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 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 fracture- related 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