Osteoporosis is a disease of progressive bone loss associated with an increased risk of fractures. The term osteoporosis literally means "porous bone." Diagnosis of osteoporosis involves a measurement of bone mineral density (BMD).
The history of BMD measurement dates back to the 1940s. At that time, bone density was measured on plain radiographs (X-rays). However, because loss of bone density is not apparent on a plain X-ray until approximately 40% of the bone is lost, different methods of BMD measurement have been developed.
The Singh index describes the trabecular patterns in the bone at the top of the thighbone (femur). X-rays are graded 1 through 6 according to the disappearance of the normal trabecular pattern. Studies have shown a link between a Singh index of less than 3 and fractures of the hip, wrist, and spine.
Radiographic absorptiometry was developed during the late 1980s as an easy way to determine BMD with plain X-ray. An X-ray of the hand is taken, incorporating an aluminum reference wedge. The X-ray is then analyzed, and the density of the bone is compared to the density of the reference wedge.
In the early 1960s, a new method of measuring BMD, called single-photon absorptiometry (SPA), was developed. In this method, a single-energy photon beam is passed through bone and soft tissue to a detector. The amount of mineral in the path is then quantified. The distal radius (wrist) is usually used as the site of measurement because the amount of soft tissue in this area is small.
SPA measurements are accurate, and the test usually takes about 10 minutes. The radioactive source gradually decays, however, and must be replaced after some time.
Dual-photon absorptiometry (DPA) uses a photon beam that has two distinct energy peaks. One energy peak is absorbed more by the soft tissue. The other energy peak is absorbed more by bone. The soft-tissue component is subtracted to determine the BMD.
DPA allowed for the first time BMD measurements of the spine and proximal femur. However, although DPA is accurate for predicting fracture risk, the precision is poor because of decay of the isotope. In addition, the machine has limited usefulness in monitoring BMD changes over time.
Dual-energy X-ray absorptiometry (DXA) works in a similar fashion to DPA, but uses an X-ray source instead of a radioactive isotope. This measurement technique is superior to DPA because the radiation source does not decay and the energy stays constant over time. DXA has become the "gold standard" for BMD measurement today.
Scan times for DXA are much shorter than for DPA, and the radiation dose is very low. The skin dose for an anteroposterior spine scan is in the range of 3 mrem.
DXA scans are extremely precise. Precision in the range of 1% to 2% has been reported. DXA can be used as an accurate and precise method to monitor changes in bone density in patients undergoing treatments.
The first generation DXA machines used a pencil beam-type scanner. The X-ray source moved with a single detector. Second-generation machines use a fan-beam scanner that incorporates a group of detectors instead of a single detector. These machines are considerably faster and produce a higher resolution image.
Measurement of BMD by quantitative computed tomography (QCT) can be performed with most standard CT scanners. QCT is unique in that it provides for true three-dimensional imaging and reports BMD as true volume density measurements.
The advantage of QCT is the ability to isolate an area of interest from surrounding tissues. QCT can, therefore, localize an area in a vertebral body of only trabecular bone, leaving out the elements most affected by degenerative change and sclerosis.
The radiation dose with QCT is about ten times that of DXA, and QCT tests may be more expensive than DXA.
Lower cost portable devices that can determine BMD at peripheral sites such as the radius, phalanges, or calcaneus are increasingly being used for osteoporosis screening. The advantage of using a portable device is the ability to bring BMD assessment to a population who otherwise would not be able to have the test. These machines are considerably less expensive than those that measure BMD in the hip and spine.
One of the problems with peripheral testing is that only one site is tested; thus, low bone density in the hip or spine may be missed. This may be a problem because of differences in bone density between different skeletal sites.
Although peripheral machines are considered accurate, doubts have been raised about their precision. Peripheral machines may not be good enough to monitor patients undergoing treatment for osteoporosis.
In postmenopausal women, differences in BMD between different skeletal sites is more common. BMD may be normal at one site and low at another site. In the early postmenopausal years, bone density in the spine decreases first because the bone turnover in this highly trabecular bone is greater than at other skeletal sites. Bone density becomes similar across the skeleton at approximately 70 years of age.
In early postmenopausal women--therefore, up to the age of 65 years--the most accurate site to measure BMD is probably the spine. In women older than 65 years, BMD is similar across the skeleton; therefore, it may not make much difference which site is measured.
Caution must be used when interpreting spine scans in elderly patients because degenerative changes may falsely elevate BMD values. BMD measurements are, however, mostly site specific, and the most accurate predictor of fracture risk at any site is a BMD measurement of the spine.
At present, peripheral BMD testing machines are good screening devices because of their portability, availability, and lower cost. However, the following patients may still need central testing, even if peripheral testing is normal:
- Postmenopausal patients not on hormone replacement therapy (HRT) who would consider HRT, bisphosphonates, or selective estrogen receptor modulators (SERMs), if low bone mass is discovered
- Patients with a maternal history of hip fracture, smoking, tallness (more than 5' 7") or thinness (less than 125 lb)
- Patients on medications associated with bone loss
- Patients with secondary conditions associated with low bone mass, such as hyperthyroidism, posttransplantation, malabsorption, hyperparathyroidism, and alcoholism
- Patients found to have high urinary collagen crosslinks
- Patients with a history of previous fragility fracture
The main purpose of obtaining a bone density test is to determine fracture risk. BMD correlates very well with risk of fracture. It is more powerful in predicting fractures than cholesterol is in predicting myocardial infarction or blood pressure in predicting stroke.
The T-score is the number of standard deviations (SD) above or below the young adult mean. The young adult mean is the expected normal value for the patient compared to others of the same sex and ethnicity. It is approximately what the patient should have been at their peak bone density at about age 20 years.
As a general rule, for every SD below normal the fracture risk doubles. Thus, a patient with a BMD of 1 SD below normal (a T-score of -1) has twice the risk of fracture as a person with a normal BMD. If the T-score is -2, the risk of fracture is four times normal. A T-score of -3 is eight times the normal fracture risk. Patients with a high fracture risk can be treated to prevent future fractures.
Other risk factors for fracture include a person's eyesight, balance, leg strength, and physical agility. Age itself is an independent risk factor for fracture, independent of bone density. Osteoporosis patients that have had a previous fragility fracture are considered to have severe osteoporosis and have a high risk for future fractures.
The Z-score is the number of SD the bone density measurement is above or below the value expected for the patient's age.
Primary osteoporosis is age-related osteoporosis, with no secondary causes.
Secondary osteoporosis occurs when underlying agents or conditions induce bone loss. Some common causes of secondary osteoporosis are thyroid or parathyroid abnormalities, malabsorption, alcoholism, smoking, and the use of certain medications especially corticosteriods.
A Z-score lower then -1.5 is suggestive of secondary osteoporosis. If secondary causes are suspected, laboratory testing should be performed to find out if there is an underlying reason for the osteoporosis. This is important because treating the underlying condition may be necessary to correct the low bone density.
The American Academy of Orthopaedic Surgeons
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