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REPLY
Body Size and Vertebral Fractures
Karen L. Margolis, MD, MPH, and
Kristine E. Ensrud, MD, MPH
1 May 2001 | Volume 134 Issue 9 Part 1 | Page 796
IN RESPONSE:
Dr. Blank notes that DXA is not a volumetric measurement of BMD. He cites evidence that BMD of the lumbar vertebrae as measured by DXA is correlated with height and weight, whereas volumetric BMD measurements are not (1). He suggests that this may explain our finding that the associations between larger body size and risks for hip, pelvis, and rib fractures are eliminated by adjustment for hip BMD since DXA measurement captures bone size as well as BMD.
To test this hypothesis, we did additional proportional hazards analyses of the associations between total body weight and risks for hip, pelvis, and rib fractures. By using the same approach as reported in Table 2 of our paper, we tested one set of models by adding height at visit 2 to the existing adjustments for age, smoking status, physical activity, history of falls, estrogen use, and health status. We then tested a second set of models that included an additional adjustment for total-hip BMD. The adjustment for height as a proxy for bone size did not affect our results or conclusions. After adjustment for height, age, smoking status, physical activity, history of falls, estrogen use, and health status, women in the lowest quartile of weight (<57.8 kg) compared with those in the upper quartile (>73.3 kg) had an increased risk for hip fracture (odds ratio [OR], 2.1 [95% CI, 1.5 to 3.0]), pelvis fracture (OR, 2.7 [CI, 1.3 to 5.6]), and rib fracture (OR, 2.0 [CI, 1.2 to 3.2]). After further adjustment for BMD, the point estimates of risk were reduced and no longer reached statistical significance (OR for hip fracture, 0.9 [CI, 0.6 to 1.3]; OR for pelvis fracture, 1.5 [CI, 0.7 to 3.3]; OR for rib fracture, 1.3 [CI, 0.7 to 2.2]).
Irrespective of the mechanism for the relationship between body size and risk for fracture, the clinical implications of our findings are unchanged. Dual-energy x-ray absorptiometry is the current standard for BMD measurement in clinical practice. The effect of adjustment for volumetric density on the association between body size and risk for various fractures remains to be investigated.
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Author and Article Information
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Hennepin County Medical Center; Minneapolis, MN 55415 (Margolis)
Minneapolis Veterans Affairs Medical Center; Minneapolis, MN 55417 (Ensrud)
1. Jergas M, Breitenseher M, Gluer CC, Yu W, Genant HK. Estimates of volumetric bone density from projectional measurements improve the discriminatory capability of dual X-ray absorptiometry J Bone Miner Res. 1995;10:1101-10. [PMID: 0007484286].
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Karen L. Margolis, Kristine E. Ensrud, Pamela J. Schreiner, Holly K. Tabor, AND for the Study of Osteoporotic Fractures Research Group*
- Annals 2000 133: 123-127.
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