Approximately
10 million people in the United States currently have osteoporosis and
about 1.5 million people suffer an osteoporosis-related fracture each
year. These fractures have a major impact not only on health and
quality of life of the individual but also on the costs of health care
delivery. Hip fractures frequently result in the end of independent
living and are associated with a mortality rate of 20% within the first
year. In 2002, annual direct care expenditures on osteoporotic
fractures were $12-18 billion.
Low bone mineral
density (BMD) in white women has been demonstrated to be a better
predictor of future fractures risk than many other commonly accepted
screening tests. Therefore, screening for low BMD to identify
individuals with dangerously low BMD is appropriate and, as pointed out
in a recent report of the Surgeon General, prevent many osteoporotic
fractures.
Populations at High Risk for Osteoporosis
The largest high-risk population comprises post-menopausal women over
65 yrs. Men develop osteoporosis about a decade later at age over 75
yrs. Other factors that increase risk for osteoporosis include low body
weight (BMI < 23), family history of osteoporosis, significant
history of smoking or alcohol excess, early menopause, testosterone
deficiency in men, and therapy that weakens bones, such as
corticosteroids.
One of the most important “red flags” of inadequate bone strength is a
fragility fracture, defined as a fracture resulting from a fall from
less than curb height. These include “silent” fractures due to collapse
of spinal vertebrae, which often are not recognized as fractures but
may present as back pain. People with a vertebral fracture,
either silent or symptomatic, have an increased risk of future
vertebral fractures and future hip fractures. |
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Common Indications for Bone Density Scans
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| Women over 65 yrs |
| Men over 75 yrs |
| Younger individuals with major risk factors: |
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History of prior fracture with minimal trauma
Low body weight (BMI <23)
Premature menopause
Testosterone deficiency in men
Chronic glucocorticoid therapy |
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Screening for Osteoporosis
All patients with major risk factors for osteoporosis should be
screened for osteoporosis by bone densitometry of the spine and
hip. Dual-energy x-ray absorptiometry (DEXA) is the imaging
method of choice in most cases. This is a very low dose x-ray scan in
which the amount of x-ray energy absorbed by bone mineral is measured
and the bone mineral content calculated. Bone mineral content divided
by bone area gives the BMD. Because DEXA measurements are not
corrected for thickness of the bone in the direction of the x-ray beam,
DXA BMD measures a combination of bone size and true bone mineral
density.
Quantitative CT (QCT) is an alternative examination for measuring BMD
that requires a somewhat greater exposure to ionizing radiation than
DEXA. QCT is generally reserved for special cases in which DEXA scans
are difficult to interpret because of scoliosis or osteoarthritis, or
when it is necessary to document the earliest changes in trabecular
bone. QCT is a three dimensional analysis, which gives BMD by volume
and which can be segmented into cortical and trabecular bone. QCT is
more sensitive than DEXA but is not as reproducible.
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Comparison of DEXA and QCT
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| DEXA |
QCT |
| Low radiation dose |
Somewhat higher radiation dose
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Bone mineral density of
area (2D) |
Bone mineral density of volume (3D); can be segmented into cortical and trabecular bone |
| High reproducibility |
Less reproducible
than DEXA |
| Accuracy may be limited by: |
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Spinal deformity
Vertebral compression fracture
Osteoarthritis |
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High accuracy |
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DEXA
images of lumbar spine and hip of 60 yr old female. The T-score from
the lumbar spine was –2.7 and the Z-score was –1.3. At the hip, the
T-score was –2.0 and the Z-score was –0.9. The patient was diagnosed
with osteoporosis.
Figure A:
Lateral scout image of the spine used to place the scans in the center
of the L1, L2, L3, and L4 vertebrae, parallel to the endplate.
Figure B:
Sample axial image through the L1 vertebra, showing placement of the
region of interest in a region of pure trabecular bone. The
densitometry “phantom” used to quantify the CT image is also shown
below the patient. |
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Bone Density Scan Interpretation
Bone densitometry results are reported in standard deviation (SD) as
T-scores and Z-scores, both of which determine the difference between
an individual’s BMD with a population mean. The T-score references a
young healthy population of the same sex whereas the Z-score population
standard is adjusted to match the patient age. A low T-score is a
measure of future fracture risk, since this risk is due to absolute
bone loss. A low Z-score indicates that the patient’s BMD is worse than
others of the same age. If so, an evaluation for a secondary cause of
osteoporosis may be indicated.
Although the World Health Organization defines osteopenia as a T-score
of –1 to –2.5, 15% of a young healthy population might have a T-score
of – 1.0 or lower on statistical grounds alone, whereas only 5% might
have a T-score of – 2.0 or lower. Osteoporosis is defined as a BMD
T-score <-2.5. However, for each SD drop in bone density, the risk of
fracture doubles. Patients with T-scores of <-2.0 should be
considered at risk for future fractures and are candidates for
treatment, as are elderly patients with T-scores of <- 1.5.
T- and Z-scores may vary considerably among skeletal sites, both
because of normal variations in the proportions of cortical and
trabecular bone, as well as variations in the rate of bone loss, which
is often greater in trabecular bone. For example, lumbar spine T-scores
are often much lower than those from the femoral neck because vertebral
bodies are largely made up of trabecular bone. Thus the spine is
generally most useful for detecting osteoporosis, if the interfering
effects of spinal osteoarthritis are avoided. If the T-scores vary from
site to site, it is generally recommended that treatment decisions be
based on the lowest T-score. Modified scans are available for special
purposes. Forearm scans are useful in patients with hyperparathyroidism
or hyperthyroidism, diseases which selectively affect cortical bone. |
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Monitoring Disease Progression
Bone
densitometry can also be used to monitor disease progression or the
effectiveness of therapy over time. Changes in posterior-anterior
lumbar spine BMD >0.03 gm/cm2 are considered to be true changes. Similarly, changes of >0.04gm/cm2 in
the lateral lumbar spine, total hip, or femoral neck are almost always
due to true changes in the patient’s skeleton. Changes are most often
seen in the lumbar spine.
Scheduling
The
MGH Department of Radiology and the Endocrine Unit are now
collaborating in the performance of bone density examinations.
Appointments at all sites can be scheduled through the Radiology Order
Entry system, http://mghroe
or by calling 617-724-9729 (Radiology) or 617-726-3839 (Bone Density Laboratory, Endocrine Unit).
Further Information
For further questions, please contact Daniel I. Rosenthal, M.D.
, Musculoskeletal Radiology, at 671-726-8784, or Robert M. Neer, M.D.
, Endocrine Unit, Department of Medicine, at 617-726-6723.
This article provided useful information about the appropriate use of imaging studies:
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References
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U.S. Department of Health and Human Services, Office of the Surgeon General. (2004). Bone Health and Osteoporosis: A report of the Surgeon General. http://www.surgeongeneral.gov/library/bonehealth/docs/exec_summ.pdf
Cummings, SR, Bates, D and Black, DM. (2002) Clinical use of bone densitometry: scientific review. JAMA 288: 1889-1897
Morris, CA, Cabral, D, Cheng, H, Katz, JN, et al. (2004) Patterns of bone mineral density testing: current guidelines, testing rates, and interventions. J Gen Intern Med 19: 783-790
Nelson, HD, Helfand, M, Woolf, SH and Allan, JD. (2002) Screening for
postmenopausal osteoporosis: a review of the evidence for the U.S.
Preventive Services Task Force. Ann Intern Med 137: 529-541
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