Transcript Bone Mineral Density Testing

Bone Mineral Density Testing
March 29, 2007
Introduction
Osteoporosis is a systemic skeletal disorder
characterized by decreased bone mass and
deterioration of bony microarchitecture
This decrease in bone mineral density (BMD)
results in fragile bones and an increased risk for
fracture with even minimal trauma
Prior to testing, osteoporosis was clinically silent
until a fracture occurred
Pathophysiology
Decreased bone mineral density is a result
of a combination of genetic and
environmental factors that affect both peak
bone mass and the rate of bone loss
These factors include medications, diet,
race, sex, lifestyle, and physical activity
Frequency
Approximately 10 million people have
osteoporosis. Another 14-18 million have
osteopenia (low bone mass)
Approximately 1.5 million fractures per
year in the United States are attributed to
osteoporosis, and more than 37,000
people die from subsequent fracturerelated complications
So Who Do We Test?
Postmenopausal women older than 65 years
Postmenopausal women younger than 65 years who
have 1 or more risk factor
Postmenopausal women who present with fragility
fractures
Women who are considering therapy in which BMD will
affect that decision
Women who have been on hormone replacement
therapy (HRT) for prolonged periods
Men who experience fractures after minimal trauma
People with evidence of osteopenia on radiographs or a
disease known to place them at risk for osteoporosis
Lab Studies
Levels of serum calcium, phosphate, and
alkaline phosphatase are usually normal in
persons with primary osteoporosis,
although alkaline phosphatase levels may
be elevated for several months after a
fracture
It is important to also check thyroid
function, and testosterone levels in men
Imaging Studies
First, obtain plain radiographs if a
decrease in bone mineral density is
suspected
Osteopenia may be apparent as
radiographic lucency but is not always
noticeable until 30% of bone mineral is lost
Plain radiography is not as accurate as
BMD testing
BMD Imaging
BMD tests are usually done on bones that
are likely to break as a result of
osteoporosis like the lower spine and hip
Can also be done on the wrist or heel
Devices that measure BMD include:
– Quantitative computed tomography
– Dual-energy x-ray absorptiometry (DEXA)
– Quantitative ultrasonography
– Radiogrammetry
Quantitative Computed
Tomography
Quantitative computed tomography measures
BMD as a true volume density in g/cm3, which is
not influenced by bone size.
This technique can be used for both adults and
children.
Disadvantages in that (1) it only determines
bone density at the spine, (2) osteophytes can
interfere with measurement, and (3) it is
associated with significant radiation exposure
and high cost
DEXA
Dual-energy x-ray absorptiometry requires less radiation,
is less expensive, and has better reproducibility than
quantitative computed tomography
Can also measure bone density at the spine and the hip.
It has become the standard method for determining bone
density.
This method can be used in both adults and children
Confounding factors in DEXA results interpretation
(falsely high bone density) include spinal fractures,
osteophytosis, and extraspinal (eg, vascular) calcification
Peripheral DEXA can be used to measure BMD in the
wrist
Quantitative Ultrasonography
Quantitative ultrasonography of the
calcaneus can be used for general
screening
However, this is not as accurate as other
methods and thus is less useful in
following response to treatment
Its advantages include low cost, portability,
and lack of ionizing radiation
Radiogrammetry
Radiogrammetry, used to measure cortical
dimensions, is usually performed on the
hand, specifically the second metacarpal
It is useful in assessing BMD in children
and is the simplest and least expensive
method
Disadvantages are that it is not as precise
as DEXA and, therefore, is less sensitive
for detecting changes over time
What Are The Results?
Results are reported as two values, T and Z
scores
T scores are the number of standard deviations
above or below what is normally expected in a
healthy young adult of the same sex
Z score is the number of standard deviations
above or below what is normally expected for
someone of the same age, sex, weight, and
ethinic origin
T Score
Above -1 indicates the bone density is
normal
Between -1 and -2.5 indicates bone
density is below normal, or osteopenia
Below -2.5 indicates osteoporosis
DEXA Images
BMD Score Report
Z Score
The Z score is help ful because it may
suggest that the patient may have a
secondary form of osteoporosis unrelated
to normal aging which is causing
decreased BMD
A score less than -1.5 should make you
investigate the cause of decreased BMD
Another Report Card
For example, if the T-score is -2.0, the BMD is lower than
average by two standard deviations. If the Z-score is 0.5, your bone density is less than the norm for people
your age by one-half of a standard deviation
QCT
QCT
QCT isolates metabolically active
trabecular bone for greater anatomic
accuracy than other methods
A series of axial scans are taken with the
patient lying on a calibration phantom
Other Tests
As mentioned earlier, ultrasound and
radiogrammetry can be used as well
These are not as accurate in determining
BMD loss but have advantages like less
radiation, smaller equipment, and they
measure BMD using smaller bones