Repeat DXAs of the Spine for Monitoring BMD

Dennis Black, PhD


November 10, 2005


Can you explain the exact nature of "degenerative changes" of the spine, which may falsely elevate spine bone mineral density (BMD) on dual x-ray absorptiometry (DXA)? Wouldn't it be unlikely that the degenerative changes would significantly change in 2 years' time, so that a repeat DXA in 2 years would at least be helpful to show interval change?

Kathleen Griffin, MD

Response From the Expert

Dennis Black, PhD 
Professor of Epidemiology & Biostatistics, University of California, San Francisco


There are various forms of artifacts that can interfere with DXA BMD, including arthritic calcifications and aortic calcifications, both of which artifactually elevate the measurement. In fact, they can interfere with longitudinal measurements, although the extent is unclear. But after about age 65 (in women), the mean spine BMD stops going down and starts to increase by 0.5% to 1% per year. This has been found among women participating in almost all longitudinal studies (both epidemiologic and randomized trials [ie, women in the placebo group]). Presumably, true bone density (the density that is correlated with bone strength) is still decreasing and the observed increases are artifactual. If the mean measured BMD is increasing by 1% per year, this implies that there are individuals in whom BMD is increasing further (perhaps 3% or 4%) while bone strength continues to decrease. It is also well known that losses at the hip accelerate in women of this age group. For these reasons, measurements of spinal DXA BMD can be very misleading in older women, and therefore it is recommended that hip BMD be the primary tool for monitoring in these individuals.


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