BONE DENSITY
Why measure bone density?
Osteoporosis is a condition in which bone is more porous than average and
therefore more prone to fracture. As this condition is symptomless until a bone is
broken, it has traditionally been difficult to pinpoint which individuals have fragile
bones prior to fracture occurring. With advances in technology and the development
of bone densitometry, it has become possible to assess an individual's bone density.
This is particularly useful because low bone density has been linked to an increased
risk of fracture. Osteoporosis can now be diagnosed prior to bones breaking, giving
individuals the opportunity to take treatments and adopt lifestyle changes which will
reduce their risk of breaking their bones.
How is bone density measured?
The most common means of measuring bone density involves a simple test called
Dual X-ray Absorptiometry (DXA). At the moment, it is the most accurate and
reliable means of assessing the strength of bones. DXA machines usually scan
some bones in the lower spine and one hip, two of the main areas at risk from
osteoporotic fractures. Other areas can also be assessed including the forearm and
the heel. This technique uses a radiation dose which is similar to natural
background radiation, less than one tenth of the dosage of a chest x-ray. The DXA
scan will take up to 10 - 20 minutes and is not in any way unpleasant. It involves
lying on a firm couch whilst an arm passes over the body taking an image of the
spine and hips. It does not involve being enclosed in a mechanical tunnel or having
an injection. Generally, clothing does not have to be removed but clothes with metal
at the hips or along the spine should be avoided (trouser zips are not a problem).
What will the results tell me?
A DXA scan produces a printout in which your bone density is compared to a
reference range of young healthy adults with average bone density. The difference
between this average and your bone density is then calculated and expressed in
terms of standard deviations (SD) and you are given a T score.
If the T score is between 0 and -1 SD - this is considered to be within normal
ranges
-1 and -2.5 SD - this is termed osteopenia
below -2.5SD - this is classed as osteoporosis
Osteopenia is the name for the category between normal and osteoporosis, when
bone is less dense than the average but not thin enough to be classed as
osteoporosis. Lifestyle adaptation in diet and exercise levels may be useful in these
cases and where relevant, use of hormone replacement therapy (HRT).
When a DXA scan is performed, a Z score is also calculated. This is a comparison
between your bone density and that of a reference range of people of your own age.
Although this is not used to diagnose osteoporosis, it is useful sometimes in the