What is bone densitometry?
Bone densitometry is a noninvasive technology that is used to measure bone mass. Bone mass, simply put, is the weight of the skeleton, overall or in specific regions. It is most important to measure bone mass in the spine, hips, and arms because these are the areas most likely to fracture when bone mass is low. The amount of bone you have in your skeleton determines how strong it is and how much trauma or force it can withstand before it fractures. Bone densitometry measurements can be useful in persons with and without osteogenesis imperfecta (OI) to estimate the risk of fracture and to assess the result of treatments.
How is it done?
The most commonly used method for measuring bone mass is called Dual Energy X-Ray Absorptiometry, or DEXA. To measure bone mass with a DEXA machine, the person lies on a flat padded table and remains motionless while the "arm" of the instrument passes over the whole body or over selected areas. While the measurement is performed, a beam of low-dose x-rays from below the table passes through the area being measured. These x-rays are detected by a device in the instrument's arm. The machine converts the information received by the detector into an image of the skeleton and analyzes the quantity of bone the skeleton contains. The results are usually reported as BMD, or "bone mineral density," the amount of bone per unit of skeletal area.
For the spine measurement, the person's lower legs rest on a styrofoam cube with the hips flexed. For the hip measurement, the toes are placed in a "pigeon-toed" position to rotate the hips and provide the largest projected area to measure. For the arm measurement, the person sits on a chair beside the machine and places an arm into a holding device while the measurement is taken. For a total body measurement, which provides individual measurements of the legs, the trunk, the pelvis, the ribs, the arms and the skull, the person simply lies flat and motionless.
It is important to remember that, if the patient moves while the measurement is taking place, errors can occur. Usually it is not too difficult for the person to remain motionless because, using the latest equipment, each measurement requires less than a minute to perform.
Why is it used?
Bone densitometry will not by itself provide a diagnosis of OI but, when combined with personal and family medical history, findings on physical examination, x-rays and biochemical testing, it can provide important information to support a diagnosis of OI. For example, if a person (usually a child not known to have OI) suffers a fracture, there may be a question about whether the trauma was sufficient to have caused the fracture. While some studies have shown that OI patients often have significantly reduced bone density, other studies indicate that bone density may be normal in OI patients who are mildly affected.
Why is bone densitometry important in OI?
One of the most consistent features of the skeletal defect in OI is low bone density, a major reason for excess skeletal fragility and fractures in this disorder. Densitometry can be helpful in assessing skeletal development in children with OI and determining the likelihood of fractures. It can also be useful to assess the possible deleterious effects of injuries, illness and/or medications in patients with OI. This technology allows physicians to assess the effects of treatments designed to build up bone mass or prevent bone loss.
Finally, it is necessary to understand that some DEXA measurements can be misleading. Skeletal deformities (such as spine curvature, compression fractures in vertebrae, or orthopaedic metal) can significantly impair or destroy the usefulness of DEXA. Furthermore, patient body size has an important effect on DEXA values and how they should be interpreted. Many physicians (even radiologists) may not appreciate the effect of body size. DEXA measurements in short stature adults or children can be artifactually low and therefore especially difficult to interpret. Consideration of body size may be necessary. Most DEXA machines now provide normal ranges for spine measurements in children, but normal ranges for other skeletal sites in children are not yet routinely available. These concerns, however, are now being addressed in the medical literature.
Is densitometry safe, and what about radiation?
DEXA technology is quite safe. Radiation exposure is exceedingly small, about the equivalent of playing outdoors all day on a summer day or flying across the United States in a jet plane. Using DEXA, a person receives less than 10% of the radiation received in a standard chest x-ray. State regulatory agencies permit routine measurement of healthy children.
Does it hurt?
Bone densitometry is not painful. There is no needle stick involved, and one cannot feel anything when the x-rays pass through the body. The hardest part is remaining still for the minute or so when the measurement is performed. The DEXA instrument does not enclose the patient.
What does it cost?
The price varies considerably in different areas. One can expect to pay between $75.00 and $150.00, depending on which machine is used and what areas of the skeleton are measured. Most metropolitan areas have several densitometers.
How can you get densitometry measurements?
To obtain a bone density measurement, one usually must be referred by a physician. While some densitometry centers will perform measurements without a doctor's request, it is helpful to have a personal physician who will interpret the results in the context of the individual's complete medical situation.
When should you be measured?
While bone densitometry measurement may be useful in the management of anyone with OI, individuals with Type I OI usually benefit most. They have the longest life span among people with OI and will be faced with the bone loss associated with menopause, immobilization, medications (such as cortisone-like drugs), etc. Thus, bone density measurements may be most useful in managing these problems, particularly menopause. With other types of OI, the skeletal defects tend to be worse, the bone density is extremely low, and the management is difficult regardless of bone density. However, bone density measurements may be useful in monitoring the skeletal condition of these patients.
How often should you be measured?
Bone density measurements are usually performed no more than once a year in adults. Bone mass changes relatively slowly in most adults, even in individuals with OI. It is not useful to perform measurements when the expected changes in bone mass are smaller than the machine can accurately detect.
Bone densitometry by DEXA or other methods can be useful in the management of patients with OI. The procedure is easy, noninvasive, safe, and convenient. The technology is accessible to almost everyone. Though it is expensive, it is becoming an integral part of the medical management of patients with osteogenesis imperfecta.
1. Reinus, W.R., McAlister, W.H., Schranck, F., Chines, A., & Whyte, M.P. (1998). Differing lumbar vertebral mineralization rates in ambulatory pediatric patients with osteogenesis imperfecta. Calcif Tissue Inc., 62:17-20.
2. Whyte, M.P. (1992). Hereditary metabolic and dysplastic skeletal disorders. In Coe F.L., Favus M.J. (Ed.), Disorders of Bone and Mineral Metabolism. Raven Press, Ltd., 219-40.
3. Kurtz, D.W., Morrish, K., Shapiro, J. (1985). Vertebral bone mineral content in osteogenesis imperfecta. Calcif Tissue Int., 37:14-18.
4. Shapiro, J. (1996). Osteogenesis imperfecta and other defects of bone development as occasional causes of adult osteoporosis. In: Marcus, R., Feldman D., Kelsey, J. (Eds.), Osteoporosis. Academic Press, 703-13.
This fact sheet was prepared by Dr. Robert Recker, Osteoporosis Research Center,
Creighton University School of Medicine, Omaha, Nebraska.
This information is brought to you by the
NIH Osteoporosis and Related Bone Diseases~National Resource Center (ORBD~NRC)
and the Osteogenesis Imperfecta Foundation
National Institutes of Health
Osteoporosis and Related Bone Diseases
National Resource Center
1232 22nd St., NW
Washington, DC 20037-1292
Tel: 800/624-BONE or 202/223-0344
Fax: 202/293-2356, TYY: 202/466-4315
The National Resource Center is supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases with contributions from the National Institute of Child Health and Human Development, National Institute of Dental and Craniofacial Research, National Institute of Environmental Health Sciences, NIH Office of Research on Women's Health, Office of Women's Health, PHS, and the National Institute on Aging. The Resource Center is operated by the National Osteoporosis Foundation, in collaboration with the Paget Foundation and the Osteogenesis Imperfecta Foundation.