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.
Risk Factors
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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