DEXA
Scanning
Dual Energy X-ray Absortiometry, or DEXA
scanning, is currently the most widely used
method to measure bone mineral density. For the
test, a patient lies down on an examining table,
and the scanner rapidly directs x-ray energy
from two different sources towards the bone
being examined in an alternating fashion at a
set frequency. The mineral density of the
patient's bone weakens, or prolongs the
transmission of these two sources of x-ray
energy through a filter onto a counter in a
degree related to the amount of bone mass
present. The greater the bone mineral density,
the greater the signal picked up by the photon
counter. The use of the two different x-ray
energy sources rather than more traditional
radioisotope studies (such that would be used
for a bone scan) greatly improves the precision
and accuracy of the measurements.
Bone-mineral density (BMD)
Bone mineral density is a measured
calculation of the true mass of bone. The
absolute amount of bone as measured by bone
mineral density (BMD) generally correlates with
bone strength and its ability to bear weight. 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.
The World Health Organization has used BMD to
define specific diagnostic categories. It should
be noted that all "normal" values of BMD are
based on Caucasian data. It is well documented
that there is significant variation in BMD
between ethnic groups. For example, African
Americans in general have a greater BMD as
compared to Caucasians of the same age and
weight. Interpretation of results must take this
difference into account
The bone densitometry test determines the
bone mineral density (BMD). Your BMD is compared
to two norms - healthy young adults (your
T-score) and age-matched (your Z-score).
First, your BMD result is compared with the
BMD results from healthy 25- to
35-year-old adults of your same sex and
ethnicity. The standard deviation (SD) is the
difference between your BMD and that of the
healthy young adults. This result is your
T-score. Positive T-scores indicate the bone is
stronger than normal; negative T-scores indicate
the bone is weaker than normal.
In general, the risk for bone fracture
doubles with every SD below normal. Thus, a
person with a BMD of 1 SD below normal (T-score
of -1) has twice the risk for bone fracture as a
person with a normal BMD. A person with a
T-score of -2 has four times the risk for bone
fracture as a person with a normal BMD. When
this information is known, people with a high
risk for bone fracture can be treated with the
goal of preventing future fractures.
T-score
The T-score shows the amount
of bone you have compared to a young adult (at
the age of 35) of the same gender with peak bone
mass T-scores are based on a statistical
measure called the standard deviation, which
reflects differences from the average score.
|
T-score |
statistically
speaking |
what it means |
diagnosis |
|
higher than -1 |
your
bone mass is within 1 standard
deviation of the avg for a 35 year
old healthy young adult women |
your
bone mass and your risk of fractures
are avg or better |
adequate bone density |
|
between -1 and -2.5 |
your bone mass is
between 1 and 2.5 standard
deviations lower than the avg for a
35 year old healthy young adult
women |
your bone mass is
lower than normal, and your fracture
risk is approximately twice as high
as avg |
osteopenia |
|
less
than -2.5 |
your
bone mass is lower than the avg for
a 35 year old healthy young adult
women by more than 2.5 standard
deviations |
your
bone mass is very low--lower than
99% of healthy young adult women,
your risk for fractures is
approximately three times higher
than avg |
osteoporosis |
|
-2.5 and below
with presence of fracture |
very low bone
density |
very high risk
of further fracture |
severe
osteoporosis |
The Z-score
Z-scores are calculated in the
same way, but the comparisons are made to
someone of your age, sex, race, height, and
weight.
Physicians may also measure a patient’s
Z-score with a bone mineral density test. The
Z-score is not used to confirm a diagnosis of
osteoporosis because a favorable BMD measurement
(compared to the average BMD measurement for the
patient’s age group) does not mean the patient
is not at risk for osteoporosis.
|
Z-score |
statistically
speaking |
what that means |
|
higher than -1 |
your
bone mass is within 1 standard
deviation of the avg for women your
age, or better. |
your
bone mass is within or above the
normal range for women your age |
|
between -1 and -2.5 |
your bone mass is
between 1 and 2.5 standard
deviations lower than the avg for
women your age |
your bone mass is
lower than avg for your age;
compared to your peers, you're in
the lowest 1 to 14% |
|
less
than -2.5 |
your
bone mass is lower by more than 2.5
standard deviations than the avg for
women your age |
your
bone mass is much lower than avg--lower
than 99% of women your age. |
Osteopenia
is not a disease,
but a term that describes low bone density.
While osteopenia is not considered a disease,
being diagnosed with it requires further
monitoring. Preventive measures should be taken
since osteoporosis may develop if bone density
loss increases
Severe
Osteoporosis
According to the
WHO (World Health Organization) Severe
osteoporosis is considered to be present when
the value for bone mineral content is more than
2.5 SDs below the mean for young adults and
there is at least one so-called fragility
fracture (a fracture assumed to be associated
with osteoporosis because it occurred as a
result of slight trauma).
The
relationship between BMD and fracture risk
In subjects with low bone mass, there is a 2
to 3 fold increase in the incidence of spinal
fractures. In subjects with a BMD in the
osteoporosis range, there is approximately a 5
times increase in the occurrence of fractures
How do
doctors interpret the test results and decide
who is a candidate for treatment?
A DEXA scan report compares the patient's
bone mineral density values with those of young
normal patient (T score) and with age matched
normal patient (Z score). By comparing a
patient's bone density against there peers, a
low score indicates there may be a reason other
than age related bone loss.
Patients risk factors for osteoporosis that
should play a part in the decision to begin
treatment include: a maternal history of a hip
fracture, any previous fracture after the age of
fifty, tall height at age of 25, poor health,
some sedatives and anticonvulsant drugs, and the
inability to rise from a chair without the use
of the arms.
National Osteoporosis Foundation's guidelines
recommend that the following people should take
or consider bisphosphonates:
- Women with a below-normal bone density
of 2.5 SD or greater and who have no history
of fractures should take bisphosphonates.
- Women with below-normal bone density 1
SD or more and have a history of fractures
should consider bisphosphonates.
How
often should DEXA scans be repeated to monitor
treatment?
Monitoring osteoporosis treatment using DXA
scan is a controversial issue. 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. The
American Medical
Association and other reputable
medical organizations have determined that
repeat DXA scans is NOT helpful in monitoring
osteoporosis treatment or prevention.
The
reasons are:
- Bone density changes so slowly with
treatment that the changes are smaller than
the measurement error of the machine. In
other words, repeat DXA scans cannot
distinguish between a real increase in bone
density due to treatment or a mere variation
in measurement from the machine itself.
- Whereas the real purpose of 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. For example,
alendronate
has been shown to decrease fracture risk by
50%, but only to increase bone density by a
few percent.
- DXA measurement taken during treatment
will not help the doctor plan or modify
treatment. For example, even if the DXA scan
shows continued deterioration in bone
density during treatment, there is not yet
research data demonstrating that changing a
medication, combining medications, or
doubling medication doses will be safe and
helpful in decreasing the future risk of
fractures.
- Very importantly, even if bone density
deteriorates during treatment, it is quite
likely that the patient would have lost even
more without treatment.
- Recent research has shown that women who
lose bone density after the first year of
hormone replacement
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 this may not be relevant to
the fracture protection of the medication
- In clinical practice, serial BMD
measures have some clinical utility in
monitoring response to therapy, but it is
important to keep in mind that
fracture-protection benefit may be realized
before BMD gains are detected
- If there is no change in BMD, it does
not mean that therapy is not working,
because treatment may have prevented bone
loss.
Don’t be concerned if your
physician doesn’t order repeated bone density
measurements every year. Due to the error of the
machine, he or she may not expect your bone
density to change in a way that is detectable in
such a short time. If you’re taking osteoporosis
medications, don’t rely too heavily on repeated
bone density testing, because it may not tell
you too much about your actual protection
against fracture.
In addition to bone densitometry
testing, your physician may recommend other
types of tests such as blood tests, which may be
used to detect the presence of renal (kidney)
disease, evaluate the function of the
parathyroid gland, evaluate the effects of
cortisone therapy, and/or assess the levels of
minerals in the body related to bone strength,
such as calcium.
BENEFITS
OF OSTEOPOROSIS TREATMENT ON FRACTURE RISK MAY
BE GREAT, EVEN WHEN IMPROVEMENT IN BONE DENSITY
IS SMALL
ORLANDO, FLORIDA—A recent study revealed that
reduction in fracture risk from risedronate, a
bisphosphonate medication commonly used to treat
patients who have osteoporosis, is seen even in
patients who have only small improvements in
bone mass as measured by bone density, according
to research presented this week at the American
College of Rheumatology Annual Scientific
Meeting in Orlando, Florida.
Researchers reanalyzed data from the HIP and
VERT studies of risedronate, which included
nearly 9,000 patients from two randomized,
double-blind clinical trials. Patients were
selected to receive risedronate or placebo based
on low bone mineral density (an indicator of
bone strength) and/or pre-existing vertebral
fracture. All patients also received daily
supplements of calcium and vitamin D if baseline
levels were low. Researchers examined the
relationship between new vertebral fractures and
changes in lumbar spine bone mineral density in
both the placebo and risedronate groups. The
results confirm that increases in bone mineral
density results in a reduction in patients’ risk
for fracture as compared to those whose bone
mineral density decreases. Patients taking
risedronate had a decreased risk of fracture
compared to those taking placebo. Further, the
results show that there was a significant
reduction in fracture risk in all patients who
had improvements in bone density, and that the
reduction in fracture risk was independent of
the magnitude of the improvement. Comparisons of
bone mineral density increases among different
therapies may not translate into meaningful
differences in reducing fracture risk.
Fracture is a leading cause of disability in
patients with osteoporosis, which affects 10
million people in the U.S., the majority of whom
are women.
“Bone density measurements have helped us
identify those who are at high risk for
fractures,” said Jonathan Adachi, MD, Professor
and Director of the Arthritis Centre at St.
Joseph’s Hospital-McMaster University in
Hamilton, Ontario, Canada, an investigator on
the study. “In following patients on therapy,
our study suggests that even moderate increases
in bone density offer treatment benefit that is
not significantly different from larger
increases. This suggests that there may be a
threshold above which further increases in bone
density have little effect on fracture benefit.
This also reinforces the need for further
research to determine how osteoporosis therapies
influence other elements of bone quality that
affect the risk of fracture, such as bone
turnover and bone micro-architecture.”
The American College of Rheumatology is the
professional organization for rheumatologists
and health professionals who share a dedication
to healing, preventing disability and curing
arthritis and related rheumatic and
musculoskeletal diseases. For more information
on the ACR’s annual meeting, see
www.rheumatology.org/annual. |
