Bone density

Bone density describes the strength of the bone. Bones are at their highest density when people are in their twenties and thirties, and then density gradually declines.

The risk of fractures is worked out by measuring bone density with a special dual-energy x-ray (DEXA®) scan. Ordinary X-rays don’t show up bone loss until a large amount of bone has been affected.

Bone density readings can also show early bone loss (known as osteopenia) which highlights the risk of developing osteoporosis.

Diagnosing osteoporosis

Bone mineral density tests

If bone mineral density testing shows that your bones are not as strong as they should be, your healthcare provider can recommend steps for you to take to help prevent a fracture.

There are two major ways that bone mineral density can be measured: by DEXA® and by ultrasound. Currently, the most accurate, comprehensive, and fastest technique for determining bone density is dual-energy x-ray absorptiometry (DEXA®), which measures bone density throughout the body within two to four minutes.

The measurements are made by detecting the extent to which bones absorb photons (atomic particles with no charge) that are generated by very low-level x-rays. Specialists use a formula based on the results of these procedures to determine if bone density has deteriorated to the fracture threshold.

Osteoporosis is diagnosed when bone mass density is measured at a 2.5 standard deviation (SD) or more below the average - or 2.5 times below the normal range. The measurement is expressed as a T score or a Z score.

Bone mineral density testing for young adults

DEXA® is simple and painless but not widely available. Simpler techniques that measure density in specific parts of the body may prove to be accurate measures of overall bone loss and be less costly. Single energy x-ray absorptiometry, for example, measures the forearm and heel. Dental x-rays of bone may even prove to be helpful.

Ultrasound techniques measure heels, fingers and leg bones; they are not as precise as absorptiometry techniques but they are still effective, inexpensive, portable and use no radiation. You put your foot into a small box, where non-ionising ultrasound waves measure bone density through the heel. The test is painless and takes only seconds to complete. New ultrasound techniques that may simplify the diagnostic process and improve accuracy are being tested.

There are other tests (blood and urine tests) that show how rapidly you're losing bone tissue but can't identify how much bone you've already lost. If your healthcare provider orders such tests, be sure to keep a record of the results over time.

Laboratory and other tests

Tests for calcium levels in the blood will usually be normal in patients with osteoporosis.

A urine test may indicate increased risk for hip fracture if it reveals high levels of the chemicals deoxypyridonoline and C-telopeptide. These substances are produced when bone is broken down. Another urine test measures bone loss by detecting a substance called N-telopeptide, which indicates bone loss (although is not associated with increased risk for fracture).

An interesting testing system under investigation uses computer techniques to analyse patterns of risk factors based on a questionnaire. In one study, it successfully predicted the presence or absence of osteoporosis in 82% of women who answered 20 questions related to risk.

Measuring bone strength with a densitometry machine

A densitometry machine determines how much mineral is present in the bone. The bone densitometer uses small amounts of x-ray to produce images of the spine, hip, or even the whole body. The x-ray is composed of two energy levels which are absorbed differently by the bones in the body.

The technical term for the method is "dual-energy x-ray absorptiometry", or DEXA®. A computer is able to determine from these differences how much bone mineral is present. The spine and hip are measured because that is where osteoporotic fractures occur most.

Common questions about bone mineral density (BMD) testing

Q. Is the test safe?

A. Even though x-rays are used, the amount absorbed by the patient is only about one-tenth of that received from a chest x-ray. Other x-ray procedures have even higher x-ray doses. The x-ray dose from the bone densitometry test is comparable to the naturally occurring radiation you are exposed to in one week.

Q. What can I expect during my bone densitometry test?

A. The bone densitometer is like a large examination table. It is padded and comfortable to lay on. Your name, age, height, weight and ethnicity will be entered into the computer before your test. This information is used to compare your results to a reference group of people with strong bones. You will be asked to lie on your back, remaining in your normal clothing.

Belt buckles, metal or thick plastic buttons, and metal jewellery will need to be removed from the region being examined. The operator will position your arms and legs for the test, which is painless and typically takes one to ten minutes. You just need to lie still and breathe normally.

Q. Does it matter which part of the body is measured?

A. The part of the body tested may depend on the type of equipment available at the testing centre in your area. The spine is often the site of choice for women in early menopause. A hip measurement is thought to be more useful for women who are older. Some centres may measure bone mass in the hand or wrist.

Ultrasound assesses bone in the heel, tibia, patella or other sites where the bones are relatively superficial. Ultrasound measurements appear to predict fracture risk as well as other measures of bone density.

Q. I thought osteoporosis was inherited. If I don't have a family history of fractures, why would I need the test?

A. Because there are no guarantees! A family history of osteoporosis is just one of the risk factors that may lead your health care provider to suggest a bone mineral density test.

Q. Why have a scan?

A. Think calcium is all you need to keep your bones strong? Think again. While calcium with vitamin D is important for bone health, it may not be all you need to prevent osteoporosis, especially if you're a woman older than 60. That's why it's important to ask your healthcare professional if a bone density test should be part of your routine health evaluation. It's a simple, painless test that can help your doctor assess the health of your bones. So if your bones are thinning, you can find out about treatment options in addition to calcium and vitamin D to help strengthen them, and continue to lead an active, independent life.

Bone densitometry rationale

In New Zealand, more than 3000 people break a hip each year. This figure has been predicted to rise to 4800 in 10 years as our population ages. About 30% of people who fracture a hip die within a year, from related complications. Another third never return to their own homes - and many who do have less mobility and less independence. More women are hospitalised with a hip fracture due to osteoporosis, than are hospitalised with breast cancer.

There are ways to help stop this happening, such as with bone densitometry to identify people at risk, which leads to preventive measures to avoid fractures, and treatment to reduce their loss of bone density.

Bone mineral density usually decreases with age and decreases more quickly in women after they reach menopause. In general, heavier people have denser bones than thinner people, men have denser bones than women, and blacks have denser bones than whites. The people at greatest risk for developing osteoporosis are those who start out with the least bone mineral density, which is usually thin, white women.

Treatment for osteoporosis includes adequate calcium and vitamin D in the diet, weight-bearing exercise such as walking and other medications. Hormone replacement therapy is now not often indicated for osteoporosis in women due to its other long term health risks.

Bone densitometry can be used to determine an individual woman's risk for osteoporosis and fracture and help in deciding whether treatment is the right course.

Who should undergo bone densitometry?

There are some different opinions on who should have bone densitometry. Basically, any person who has several [listed] risk factors should consider having a diagnostic bone density test. But testing is particularly beneficial to postmenopausal women who have one or more risk factors. Special bone mass measurement requirements apply to people taking government-subsidised medication in New Zealand.

The American College of Rheumatology recommends a scan for:

• All women who are at or are experiencing or have experienced menopause, women with a family history of osteoporosis
• Women athletes who do not menstruate due to excessive exercise
• Men or women who have taken corticosteroid drugs for long periods
• Men or women with abnormalities of the vertebrae or evidence of weakened bones
• Men or women with primary hyperparathyroidism (a condition where the parathyroid glands in the neck are overactive)

On the other hand, the USA National Osteoporosis Foundation recommends that bone densitometry be used to screen only people who are at risk for osteoporosis and when the results could influence a treatment decision. It recommends it for:

• Postmenopausal women under the age of 65 who have one or more additional risk factors for osteoporosis (besides menopause)
• All women aged 65 and older, regardless of additional risk factors
• Postmenopausal women who presently have fractures
• Women who are considering therapy for osteoporosis.

Depending on bone densitometry findings, you may have the test repeated in a year or a few years to follow your condition.

Arranging a bone density test

In New Zealand there are about 30 clinics (mostly private) which offer DEXA® scans. The price ranges from about $85 to $150, most costing about $100. Ask your local public hospital if they offer a free service, if money is a problem for you.

Some clinics require you to be referred by a general practitioner so the clinic doctor can send your scan results back to your GP - as well as to you - in case you need treatment for low bone mass. In other cases you can make an appointment without a referral letter from your GP.

DEXA® scans for bone mass measurement are available at a number of radiology and bone densitometry clinics and hospital endocrinology departments around New Zealand.

Original material supplied by LivingWith. Edited by everybody.