Bone Remodelling

Bones are living, growing tissue. During our lifetime, bone is constantly being renewed. The old bone is removed and the new bone is laid down. This process is called bone remodelling. We are so efficient at remodelling that we are able to replace our entire skeleton every 10 years!

Remodelling has two purposes. First, by remodelling, the bone is able to repair microfractures – tiny microscopic cracks that occur in the bone with daily activity. This repairing of microfractures helps to prevent osteoporosis fractures. Second, remodelling allows the bone to serve as a source of minerals important to the function of many organ systems.

There are two procesesses involved in remodelling:

1. Resorption
   Cells called osteoclasts dissolve some tissue on the bone's surface, creating a small cavity. This process usually takes place over a few days.

2. Formation
   Cells called osteoblasts fill the cavities with a soft protein containing collagen. This hardens when minerals, especially calcium, are deposited on it. Although many systems in our body need calcium to function properly, most of the calcium is in our bones and teeth. Bone formation occurs over the course of several months.

Typically bone formation and bone resorption occur at equal rates so that the quantity and quality of bone are maintained. When the rate of resorption exceeds the rate of formation, which occurs as we age and with estrogen deficiency, the quantity and quality of bone decrease.

Below is an image of normal bone (on the left), where bone formation and bone resporption are equal; and an image (on the right) of porous bone, where resorption exceeds formation, such as we see in people who have osteoporosis.

Normal Bone	Porous Bone

What Helps Bones to Grow?

Hormones (estrogen in women, testosterone in men), an adequate intake of calcium and vitamin D, and weight-bearing exercise are all important to enhance the formation of bone. If the bone removed by resorption is completely replaced, then the amount of bone does not change, and bones stay strong.

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