Measurement of bone mass
Bone mass, measured by bone mineral densitometry, is used to establish the diagnosis of osteoporosis, to predict the risk of subsequent fractures, and to monitor changes in bone mass during therapy for osteoporosis. No clinical finding, laboratory test, or other radiographic examination is able to reliably identify individuals with osteoporosis. Conventional radiographic techniques are not sensitive enough to diagnose osteoporosis as they do not reliably detect bone loss until 30% to 40% of bone mineral is lost. Although bone densitometry may determine low bone mass, it cannot identify the etiology of the bone loss. Thus, bone densitometry must be used with a complete clinical evaluation, laboratory testing, and other diagnostic studies to determine the cause of and the most appropriate treatment for osteoporosis.
2. Is bone mass the only parameter that determines whether a bone will fracture?
Although decreased bone mass is the primary determinant of whether a bone will fracture, bone architecture and geometry are also important factors contributing to bone strength. The relationship between bone mass and fracture risk is more powerful than the relationship between serum cholesterol concentration and coronary artery disease. A decrease in bone mass of one standard deviation (SD) doubles the risk of fracture. In comparison, a decrease in the cholesterol concentration of 1 SD increases the risk of coronary artery disease by only 20% to 30%.
3. How does bone densitometry measure bone mass?
All bone densitometry techniques determine the amount of calcium present in bone by utilizing an ionizing radiation source (either from a radionuclide or from an x-ray tube) and a radiation detector. Bone densitometry is based on the principle that bone absorbs radiation in proportion to its bone mineral content. The bone mineral content of the bone (or a region of interest within a bone) is then divided by the measured area. The result is the bone mineral density (BMD), expressed in grams per unit area (g/cm2). This BMD is not a true volumetric density (g/cm3) but rather an areal density. In this chapter, bone mass and BMD are used interchangeably.
4. What techniques are available to measure bone mass?
The techniques available to measure bone mass include dual-energy x-ray absorptiometry (DXA) and quantitative computed tomography (QCT). Central DXA measures bone mass of the hip, spine, and whole skeleton, whereas peripheral DXA measures bone mass of the forearm only. Another technique, quantitative ultrasound (QUS), transmits ultrasound waves through the bone. The more complex and denser the bone structure, the greater will be the attenuation of the ultrasound wave. Thus, QUS may determine both density and structure of the bone. Table 10-1 compares these bone mass measurement techniques.
5. What is the preferred method for measuring bone mass?
DXA is the preferred method for measuring bone mass. It has the best correlation with fracture risk, requires relatively short scanning times (< 5 minutes), determines bone mass in all areas of the skeleton with high accuracy and reproducibility (precision), and is associated with a small radiation exposure. DXA does not require replacement of the radiation source. Drawbacks of DXA are the cost of the equipment, exposure of patients to ionizing radiation (no matter how small a dose), and cost of the test (in comparison with some other methods).
6. What are the indications for the measurement of bone mass?
Widespread BMD screening for osteoporosis is not recommended at this time. The United States Preventive Services Task Force (USPSTF) has recommended screening for osteoporosis of women of all racial and ethnic groups age 65 or greater and in women 50 to 65 years of age whose 10-year risk for any osteoporotic fracture is 9.3% or greater (determined by the FRAX fracture assessment tool; see later). The USPSTF concluded that for men, evidence of the benefits of screening for osteoporosis is lacking and the balance of benefits and harms cannot be determined.
The National Osteoporosis Foundation (NOF) recommends BMD testing for the following:
Women age 65 and older and men age 70 and older regardless of clinical risk factors
Adults who have a fracture after age 50
Anyone being considered for pharmacologic therapy for osteoporosis
Anyone being treated for osteoporosis, to monitor treatment effect
Anyone not receiving therapy in whom evidence of bone loss would lead to treatment
Postmenopausal women who are discontinuing estrogen should be considered for bone density testing.
In addition, DXA is also being increasingly used to study bone status in pediatric and adolescent patients, to perform vertebral fracture assessment, and to determine body composition.
7. What do bone mass measurements mean?
The bone densitometry report gives the absolute bone mass measurements (in g/cm2), which do not provide clinically useful information unless these values are compared with those of reference populations. To do this, the BMD report provides additional pieces of information: a T-score and a Z-score (Fig. 10-1).
The T-score is the number of SDs that the patient’s BMD is above or below the mean BMD of a normal young adult gender-matched population. This population represents the optimal or peak BMD for the patient. A patient whose BMD is 1 SD below that of the young reference population has a T-score of −1.0. At the spine, 1 SD represents about 10% of the bone mass. Thus, someone with a T-score of −1.0 has lost about 10% of his or her bone mass or has fallen short by 10% of achieving an optimal peak bone mass. Because the T-score is a good predictor of future fractures, it is used to diagnose osteoporosis.
9. What do Z-scores tell us about the patient?
