BONE MINERAL DENSITY

Considerable progress in the development of methods for assessing the skeleton now makes it possible to detect osteoporosis noninvasively and early. Several techniques are available to measure BMD noninvasively. The choice of BMD method should be based on the anatomic sites available for study, accessibility of technology to the patient, and cost. BMD measurements can be used to establish or confirm a diagnosis of osteoporosis. BMD has a continous, graded, inverse relationship to the risk of fracture: the lower the BMD, the greater the risk of fracture.

In order to standardize values from different densitometers, results are reported as standard deviations (SD) above or below the peak bone mass for the patient’s age and sex matched reference (Z-score) or sex-matched normal young adult reference (T-score). T-score is the most clinically relevant value on the BMD report and can help confirm a diagnosis of osteoporosis.

WHO Definitions

In 1994, an expert panel of the World Health Organization (WHO) proposed criteria for the diagnosis of osteoporosis in adult women.¹¹ For each standard deviation reduction in bone mineral density, fracture risk increases by approximately 1.5 to 3 fold; below 2 standard deviations, it increases exponentially.¹¹ The criteria take advantage of the fact that fracture risk (the primary clinical problem in osteoporotic patients) increases with decreasing bone mineral density. The reference points for the various diagnostic classifications are listed here and are referred to as the "T score." This method characterizes osteoporosis on the basis of a reference range of mean young adult bone mineral density. By using this approach, the incidence of osteoporosis can be seen to increase with age, as bone mineral density declines below the threshold for the osteoporosis diagnosis.¹¹

Although using a T score of -2.5 (i.e., 2.5 SD below the young adult mean for bone mineral density) as a cut off point for an osteoporosis diagnosis is clinically relevant when looking at the population as a whole, it should be remembered that there is a large overlap between bone mineral density in patients with and without fracture. Therefore, it is important to keep the individual patient's characteristics in mind and use the above criteria only as a guide in establishing the diagnosis.

This figure depicts BMD in women as a function of age. The proportion of patients with a BMD measurement of 2.5 standard deviations or more below the young adult mean increases exponentially with age. (Adapted from reference 11)

MEASUREMENT TECHNIQUES

Bone densitometry is the best available method for diagnosing osteoporosis and identifying those individuals at risk. This table gives an overview of the methods used to measure bone mineral density (BMD). The basic function of each of these methods is to measure the amount of bone mineral present which is an important determinant of bone strength.

DXA

The traditional methods are based on the principle of x-ray absorptiometry. These include dual-energy x-ray absorptiometry (DXA), peripheral site DXA (pDXA), single-energy x-ray absorptiometry (SXA), and radiographic absorptiometry (RA). In addition to these approaches is qualitative computed tomography (QCT) and peripheral QCT (pQCT), which utilize a computed tomography scan to calculate bone mineral density. All these techniques have proven ability to assess osteoporotic fracture risk, but differ in the sites of the body that they measure and the required equipment to perform the measurement.¹⁵-¹⁸ Quantitative ultrasound (QUS) is fairly inexpensive, portable, and is radiation free.

pDXA

The site typically measured is the heel, and is positioned between two ultrasound transducers. Bone mass is determined by the transmission of sound waves passing through the bone; the fewer that pass through, the denser the bone.¹⁹

Guidelines published by the Society for Clinical Densitometry state that any available method is acceptable, and the choice of technique and site for measurement should be at the discretion of the physician.¹⁸ However, evidence suggests that direct measurement of the hip and spine may be more predictive of fracture risk at those sites.¹⁹

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