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An increase in BMD in a single vertebra may indicate things are getting worse, not better
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In May 2004 a 65-year-old man fell 8 ft. when the ladder on which he was standing slipped. He was lucky that his only injury was a fractured vertebra. A bone mineral density study was done with the following T-scores: L1 -3.2, L2 -3.1, L3 -2.9, L4 -1.3.
It is easy to correctly identify L4 as the fractured vertebra.
Firstly, the technologist performing the scan should be able to identify the anatomic abnormality on the image, as should the person reporting the scan. Secondly, in a crushed vertebra the same amount of bone mineral is now present in a smaller area (and volume) such that mass per unit area is increased — hence the “higher” BMD and “better” T-score. The data from L4 should not have been included in the original report and the report should have included a comment to this effect.
Fast forward to June 2008 after four years of therapy for osteoporosis. T-scores were L1 - 2.8, L2 -2.6, L3 -0.9, L4 (still erroneously being included in the report) -0.5. The changes in L1 and L2 most likely represent a true improvement in BMD. However, plane radiographs confirmed that there is now a new fracture at L3 and a worsening of the fracture at L4.
At another time I will discuss the semantics of whether a fracture occurring on therapy represents a treatment failure. Today, I want to stress that the T-scores throughout the four evaluated lumbar vertebra should be within one T-score of each other and greater variability should alert you to the need to obtain X-rays to find out what might be going on.
The print out from many DXA instruments includes the disclaimer that “the images should not be used for diagnostic purposes.” As someone (I don’t remember who) once put it — “This is not a recommendation to ignore the images.” When you get a DXA report on your patients make sure that the report includes not just the final diagnostic impression (osteoporosis, low bone mass or normal) but includes the images. Faxed copies of these reports are not easy to read but your request for them should at least remind the person providing the report to look at the images and comment on them!