop.

Bone thickness is dependent on the load it has to carry. Athlete bones are heavier than nonatheletes. If a person breaks a leg, wears a cast and predominately walks on the “good” leg. Good leg bones become denser with calcification. The leg bones in the cast is thinner from the lost of calcification from non-use.

Bone Repair - simplified

It takes three cells to repair the bones: the osteoclasts and osteoblasts (defined above) and a chondroblast cell that forms new cartilege.

Immediately after a bone breaks, the body creates a plan to put it back together. Doctors divide the overall process into four phases.
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The first phase is a “fracture hematoma” in which the break causes the blood vessels running down the length of the bone to clot. This is the bone stabilizer. The clot serves two purposes: blood no longer reaches the jagged bone edge. This causes the bone cells to die quickly. The resultant swelling and inflammation is due to the cells removing dead and damaged tissue. Tiny blood vessels develop in the fracture hematoma. This function jumpstarts the healing process.

The second phase begins after several days. Tougher tissue develops around the fracture hematoma, which transforms the fracture into a soft callus (like the kind that develops on a heel from shoes that rub). Fibroblast cells (critical for wound healing and used for connective tissue) produce fibers of collagen (the main protein in bones). Chrondroblasts cells begin to produce fibrocartilage. Fibrocartilage makes the callus thicker and stronger. This bridges the gap between the two pieces of bone. This phase lasts three weeks.

The third phase is when osteoblasts move in and produce bone cells that change the callus into a bone callus. This hard shell lasts three or four months and provides protection and stability for the bone to enter into the final stage of healing.

The final stage is when the bone strengthens. This may take up to a year.

This covers a simple fracture healing itself. It is best to see a doctor if you believe you have a fracture to avoid complications.

Source: http://health.howstuffworks.com/human-body/systems/m usculoskeletal/heal-broken-bones1.htm

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