When assessing bone status for the purpose of determining if a patient is normal, osteopenic, or osteoporotic, the skeletal sites (called regions of interest [ROI]) typically scanned are the proximal femur, lumbar spine, and radius. The BMD of the patient is then compared to a manufacturer-provided normative database of young adults (the logic being that the BMD in the young adult normative population represents maximal peak bone mass). Total body BMD and body composition can also be quantified (grams of lean and fat mass), and custom scans can be designed for other skeletal sites. Specifically, a patient’s BMD is compared to a database of sex- and age-adjusted normal values, and the deviation from normal is expressed as a T-score (the number of standard deviations the patient's BMD is above or below the average BMD of the young adult reference population) and Z-scores (the number of standard deviations a patient's BMD is above or below the average BMD of a sex- and age-matched reference population).3 The International Society for Clinical Densitometry (ISCD) has developed and published well-accepted guidelines used to assist in acquiring high-quality DXA scans and for the diagnosis of osteoporosis using BMD. The accuracy and, especially, the precision of DXA scans can be remarkable when they are performed by trained technologists, and thus, serial scans can be performed to monitor BMD and body composition changes with aging or in response to treatment.
Because of the nature of the scan mechanics and speed, the effective radiation dose with DXA is very low, expressed in microSieverts.4,5 Generally, the radiation exposure from a series of the lumbar spine, proximal femur, and distal radius is about the same as daily background radiation. Even total body scans present very low exposure due to the scan speed at which any 1 body part is exposed for only a fraction of a second.
BENEFITS OF USING DXA FOR THE ORTHEOPEDIST
At the time of this writing in 2018, the presumption could be made that most physicians in the specialties of internal medicine, rheumatology, endocrinology, radiology, and orthopedics were familiar with the capabilities of DXA to assess BMD for the purpose of diagnosing osteoporosis. However, DXA is likely underused for other purposes, as orthopedists may be unaware of the full capabilities of DXA. Printouts after a scan contain more information than simply BMD, and there are more features and applications of DXA that can potentially be useful to orthopedists.
BONE SIZE
Data from a DXA scan are expressed not only as g/cm2 (BMD) but also as total grams in the ROI (known as bone mineral content, abbreviated as BMC), and cm2 (area of the ROI). These data may appear on a separate page, being considered ancillary results. The latter 2 variables are rarely included on a report sent to a referring physician; therefore, awareness of their value is probably limited. However, there are instances where such information could be valuable when interpreting results, especially bone size.6,7 For example, on occasion, patients present with osteopenic lumbar vertebrate but larger than normal vertebral size (area). Many studies have shown that bone size is directly related to bone strength and thus fracture risk.8,9 Although an understudied phenomenon, large vertebral body size could be protective, counteracting a lower than optimal BMD. Further, because the area of the ROI is measured, it is possible to calculate the bone width (or measure directly with a ruler tool in the software if available) for the area measured. This is especially feasible for tubular bones such as the midshaft of the radius, or more specifically, the classic DXA ROI being the area approximately one third the length of the radius from the distal end, the radius 33% region (actually based on ulna length). Consequently, it is possible to use the width of the radius 33% ROI in addition to BMD and T-score when assessing fracture risk.
CASE STUDY
A 60-year-old man had a DXA series of the lumbar spine, proximal femur, and whole body. His total body T-score was 0.6 (normal), and his total proximal femur T-score was −0.8 (normal), but his lumbar spine vertebrae 2 to 4 T-score was −1.9. As the patient was osteopenic based on the lumbar spine T-score, some physicians may have initiated antiresorptive therapy, especially if other risk factors for fracture were present. Further examination of the ancillary results of the DXA scan revealed that the vertebral body height T-score was a remarkable 1.11 and 1.53 after adjustment for stature (automatic software calculation). These results suggested that the patient had vertebral bodies of above average size, which theoretically would be protective against fracture even though the BMD T-score was below normal. For this patient, this finding mitigated immediate concern about the lumbar spine T-score of −1.9. Although vertebral body size is not typically used in assessing fracture risk, it is useful information that could be factored into the decision to start treatment or watch for further change with aging.
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