BONE DENSITOMETRY - a patient's guide
Prof Ian Holdaway - Endocrinologist

What is bone density?

Bone mineral density (BMD) is a measure of the amount of calcified tissue in bones and is essentially an estimate of bone strength. 'Thin' bones are bones with less calcified tissue in a given volume of bone. Such bones remain the same size but contain less protein and less calcium, and are weaker and more likely to fracture than normal.

How does bone density relate to osteoporosis?

Bone mineral density is used in the diagnosis of osteoporosis, which is a state where there is a lowered amount of supporting protein (mainly collagen) and calcium in bone which makes it structurally weak and liable to fracture. Reduced bone density is thus usually due to osteoporosis, but occasionally can be due to other causes such as reduced mineralisation (calcification) of bone due to deficiency of vitamin D or calcium (a condition called osteomalacia).

Bone density is relatively low in childhood but increases to a maximum ('peak bone density') in young adulthood.

What are the causes of low bone density?

Bone mineral density falls with age in both men and women. Whereas the decline with age is gradual in men, there is a more pronounced fall in women at and after the menopause due to a decrease in the important ovarian hormone, oestrogen, at that time.

Oestrogen has an important role in women in maintaining bone strength. Thus age and menopause are important causes of reduced bone density and contribute importantly to the risk of osteoporosis.

Other factors that can cause a reduction in bone density include some medications e.g. Prednisone (used for treatment of a variety of conditions including some forms of arthritis and asthma), chemotherapy, and some treatments for epilepsy. High alcohol intake, smoking, low vitamin D and/or calcium intake, and prolonged inactivity can all reduce bone density, as can disorders such as thyroid gland overactivity, excess parathyroid hormone production (primary hyperparathyroidism) etc. Inherited factors, not well understood, also influence BMD, so a family history of osteoporosis or frequent fractures could indicate a familial tendency to reduced bone density.

What problems does a low bone density cause?

Reduced bone density increases the risk of fracture with minor accidents. Obviously, heavy falls or major accidents can lead to fracture in any individual regardless of bone strength, but those with reduced bone density are liable to fracture with minimal trauma. Osteoporosis is not, of itself, painful and is thus a 'silent' disorder until fracture occurs. Detection of osteoporosis by measurement of BMD is very helpful in order to assess fracture risk and to serve as a baseline to assess the response to treatment.

How is bone density measured?

BMD is best measured by a technique called DEXA scanning (dual energy x-ray absorptiometry). This involves the subject lying on a couch for 5-10 minutes while a x-ray gantry passes over the area to be assessed (usually the lower spine and hip), although the forearm can also be measured.

The technique is painless and very safe since x-ray exposure is only about 1% of that from a chest x-ray. Two different x-ray energies are used which are differentially absorbed by bone compared with soft tissue (muscle and fat) so the density of the bone can then be calculated. The precision of the technique is good, with an error of only 1-2% between repeated measurements, so changes in BMD greater than this over time represent genuine alterations.

Bone density can also be assessed in the spine by computerised tomographic (CT) scanning, but is less precise and uses higher amounts of x-ray than DEXA. Ultrasound of the heel provides a measure of bone 'stiffness' and has been used as a convenient surrogate method to assess risk of fracture at the hip. Good quality ultrasound can be used for screening purposes although there is doubt over the reliability of some machines and the technique is not suitable for following the response to treatment.

How are bone density results reported?

The absolute density value is reported as grams of bone per unit area (e.g. 1.1g/cm2 ), together with a statement of how this relates as a percentage to the bone density of the 'normal' population of the same sex, age, build and ethnicity.

The normal data have been obtained from large population surveys. Thus, a reading may be reported as '5% above average for age' or '8% below average for age'.

In general, readings 10% or more below average for age suggest an important reduction in bone density requiring tests to exclude possible underlying illnesses affecting BMD (low vitamin D etc). However, the most important calculation made from the BMD result is to estimate how far the reading is above or below the mean value for normal young adults. In the case of low BMD this can be expressed statistically as the number of standard deviations the measurement lies below the young adult mean value. This number is called the 'T score', and standardises the measurement. The World Health Organisation has defined a T score between -1 and -2.5 as being osteopaenia (mildly to moderately reduced bone density), and a T score of -2.5 or less as osteoporosis.

Importantly, the risk of fracture (relative to the normal young adult population) increases by a factor of 2 for every SD reduction below the young adult average value. Thus, a T score of -3 indicates a 2 x 2 x 2 (8) fold increase in fracture risk. It is important to realise that T-score analysis has been established for DEXA measurements and does not apply to measurements by ultrasound or CT.

Does a low bone density reading mean that I will have a fracture in due course?

Not necessarily. Fractures occur with falls or accidents so someone with a low BMD may never have a fracture if no fall or accident occurs. Falls are more common in the elderly, so special care may be needed to prevent falls in elderly individuals with low bone density e.g. muscle strengthening exercises, walking aids, hip protectors to cushion the effect of falls etc.

Can a low bone density be treated?

A number of treatments can help restore bone density when this has been found to be low. It is important to maintain a generous intake of calcium (normally as a supplement of 500-1000 mg per day taken as calcium tablets) and, if elderly or out of the sun a good deal, a supplement of vitamin D (normally using 1 or 2 multivitamin tablets each day).

Female hormone (oestrogen) in post menopausal females or male hormone (testosterone) in males is very effective at improving bone strength. This is popularly known as hormone replacement treatment or HRT.

Medications called bisphosphonates are also often very good at increasing bone density and can be given orally (e.g. etidronate, trade name Didronel), or intravenously (pamidronate or clodronate). A new oral bisphosphonate (alendronate, trade name Fosamax) is also available but is not currently funded under the pharmaceutical schedule and costs, on average, about $800 per year.

An activated form of vitamin D, calcitriol (trade name Rocaltrol), has also been found to be helpful in some patients, particularly those with osteoporosis due to taking Prednisone. General practitioners or specialist physicians are able to advise on suitable options of treatment for individual patients.

Should bone density measurements be repeated?

There are guidelines published concerning follow-up BMD measurements. In those with low bone density needing treatment it is wise to repeat the measurement after 1-2 years to ensure the treatment is working and BMD is improving.

In those with mild reductions of BMD the measurement can be repeated after 3-5 years to check if there has been any further reduction, in which case treatment may be needed.

Those with bone density at or above average levels should normally have a repeat measurement done at a longer interval (5-10 years) to ensure there is no undue acceleration of bone loss over time.

Where can a bone density measurement be done?

A number of DEXA bone scanners are available through the country in the private sector, and measurements are covered by medical insurance. Costs are usually around £100-£150.