How is bone mineral density (BMD) measured in patients with osteoporosis?

Updated: Jan 19, 2021
  • Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Felix S Chew, MD, MBA, MEd  more...
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The most accurate way to diagnose osteoporosis is by measuring bone mass. Bone mineral density (BMD) is determined by measuring the amount of bone mineral (calcium hydroxyapatite) per unit volume of bone tissue. X-rays or gamma rays are often used to quantify BMD. In quantitative terms, BMD is the amount of calcium hydroxyapatite, or Ca10 (PO4)6 (OH)2, per unit volume of bone tissue examined. Common methods include conventional radiography, quantitative CT (QCT), single-photon absorptiometry (SPA), dual-photon absorptiometry (DPA), quantitative ultrasonography (QUS), [12] and dual-energy x-ray absorptiometry (DEXA). DEXA scans can be used to detect small changes in bone mass by comparing the patient's bone density to that of healthy adults (T score) and to age-matched adults (Z score). [8, 13, 14, 15, 16, 17, 2, 9, 3, 18, 19, 11]

A number of methods have been developed for the in vivo determination of bone density in patients at risk for osteoporosis. Two of the most frequently used methods are based on measuring the attenuation of a beam of electromagnetic radiation or ultrasound when it passes through the bone. Ultrasonic measurement of velocity through the bone has also been used to determine bone density. [20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33]

Currently, DEXA is the most accurate and recommended method for BMD measurement. It is a sensitive technique and can detect changes in bone density only 6-12 months after a previous measurement is obtained. Density measurements of the spine or hip are used. The procedure takes approximately 20-30 minutes. The radiation exposure is low at approximately 2.5 mrem.

Bone biopsy may be useful in unusual forms of osteoporosis, such as osteoporosis in young adults. Biopsy provides information about the rate of bone turnover and the presence of secondary forms of osteoporosis, such as myeloma and systemic mastocytosis. Patients with a high turnover usually respond better to antiresorptive drugs than to other treatments. Bone turnover can also be evaluated by estimating certain biochemical markers, such as osteocalcin and deoxypyridinoline. [34] Biochemical markers can be more useful than bone density for monitoring treatment, as changes in bone density may not be detected for 2 years. [35, 36]

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