Osteoporosis and low bone mass affect an estimated 43.6 million Americans (National Osteoporosis Foundation “America’s Bone Health” 2002), the majority of whom are women. As a result, this population is at an increased risk for fractures, particularly of the hip and the spine. The treatment and management of osteoporosis is discussed in detail elsewhere on this site. This article will focus on the role of bone mineral density (BMD) evaluation.

How does osteoporosis occur?

In order to understand the role of bone mineral density scanning, it is important to know a little about how osteoporosis occurs. Bone is constantly being remodeled. This is the natural, healthy state of continuous uptake of old bone (resorption) followed by the deposit of new bone. This turnover is important in keeping bones healthy and in repairing any minor damage that may occur with wear and tear. The cells that lay new bone down are called osteoblasts, and the cells responsible for resorption of old bone are called osteoclasts. Osteoporosis occurs as a result of a mismatch between osteoclast and osteoblast activity. This mismatch can be caused by many different disease states or hormonal changes. It is also commonly a result of aging. In osteoporosis, osteoclasts outperform osteoblasts so that more bone is taken up than is laid down. The result is a thinning of the bone with an accompanying loss in bone strength and a greater risk of fracture. A thinning bone results in a lower bone density or bone mass.

There are two major types of bone. Cancellous bone (also known as trabecular bone) is seen in areas such as the spine and wrists. This type of bone normally undergoes a rapid rate of turnover. As a result, if osteoclast and osteoblast activity become mismatched, cancellous bone is affected rapidly. Cortical bone is located in the arms and legs. This type of bone is metabolically slower than cancellous bone, and is therefore less affected by alterations in bone turnover. There is a normal rate of decline in bone mass with age in both men and women. For women, in addition to age, the menopause transition itself causes an extra degree of bone loss. This bone loss is greatest in the first 3 to 6 years after menopause. Since women generally have a lower bone mass to begin with in comparison with men, the ultimate result is a higher risk of fracture in postmenopausal women as compared to men of the same age. Nevertheless, it is important to remember that men may also be at risk for osteoporosis, especially if they have certain illnesses, a low testosterone level, are smokers, take certain medications, or are sedentary.

What is "bone mineral density" (BMD)?

The absolute amount of bone as measured by bone mineral density (BMD) testing generally correlates with bone strength and its ability to bear weight. The BMD is measured with a dual energy x-ray absorptiometry test (referred to as a DXA scan). By measuring BMD, it is possible to predict fracture risk in the same manner that measuring blood pressure can help predict the risk of stroke.

It is important to remember that BMD cannot predict the certainty of developing a fracture. It can only predict risk. It is important to note that a bone density scan, or test, should not be confused with a bone scan, which is a nuclear medicine test that is used to detect tumors, cancer, fractures, and infections in the bone.

The World Health Organization is the source of the commonly-accepted definitions for osteoporosis (WHO Technical Report Series #843, Geneva 1994):

Normal: A value for BMD statistically within 1 standard deviation of the young adult peak bone mass. The report shows a T score > -1 signifying a BMD within the normal range.

Low bone mass (medically termed osteopenia): A value for BMD statistically less than 1 standard deviation but more than 2.5 standard deviations below that of an average young adult. The report shows a T score between –2.5 and £ -1, which signifies an increased fracture risk but does not meet the criteria for osteoporosis.

Osteoporosis: A value for BMD statistically more than 2.5 standard deviations below that of the average peak young adult bone mass. BMD in this range signifies an even higher fracture risk than osteopenia. The report shows a T score £-2.5.

Based on the above criteria, it is estimated that 40% of all postmenopausal Caucasian women have osteopenia and that an additional 7% have osteoporosis (Siris et al JAMA 2001).

Why is BMD measurement important?

Determining a person's BMD helps a doctor decide if a person is at increased risk for osteoporosis-related fracture. The purpose of BMD testing is to help predict the risk of future fracture. The information from a BMD is used to aid a decision as to whether prescription medicine therapy is needed to help reduce the risk of fracture.

What is the relationship between BMD and fracture risk?

In subjects with low bone mass (as defined above) at the hip or the spine (the 2 areas traditionally measured with DXA scanning), there is a 2 to 3 fold increase in the incidence of any osteoporotic fracture. In other words, low bone density at the measured areas of the spine and hip can even predict future osteoporotic fractures at other parts of the body besides the spine and hip (Marshall et al BMJ 1996). In subjects with a BMD in the osteoporosis range, there is approximately a 5 times increase in the occurrence of osteoporotic fractures.

Who should have BMD testing?

At present, the National Osteoporosis Foundation (Pocket Guide to prevention and treatment of osteoporosis, 2003) and the U.S. Preventive Services Task Force (Annals of Internal Medicine 2002), the 2 most prominent guidelines about BMD testing, have recommended that all women over the age of 65 be tested, regardless of risk factors. The guidelines differ a little bit regarding younger women because of the lack of reliable scientific information. However, in general, testing is recommended for all postmenopausal women between the ages of 50 and 65 years who have risk factors for osteoporosis other than menopause (these include a previous history of fractures, low body weight, cigarette smoking, and a family history of fractures). In addition, there are several medications and medical conditions that a doctor reviews to determine if a person is at an especially high risk for fracture. It is also advised that anyone seeking therapy for osteoporosis be tested. These are guidelines only, and it should be remembered that testing is only indicated if it will influence treatment decision. For example, is the person willing to be treated if the results indicate an increased fracture risk?

Although the complete list is quite extensive, here are some characteristics that have been linked with increased risk of osteoporotic fracture (Pocket Guide to prevention and treatment of osteoporosis, 2003) and might warrant BMD testing in women under age 65 years:

• Personal history of fracture as an adult
• History of fracture in 1st degree relative
• Body weight <127 lbs.
• Advanced age
• Current cigarette smoking
• Use of corticosteroid therapy > 3 months
• Impaired vision
• Estrogen deficiency at early age (<45 years)
• Dementia
• Poor health/frailty
• Recent falls
• Lifelong low calcium intake
• Low physical activity
• Alcohol intake >2 drinks/day

How is BMD measured?

Dual X-ray absorptiometry (DXA) is the preferred technique for measuring BMD. [DXA has also been called dual energy X-ray absorptiometry, or DEXA.] DXA is relatively easy to perform and the amount of radiation exposure is low. A DXA scanner is a machine that produces 2 x-ray beams, each with different energy levels. One beam is high energy while the other is low energy. The amount of x-rays that pass through the bone is measured for each beam. This will vary depending on the thickness of the bone. Based on the difference between the 2 beams, the bone density can be measured.

At present, DXA scanning focuses on 2 main areas -- the hip and spine. Although osteoporosis involves the whole body, measurements of BMD at one site can be predictive of fractures at other sites. Scanning generally takes 10 to 20 minutes to complete and is painless.

What are other methods of measuring BMD?

There are small DXA scanners called peripheral DXA machines. Regular DXA machines have a standard reference (called NHANES III) that can be used for all machines no matter the manufacturer. However, peripheral DXA machines do not yet have a uniform reference standard for the normal peak young adult bone mass that can apply to all machines and all manufacturers. This is necessary for peripheral DXA to be ready for more widespread use. Efforts are in progress to make the peripheral DXA technique more standardized.

Quantitative computed tomography (QCT) can be used to assess BMD. A standard CT scanner is used in this method. However, the amount of radiation exposure is higher than with DXA and the cost is greater. For these reasons, QCT is not in general clinical use.

Ultrasound is a relatively new diagnostic tool to measure BMD. There is no radiation source with this procedure. An ultrasound beam is directed at the area being analyzed. The scattering and absorption of the waves allow for an assessment of bone density. The results are not as precise as with the other methods mentioned. This technique is relatively new and there is considerable research being conducted in this area. Since ultrasounds can easily be performed in a physician's office, this method may become valuable for screening larger populations if its accuracy becomes more refined.

How often should DXA scans be repeated to monitor treatment?

Monitoring osteoporosis treatment using DXA scans is highly controversial. Some doctors recommend DXA scanning at 1 to 2 year intervals to monitor changes in bone density during treatment. But recent scientific evidence questions the usefulness of such interval monitoring. Reasons why repeating bone density scans is extremely tricky include:

1. Bone density changes so slowly that the changes may be smaller than the measurement error of the machine. In other words, repeat DXA scans cannot distinguish between a “real” increase in bone density or a mere variation in measurement from the machine itself. Typically, BMD changes 1%/yr, which is less than the error of a DXA machine (usually in the range of 3%). Changes of less than 2 to 4% in the vertebrae and 3 to 6% at the hip from test to test can be due to the precision error of the method (National Osteoporosis Foundation Physician’s Guide to Prevention and Treatment of Osteoporosis 2003).
2. Whereas the real purpose of prescription osteoporosis treatment is to decrease future bone fractures, there is no good correlation between increases in bone density as measured by DXA with decreases in fracture risks with treatment. There are multiple examples of this in recent clinical studies. For example, the improvement in BMD only accounted for 4% of the reduction in spine fracture risk with raloxifene (Sarkar et al JBMR 2001), 16% of the reduction in spine fracture risk with alendronate (Cummings et al Am J Med 2002), and 18% of the reduction in spine fracture risk with risedronate (Watts et al J Clin Densitom 2004). Thus, improvement in BMD does not indicate the amount of the anti-fracture benefit of osteoporosis medication. Prescription medication may decrease a person’s risk of fracture even when there is no apparent increase in BMD (National Osteoporosis Foundation Physician’s Guide to Prevention and Treatment of Osteoporosis 2003). Physicians and non-physicians alike are often surprised to learn this information.
3. Even if the DXA scan shows continued deterioration in bone density during treatment, no research data exists demonstrating that changing a medication, combining medications, or doubling medication doses will be safe and helpful in decreasing the future risk of fractures compared to just continuing the same medication.
4. Even if a person’s bone density deteriorates during treatment, it is quite likely that the person would have lost even more bone density without treatment.
5. Recent research has shown that women who lose bone density after the first year of menopausal hormone therapy will gain bone density in the next two years, whereas women who gain in the first year will tend to lose density in the next two years of therapy. Therefore, bone density during treatment naturally fluctuates and may not be indicative of the fracture protection of the medication.

What is the cost of DXA?

The cost for DXA scanning varies depending on insurance policies and coverage. In general, a patient without coverage paying cash can expect to pay approximately 200 U.S. dollars for the procedure.

What about the accuracy of BMD testing in the doctor's office using smaller equipment?

There are several devices that are smaller than the standard DXA scanner that are being used in doctors offices to screen for low bone density. Very little scientific data is available about these smaller units. Most of the information comes directly from the equipment manufacturers themselves. Many of these models test peripheral bones in the feet or hands. Other units use ultrasonography. These techniques can be less accurate than BMD testing performed with state of the art equipment. Additionally, office testing equipment can range dramatically in price and quality.

In general, these devices may be reasonable to measure overall fracture risk, but are not useful in monitoring therapy. Their use might be limited to screening and results would require confirmation using DXA. In addition, expertise in using the equipment and interpreting the data can vary. At present, it is difficult to comment on these other methods of BMD testing.

Summary

Osteoporosis is a disease that results in a significant risk of fracture. The consequences of fracture can include hospitalization, immobility, and a decrease in the quality of life. From a larger perspective, it is a costly disease in terms of the healthcare system and time lost from work. Early detection and therapy is the mainstay for trying to prevent these complications. BMD testing results correlate well with the risk of fracture and the testing is easily performed in a time efficient manner without any discomfort. Although many methods of BMD testing exist, the best currently is DXA scanning. It is imperative that testing ultimately be done using state of the art equipment with capable highly-trained personnel and a doctor well versed in interpreting the results.

Bone Density Scan At A Glance

• 40% of postmenopausal women in the U. S. have osteopenia (low bone density). An additional 7 % have osteoporosis (Siris et al JAMA 2001).
• In 1995, osteoporosis-related fractures were associated with over 400,000 hospitalizations (Gelbach et al Osteoporosis International 2003), stressing the importance of early detection and appropriate prescription therapy.
• Bone mineral density (BMD) estimates the true mass of bone.
• BMD analysis is recommended for women between ages 50 and 65 with risk factors and for all women over the age of 65. In addition, men and women taking certain medications or having certain diseases should consider testing.
• By measuring BMD, it is possible to predict fracture risk in the same manner that measuring blood pressure can help predict the risk of stroke (Marshall et al BMJ 1996).
• Dual X-ray absorptiometry (DXA) is quick, painless and the preferred method to measure BMD.
• Osteoporosis has many available prescription and non-prescription treatment options once the diagnosis is made.

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