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Bone mineral density basics

Bone mineral density is a measure of bone strength. What it actually measures is the amount of mineral contained in a cubic centimetre of bone. This is usually done with a special type of X-ray such as a dual energy X-ray absorptiometry test (DXA).

Minerals, such as calcium and phosphorus, help keep bones strong. Bones that have less mineral content per cubic centimetre (low bone mineral density) are weaker than bones that have lots of mineral content per cubic centimetre (high bone mineral density). Weak bones are a major cause of fractures – such as a broken hip – in older women.  

Bone mineral density is often expressed as a T-score, which compares a patient’s bone mineral density against that of the average 30-year-old (when bone mineral density is usually highest), adjusted for sex and ethnic background. If your score is above zero, it means your bone density is higher than that average 30-year-old’s. If your score is negative, it means your bone density is lower than that. A score between 1 and -1 is normal. Lower than -1 suggests low bone density, and a score lower than -2.5 indicates osteoporosis.

Bone mineral density changes throughout life. In women, bone mass (the amount of bone in your skeleton) peaks around age 30, and then remains pretty constant until menopause. But in the first few years after menopause, bone mass drops dramatically, and then continues to decrease more slowly after that. Hormone levels can affect bone density, so the fast decline in bone mass at menopause corresponds to the drop in estrogen that women experience at this time.

Bone maintenance

You use your skeleton every day, so naturally there will be some wear and tear on your bones. That’s why your body has a natural maintenance system for bone upkeep: your bones are constantly under renovation, with old bone being broken down and new bone built up in its place. Ideally, the processes of breaking down and building bone are balanced, with new bone replacing lost bone at a balanced rate, so the bone itself remains strong.

This process is called remodelling, and it is happening constantly. In fact, remodelling replaces your entire skeleton about once every 10 years.

Two types of specialized cells are responsible for most of the bone breakdown and buildup:

Osteoclasts break down cells on the surface of bone at spots that have become weak, such as tiny cracks. This is called resorption.

Osteoblasts then fill the spaces left where the bone has been dissolved, leaving a matrix composed mostly of collagen (a protein). That’s where the minerals come in: as minerals such as calcium are deposited over the matrix, it hardens. This is called mineralization.

When the building process isn’t keeping up with the breakdown process, there isn’t enough new bone to replace all of the lost bone. That means that the mineral content per square inch (bone mineral density) decreases.

Bone health

Low bone mineral density is the major feature of osteoporosis, and increases the risk of fractures.

Factors that affect bone mineral density include:

  • Hormones – Certain hormones, such as estrogen, help the body build healthy bone. That’s why women often experience a drop in bone mineral density around menopause, when estrogen levels drop.
  • Age – Bone mineral density decreases as we get older.
  • Exercise – Weight-bearing exercise – even walking – helps build bone strength.
  • Calcium and vitamin D – Calcium is a major component of bone, and vitamin D is crucial to bone remodelling.
  • Lifestyle choices – Smoking and high alcohol or caffeine intake may affect bone mineral density.
  • Illness and medications – Some conditions, such as liver disease and hyperthyroidism, can affect bone health. Certain medications, including corticosteroids and blood thinners, are linked to low bone mineral density.
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  • A publication of:
  • Women's College Hospital