Bone Density Scan
Osteoporosis and low bone mass affects an estimated 23.4 million Americans, the majority of whom are women. As a result, this population is at an increase risk for fractures, particularly of the hip and the spine. The treatment and management of osteoporosis is discussed in detail elsewhere on this site. This article will focus on the role of bone mineral density (BMD) evaluation.
How does osteoporosis occur?
In order to understand the role of bone mineral density scanning, it is important to know a little about how osteoporosis occurs. Bone is constantly being remodeled. This is the natural, healthy state of continuous uptake of old bone (resorption) followed by the deposit of new bone. This turnover is important in keeping bones healthy and in repairing any minor damage that may occur with wear and tear. The cells that lay new bone down are called osteoblasts, and the cells responsible for resorption of old bone are called osteoclasts. Osteoporosis occurs as a result of a mismatch between osteoclast and osteoblast activity. This mismatch can be caused by many different disease states or hormonal changes. It is also commonly a result of aging. In osteoporosis, osteoclasts outperform osteoblasts so that more bone is taken up than is laid down. The result is a thinning of the bone with an accompanying loss in bone strength and a greater risk of fracture. A thinning bone results in a lower bone density or bone mass.
There are 2 major types of bone. Cancellous bone (also known as trabecular bone) is seen in areas such as the spine and wrists. This type of bone undergoes a rapid rate of turnover. As a result, if osteoclast and osteoblast activity becomes mismatched, cancellous bone is affected rapidly. Cortical bone is located in the arms and legs. This type of bone is metabolically slower than cancellous bone, and is therefore less affected by alterations in bone turnover.
There is a normal rate of decline in bone mass with age. In women, this bone loss is greatest in the first 3 to6 years after menopause . This loss is particularly damaging to cancellous bone. Since women generally have a lower bone mass to begin with in comparison with men, the ultimate result is an increase in the risk of fracture in postmenopausal women as compared to men of the same age. It is important to remember that men may also be at risk for osteoporosis, especially if they have other illnesses, a low testosterone level, are smokers, or are sedentary.
What is "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 bone mineral density to define specific diagnostic categories:
Normal: A value for BMD statistically within 1 standard deviation of an young adult. These people fall within the normal range.
Low bone mass: A value for BMD statistically more than 1 standard deviation but less than 2.5 standard deviations than an average young adult. These people have an increased fracture risk but do not meet the criteria for osteoporosis.
Osteoporosis: A value for BMD statistically greater than 2.5 standard deviations below an average young adult.
By these criteria, it is estimated that 30% of all postmenopausal Caucasian women have osteoporosis and that almost 60% have low bone mass.
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.
Why is BMD measurement important?
Determining a person's BMD helps a doctor decide if therapy for osteoporosis is needed. In addition, if therapy is started, subsequent BMD measurements are used to monitor the effectiveness of treatment. The purpose of BMD testing is to: - Help predict the risk of future fracture.
- Measure the amount of bone mass.
- Monitor the effectiveness of treatment.
What is the relationship between BMD and fracture risk?
In subjects with low bone mass (as defined above), 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.
Who should have BMD testing?
At present, the National Osteoporosis Foundation has recommended that testing be performed on all postmenopausal women under the age of 65 who have risk factors for osteoporosis (these include a previous history of fractures, low body weight, cigarette smoking, and a family history of fractures). In addition, it is recommended that all women over the age of 65 be tested, regardless of risk factors. It is also advised that anyone seeking therapy for osteoporosis be tested. These are guidelines only, and it should be remembered that testing is only indicated if it will influence treatment decision. For example, is the patient willing to be treated if the results are positive?
How is BMD measured?
Dual X-ray absorptometry (DXA) is the preferred technique for measuring BMD. [DXA is also called dual energy X-ray absorptometry or DEXA.] DXA is relatively easy to perform and the amount of radiation exposure is low. A DXA scanner is a large machine that produces 2 x- ray beams, each with different energy levels. One beam is high energy while the other is low energy. The amount of x-rays that pass through the bone is measured for each beam. This will vary depending on the thickness of the bone. Based on the difference between the 2 beams, the bone density can be measured.
At present, DXA scanning focuses on 2 main areas -- the hip and spine. Because osteoporosis involves the whole body, measurements of BMD at one site are usually predictive of fractures at other sites. However, by directly measuring the areas of particular interest, such as the hip and the spine, a direct observation can be made. For example, hip measurements provide a better prediction of hip fracture risk than measurements taken at other skeletal sites. In general, DXA scanning is performed on the hip (including an area of the femur called the "Wards triangle") and the spine. Scanning generally takes 10 to 20 minutes to complete.
Older techniques used a single beam x-ray that worked well for sites such as the wrist. However, to measure the hip, a single beam is not optimal since there is so much surrounding tissue. Single energy sources cannot correct for variations in soft tissue thickness, so their use is limited. Another older technique known as dual photon absorptometry (DPA) works on the same principles as DXA. However, it is slower and requires a larger radiation exposure.
What are other methods of measuring BMD?
Quantitative computed tomography (QCT) can be used to assess BMD. A standard CT scanner is used in this method. However, the amount of radiation exposure is higher than with DXA and the cost is greater.
Ultrasound is a relatively new diagnostic tool to measure BMD. There is no radiation source with this procedure. An ultrasound beam is directed at the area being analyzed. The scattering and absorption of the waves allow for an assessment of bone density. The results are not as precise as with the other methods mentioned. This technique is relatively new and there is considerable research being conducted in this area. Since ultrasounds can easily be performed in a physician's office, this method may be valuable for screening larger populations once its accuracy has been refined.
It must be realized that all tests of BMD have some margin of error. Follow-up testing is often helpful to interpret the significance of the results for an individual patient.
How often should DXA 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.
What is the cost of DXA?
The cost for DXA scanning varies depending on insurance policies and coverage. In general, a patient without coverage paying cash can expect to pay approximately 200 U.S. dollars for the procedure.
What about the accuracy of BMD testing in the doctor's office using smaller equipment?
There are several devices that are smaller than the standard DXA scanner that are being used in doctors offices to screen for low bone density. Very little scientific data is available about these smaller units. Most of the information comes directly from the equipment manufacturers themselves. Many of these models test peripheral bones in the feet or hands. Other units use ultrasonography. These techniques can be less accurate than BMD testing performed with state of the art equipment. Additionally, office testing equipment can range dramatically in price and quality.
In general, these devices may be reasonable to measure overall fracture risk, but are not useful in monitoring therapy. Their use might be limited to screening and results would require confirmation using the more reliable BMD tests described above. In addition, expertise in using the equipment and interpreting the data can vary. At present, it is difficult to comment on these smaller methods of BMD testing.
Summary
Osteoporosis is a disease that results in a significant risk of fracture. The consequences of fracture can include hospitalization, immobility, and a decrease in the quality of life. From a larger perspective, it is a costly disease in terms of the healthcare system and time lost from work. Early detection and therapy is the mainstay for trying to prevent these complications. BMD testing results correlate well with the risk of fracture and the testing is easily performed in a time efficient manner without any discomfort. Although many methods of BMD testing exist, the best to date is DXA scanning. It is imperative that testing ultimately be done using state of the art equipment with capable personnel and a doctor well versed in interpreting the results. - Up to 30% of postmenopausal women in the U. S. have loss of bone density to the level of osteoporosis. An additional 54% have sub- optimal bone density.
- At least 90% of hip and spine fractures in elderly women can be attributed to low bone density.
- In 1995, osteoporosis-related fractures were associated with over 400,000 hospitalizations and over 2.5 million doctor's visits.
- Bone mineral density (BMD) is a measured calculation of the true mass of bone.
- BMD analysis is recommended for women under the age of 65 with risk factors and for all women over the age of 65.
- 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.
- Dual X-ray absorptometry (DXA) is quick, painless and the preferred method to measure BMD.
- Osteoporosis has many available treatment options once the diagnosis is made.
How to Order | Price
List | Refill Orders |
Order Tracking
Home |
About Us | Contact
Us | FAQ
Buy cheap canadian prescriptions from a Canadian Internet Pharmacy leader.
cheap online pharmacy canadian pharmacy prescription drugs online discount medicine pharmacies
online mail order prescription medication canadian online pharmacy search for online drugs
canada pharmacies discount canadian medication drugstore Plavix Fosamax Lipitor Celebrex
Actonel Flomax Glucophage Premarin Prevacid Evista .
|
