The term “metabolic bone disease” refers to conditions producing diffuse reductions or increases in bone density that affect bone integrity. Conditions causing reduced bone density include osteopenia/osteoporosis and osteomalacia. Osteoporosis is characterized by reduction in both bone matrix and mineral, whereas in osteomalacia the bone matrix is intact but bone mineral is decreased. Conditions causing increased bone density are less common and include osteopetrosis, fluorosis, fibrous dysplasia, and Paget disease of bone. Metabolic bone diseases may cause bone pain and fractures.
Dual-energy x-ray absorptiometry (DXA) is used to determine the BMD of the lumbar spine and hip. DXA bone density of the nondominant distal radius is also helpful for patients with hyperparathyroidism, for men receiving androgen deprivation therapy, and for patients with conditions causing other DXA measurement artifacts. Bone densitometry should be performed on all patients who are at risk for osteoporosis or osteomalacia, including all postmenopausal women aged 65 years and older and all men aged 70 years and older. DXA screening should also be considered for younger postmenopausal women with elevated risk, especially women with early menopause and those with a family history of osteoporosis. It should also be performed in younger patients who have had pathologic fractures or radiographic evidence of diminished bone density. DXA delivers negligible radiation.
BMD is typically expressed in g/cm2, for which there are different normal ranges for each bone and for each type of DXA-measuring machine. The “Z score” expresses an individual’s BMD as the number of standard deviations from age-matched, race-matched, and sex-matched means. The “T score” reports BMD as the number of standard deviations from young sex-matched means. Patients with a low T score are said to have “osteopenia” or “osteoporosis,” although osteomalacia is also frequently present. Any BMD classification is somewhat arbitrary and there is no BMD fracture threshold; instead, fracture risk increases about twofold for each standard deviation drop in BMD. The World Health Organization has established criteria for defining osteopenia and osteoporosis based on the T score: T score greater than or equal to –1.0, normal; T score –1.0 to –2.5, osteopenia (“low bone density”); T score less than –2.5, osteoporosis; T score less than –2.5 with a fracture, severe osteoporosis.
The inherent problem with DXA BMD is that it is an imperfect approximation for bone tensile strength and fracture risk; about two-thirds of patients who sustain hip fractures have “osteopenic” or “normal” BMD, rather than osteoporosis. Another problem is that each different type of DXA-measuring machine produces significantly different BMD results in a given individual; it is therefore important to use the same DXA machine when comparing serial DXA BMDs. Also, artifacts and technical problems with DXA are common, resulting in erroneous BMD reports. Unexpected dramatic decreases or increases in reported BMD should elicit suspicion of an error and prompt a repeat DXA measurement with a different scanner.
Vertebral BMD errors can be caused by anatomic variables, such as ...