APPLICATIONS OF BONE DENSITOMETRY FOR OSTEOPOROSIS

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Osteoporotic fractures are now recognized as one of the most serious problems in public health. Among white women aged 65 to 84 years, 90% of hip and spine fractures, 70% of forearm fractures, and 50% of fractures at other sites are caused by osteoporosis.43 In the United States in 1995, the total health care costs attributable to osteoporotic fractures exceeded $13 billion, of which two thirds was attributable to hip fractures.51 One quarter of patients sustaining hip fracture die within 1 year of the fracture,30 and survivors frequently experience disability and loss of independence.35

Increased recognition of the morbidity and mortality attributable to osteoporosis has led to a major effort by the pharmaceutical industry to develop new therapeutic strategies for fracture prevention.5, 14, 40 At the same time, there has been a rapid evolution of new radiologic techniques for the noninvasive assessment of skeletal integrity.19, 24 Scans to measure bone mineral density (BMD), often performed in the spine or hip, are widely regarded as an essential part of the evaluation of patients at risk for osteoporosis. A technique closely associated with the recent growth in bone densitometry is dual X-ray absorptiometry (DXA).60 Associated with the advantages of high precision, low radiation dose, and stable calibration, DXA is an excellent modality to assist in the diagnosis of osteoporosis and aid decisions in treatment.

Despite the widespread popularity of DXA scanning of the spine and femur, there is continuing interest in new techniques for assessing the peripheral skeleton.22 In recent years, single-photon absorptiometry (SPA) scanning of the distal forearm has been updated by replacing the 125I radionuclide source with an X-ray tube. Another new peripheral technique is quantitative ultrasound (QUS) scanning of the calcaneus.22, 25, 45 Bone ultrasound systems use frequencies in the range of 0.2 to 1.0 MHz and measure broadband ultrasonic attenuation (BUA) and the speed of sound (SOS) in the heel. Advantages of QUS devices are their lack of ionizing radiation and the fact that they are cheaper and more portable than X-ray systems. Although DXA is currently the more widely accepted procedure, growing evidence suggests that the QUS scan is an effective substitute for BMD measurement in the calcaneus38 and is predictive of fracture risk.2, 26 It is expected that the United States Food and Drug Administration (FDA) will soon give approval for the routine clinical use of certain QUS devices.

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BONE DENSITOMETRY AND FRACTURE RISK

Osteoporosis is widely accepted as “a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture.”11 Because of the widespread availability of bone densitometry systems, the diagnosis of osteoporosis increasingly is being based on bone density measurements. The World Health Organization (WHO) working group has defined osteoporosis as a BMD value 2.5 or greater standard

Dual X-Ray Absorptiometry

The fundamental principle behind DXA is measurement of the transmission through the body of x-rays with two different photon energies. Because of the dependence of the attenuation coefficient on atomic number and photon energy, measurement of the transmission factors at two energies enables the area densities (i.e., mass per unit projected area) of two different types of tissue to be inferred. In DXA scans, these are taken to be bone mineral (hydroxyapatite) and soft-tissue, respectively.

A DXA

INTERPRETATION OF BONE DENSITOMETRY STUDIES

Primary care physicians and other clinicians requesting DXA scans usually have little or no expertise in interpreting the computer printout from bone densitometry studies. Thus, radiologists and others issuing clinical reports on the results of scans need to present a clear interpretation that provides the requesting physician with the necessary information on which to base decisions. Among the difficulties encountered in this task is the fact that there is little consensus in regards to how

Bone Densitometry and Fracture Risk

During the past 20 years, epidemiologic studies evaluating the association between BMD measurements and fracture risk have influenced the growth of the clinical applications of bone densitometry and have encouraged the rapid evolution of new techniques. Such studies have confirmed that BMD measurements in the peripheral and axial skeleton and QUS measurements in the calcaneus are predictive of fracture risk, with risk ratios in the range 1.5 to 2.5. Additional studies will be important in the

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    Address reprint requests to Glen M. Blake, PhD, Department of Nuclear Medicine, Guy's Hospital, St. Thomas Street, London SE1 9RT, United Kingdom

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