Why and how to measure bone density
The bone is not a solid lump of calcium but rather a dynamic organ constantly repairing and re-moulding itself in response to growth, stress, injury, our biochemistry and hormones. As a result, bone is strongly influenced by environmental and lifestyle factors. Osteoblasts, cells which form bone, and Osteoclasts, cells which reabsorb bone, work together to maintain bone health. As you can see from the graph, bones continue to increase in thickness (or density) until our late twenties, before plateauing during our mid-thirties. As we get older, the amount of bone removed by osteoclasts is more than the amount made by osteoblasts, and our bone density then begins to slowly decrease.
Over time, the hard, outer-shell of bone thins and the holes in cancellous bone get larger. If this level of loss is greater than what is typically expected for a person’s age, this is called ‘bone loss’ or ‘bone thinning’, or in medical terms, osteopenia (mild loss) or osteoporosis (significant loss). These conditions are associated with an increased risk of the bones becoming more fragile and breaking (fracturing) in response to much less trauma. The adjacent picture shows the difference between normal bone on the left and thinned bone on the right.
How to measure bone density?
Sometimes osteoporosis can be seen on a plain x-ray, but the most reliable test is a bone density scan (DEXA). This test does not require an injection, is painless, and safe due to the amount of radiation used being much less than a normal x-ray. You just have to lie on a bed for a few minutes, and the machine provides a scale of bone loss known as a T-score:
- 0 to -1 normal
- -1 to -2 osteopenia
- < -2 osteoporosis
Who should have DEXA scans?
Elderly people are advised to have a DEXA scan, as lifestyle factors and specific treatments are available to reduce the risk of progression and fracture. People with specific risk factors should be tested more frequently. Arranging a DEXA scan in the UK usually begins with a discussion with your General Practitioner.