Osteoporosis targets women much more often than men, and it becomes more common after menopause and with advancing age. As a result, healthcare providers recommend bone density testing for women who have been through menopause and are at least 65 years old. In addition, there are certain characteristics that put people at higher risk for fracture, so healthcare providers sometimes recommend testing in men or women younger than 65 who have one or more risk factors
Bone density testing is used to assess the strength of the bones and the probability of fracture in persons at risk for osteoporosis. The test, referred to as bone densitometry or bone mineral density scan (BMD), is a simple, noninvasive procedure that takes just minutes.
What is Osteoporosis?
Osteoporosis, meaning “porous bone”, is a bone disorder that decreases bone mass. As we age, our bones may lose bone mass and osteoporosis leads to an increased risk of fractures (breaks). Osteoporosis can lead to pain, height loss due to a backbone that curves forward, or fractures. Called the “silent crippler,” osteoporosis may progress without symptoms or pain until a fracture occurs.
Hip or spine fractures are a serious consequence of osteoporosis and may result in loss of independence and the need for nursing home care. Medical complications after these fractures can be life threatening and the risk for death increases after hip or spine fractures.
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, which 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, change in normal hormones as occurs after menopause, and with diets low in calcium and vitamin D. 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 the inner, softer portion of the bone, and cortical bone is the outer, harder layer of bone. Cancellous bone undergoes turnover at a faster rate than cortical bone. As a result, if osteoclast and osteoblast activity become mismatched, cancellous bone is affected more rapidly than cortical bone. Certain areas in the body have a higher ratio of cancellous bone to cortical bone such as the spine (vertebrae), the wrist (distal radius) and the hips (femoral neck).
Most of a person's bone mass is achieved by early adulthood. After that time, the bone mass gradually decline throughout the rest of a person's life. 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 three to six years after menopause. Women can lose up to 20% of the total bone mass during this time. 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. The best method to prevent osteoporosis is to achieve as high a bone mass by early adulthood with a proper diet and regular exercise. Unfortunately, osteoporosis is not often considered during this time in a person's life.
In premenopausal women, estrogen produced in the body maintains bone density. Following the onset of menopause, bone loss increases each year and can result in a total loss of 25%-30% of bone density in the first five to ten years after menopause. Your doctor can help you decide when and if you need a bone density test. In general, this testing is recommended for women 65 and older along with younger postmenopausal women who have further risk factors for osteoporosis.
Some of the confusion about the test is understandable because official recommendations and advice from physicians on when to first get tested isn't in perfect agreement.
For instance, the National Osteoporosis Foundation as well as the American Association of Clinical Endocrinologists recommends all women aged 65 and over, as well as women and men after age fifty who experience fractures, get a bone density test. They also suggest that younger women who have gone through menopause and have one or more risk factors (such as family history of spine fractures) get tested, too.
Despite those guidelines, many physicians say that all average, healthy women should get a bone density test when they enter menopause, says Laura Tosi, MD, director of the bone health program at Children's National Medical Center in Washington. That makes sense, she says, because bone loss tends to speed up in the years after menopause, so getting a baseline idea of where you stand as you enter menopause gives you something to compare later scans to.
And some women should get the test even earlier, Tosi says. For instance, a woman who is 40 or so and suffers a "fragility" fracture -- a bone break that occurs when you fall from a standing height (about 5.5 feet or less) -- should get a bone density test, Tosi says. That type of fracture, she reasons, doesn't occur to strong bones.
Women who have been on high-dose corticosteroid medications to treat autoimmune disease such as lupus, along with women who have thyroid disease, should consider a bone density test, too, Tosi says, because they are more likely than others to have lower bone density.
The list of the some risk factors, emphasizing the importance of the bone density tests in women are as follows:
- A history of bone fractures as an adult or having a close relative with a history of bone fractures
- Tobacco Smoking
- Vitamin D deficiency, which can occur as a result of certain medical conditions
- Excessive alcohol or caffeine consumption (three or more servings a day)
- Weight loss or low body weight (less than 127 lbs or 58 kg); small-boned body frame
- Early menopause or late onset of menstrual periods
- Physical inactivity
- Long-term use of steroid medications, such as prednisone or phenytoin, (Dilantin). known to cause bone loss
- Low estrogen levels
- Rheumatoid arthritis
- A disorder strongly associated with osteoporosis, such as diabetes, untreated hyperthyroidism, hyperparathyroidism, early menopause, chronic malnutrition or malabsorption, or chronic liver disease
Types of Bone Density Tests
There are several different types of bone density tests.
Dual-energy x-ray absorptiometry (DXA) — Experts agree that the most useful and reliable bone density test is a specialized kind of x-ray called dual-energy x-ray absorptiometry, or DXA. DXA provides precise measurements of bone density at important bone sites (such as the spine, hip, and forearm) with minimal radiation.
The most recommended DXA tests are of the hip and spine because measurements at these sites are effective for predicting osteoporotic fracture at any site, choosing candidates for therapy, and for monitoring response to therapy. If you are unable to lie on an examination table, it is impossible to measure the spine and hip bone density. Instead, you can sit beside the DXA machine for a scan of your wrist area. When the hip and spine cannot be measured, the diagnosis of osteoporosis can be made using a DXA measurement of the wrist.
Quantitative computerized tomography — This is a type of CT that provides accurate measures of bone density in the spine. Although this test may be an alternative to DXA, it is seldom used because it is expensive and requires a higher radiation dose.
Ultrasonography — Ultrasound can be used to measure the bone density of the heel. This may be useful to determine a person's fracture risk. However, it is used less frequently than DXA because there are no guidelines that use ultrasound measurements to diagnose osteoporosis or predict fracture risk. In areas that do not have access to DXA, ultrasound is an acceptable way to measure bone density.
DXA Test Procedure
Unlike a bone scan, bone densitometry testing does not involve the administration of radioactive contrast material into the bloodstream. The central bone density device is used in hospitals and medical offices, while the smaller peripheral device is available in some drugstores and in screening sites in the community. The DEXA scan involves a much smaller radiation exposure than a standard chest x-ray.
During DXA, you lie on an examination table. An x-ray detector scans a bone region, and the amount of x-rays that pass through bone are measured and displayed as an image that is interpreted by a radiologist or metabolic bone expert. The test causes no discomfort, involves no injections or special preparation, and usually takes only 5 to 10 minutes. The x-ray detector will detect any metal on your clothing (zippers, belt buckles), so you may be asked to wear a gown for the test.
The amount of radiation that's used is minimal, amounting to roughly the same radiation that an average person gets from the environment in one day. After the test is completed and the doctor interprets the results, you will be given a score that speaks to the condition of the bones.
DXA Test Results
The results of a bone density test are expressed either as a "T" or a "Z" score. T-scores represent numbers that compare the condition of your bones with those of an average young person with healthy bones. Z scores instead represent numbers that compare the condition of your bones with those of an average person your age. Of these two numbers, the T-score is usually the most important. T-scores are usually in the negative or minus range. The lower the bone density T-score, the greater the risk of fracture is.
Normal bone density — People with normal bone density have a T-score between +1 and -1. People who have a score in this range do not typically need treatment, but it is useful for them to take steps to prevent bone loss, such as having adequate amounts of calcium and vitamin D and doing weight-bearing exercise.
Low bone mass (osteopenia) — Low bone mass (osteopenia) is the term healthcare providers use to describe bone density that is lower than normal but that has not yet reached the low levels seen with osteoporosis.
A person with osteopenia does not yet have osteoporosis, but is at risk of developing it. People with osteopenia have a T-score between -1.1 and -2.4.
Osteoporosis — People with osteoporosis have a T-score of -2.5 or less. Larger numbers (e.g., -3.2) indicate lower bone density because this is a negative number.
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