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Bone scans

MagazineOctober 1994 (Vol. 5 Issue 7)Bone scans

Q:Any woman suffering as my mother did (see Case Study, p 10) should not be dismissed, but we both regularly meet women who are in pain but who are not receiving treatment, although they fulfill many criteria which would indicate osteoporosis

Q:Any woman suffering as my mother did (see Case Study, p 10) should not be dismissed, but we both regularly meet women who are in pain but who are not receiving treatment, although they fulfill many criteria which would indicate osteoporosis. Docto

Perhaps women at risk should be screened at 35 with a bone scan to give a benchmark against which any deterioration could be measured and then treatment would be less subjective. B L, Kettering.

A:That might be a good idea if we had a test that could be relied upon to

deliver an accurate result. The problem is, many medical experts agree, that even the latest techniques in bone scanning should be interpreted with caution. According to a recent BMJ editorial (9 April 1994), Susan M Ott, associate professor in the Division of Metabolism, University of Washington in Seattle, argued that changes in bone mass may not signify anything.

The instruments are imprecise, multiple measurements may be wrong, even the assumptions upon which we scan bone are open to question eg, the very notion that bone density has a volume density or that we can treat it and effectively reverse bone loss.

The latest souped up bone scan is the "dual energy x-ray absorptiometry" a fancy sort of x-ray. Even this new technique has its major problems, points out Ms Ott. "A walk around the room causes the measurement to change by up to 6 per cent (at the hip), which corresponds to six years of bone lost at the usual rate." There is also the problem of alterations in machine quality control and operator error, which have proved to be such a bugbear with mammograms.

The current propensity to measure many different areas of the body concurrently one shot of the top of the leg produces five separate measurements, she says increases the risk of a false positive.

David M Reid, a rheumatologist at City Hospital in Aberdeen, Scotland, and his colleagues also urged caution, particularly when measurements are made at the hip. "Apparently dramatic changes can be taken as indicating improvement or dramatic bone loss but may simply be due to the precision of the measurement and poor repositioning technique," they wrote (BMJ, 11 June 1994).

Perhaps, most significantly, says Ms Ott, measuring bone mass may be a useless exercise because bone mass and bone strength aren't necessarily correlated. This has significance for women who are given drugs in an attempt to reverse osteoporosis. For instance, fluoride causes bone mass to increase dramatically, but decreases its strength hence the increase in osteoporosis among elderly populations in highly fluoridated communities. Similarly, some drugs may increase bone mass by 5 per cent because bone structure has been damaged and doesn't get strengthened with the drug.

It's important to understand that bone in healthy individuals is a dynamic entity, constantly undergoing interior remodeling. Two sets of cells are responsible: osteoclasts the construction workers which rip down the worn out bone; and osteoblasts the architects which utilize calcium, magnesium, boron and other minerals to build up the healthy new tissue.

This process is called "resorption". All the usual drugs for osteoporosis like estrogen and calcitonin or etidronate (called "antiresorbing drugs") do is to lower this process of turnover and renewal, preventing the hardhat osteoclasts from doing their job. Eventually, says Ott, there is no further bone formation.

"The total amount of new bone formed is limited by the initial rate of bone formation," says Ott a "remodelling barrier", she claims, is 5-10 per cent. Hence why long-term studies of treating osteoporosis with estrogen show that as soon as a post-menopausal woman stops taking

estrogen, her bone mass quickly declines to virtually the same level

as women who have never taken the drug.

Ott's point is borne out in one study of women taking etidronate (The New England Journal of Medicine, 12 July 1990). The study recorded higher rates of fractures (34 among 212) in the groups receiving the drug, compared to the 28 of 211 who did not receive etidronate. Furthermore, those taking the placebo had an increase in spinal bone mineral density during the two-year study period a measurement that may, of course, be meaningless.

Several years earlier, during a debate in the correspondence section of The Lancet over the validity of bone density screening (29 September 1990) Dr Albert van Hemert of the Department of Clinical Epidemiology at Leiden State University Hospital, Netherlands, referred to his own study as demonstrating that the presence or absence of low bone density is a useless indicator of risk of fractures or osteoporosis. A study of 1014 middle-aged women over nine years was only at best half right; for instance, the group considered at high risk of osteoporosis only suffered 40 per cent of all fractures among the group. "Bone density screening cannot be expected to yield better predictions because of the large overlap in bone density between fracture and non-fracture groups," concluded van Hemert.

Besides the problems of inaccuracy, it's wise to remember that this test and CAT (computerized axial tomography) blast the patient with high doses of x-rays, which cause cumulative damage, including cancer, over time.

Bone scans may have a one-time use to help in diagnosing women suspected clinically of osteoporosis but appear to be too variable to be relied upon as a general screening test for women without symptoms. As a marker for future osteoporosis, pre and post-menopausal women may be better off testing their red cell magnesium levels. Dr Guy Abrahams and others have discovered that low magnesium, rather than a low calcium level per se, is one central factor predisposing women to osteoporosis (J Nutr Med, 1991; 2: 165-78). In his study, Dr Abraham gave magnesium to 19 women taking hormone replacement therapy. After eight months, women taking the supplements had an increase in bone density of 11 per cent (if the bone scans, of course, were accurate), while there was no increase in the women taking the hormones alone. In the supplement group, bones continued improving after two years. Other researchers have discovered an important role for boron in preventing osteoporosis; Dr Ellen Grant recommends 3mg boron for post-menopausal women.

Besides supplements, the best preventive therapy is for women to engage in regular weight bearing exercise.


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