Effect of Paget's disease of bone on areal lumbar spine bone mineral density measured by DXA, and density of cortical and trabecular bone measured by quantitative CT

Although bone density may be increased in bone that is affected by Paget's disease, density changes in cortical and trabecular bone and the effect on bone that is apparently unaffected by Paget's disease are relatively unexplored. We have investigated 81 vertebrae (28 affected, 53 unaffected) in 27 patients with Paget's disease, by dual X-ray absorptiometry (DXA) and by quantitative CT (QCT) bone density measurements of trabecular and cortical bone. DXA bone density was high (mean z-score=1.62, p<0.001) in vertebrae affected by Paget's disease, but not significantly different from normal in unaffected vertebrae (mean z-score=0.07, ns). Mean QCT z-score in Paget's vertebrae was 2.07 (p=0.009) for cortical bone and 1.37 (p.008) for trabecular bone. DXA correlated with QCT cortical values in affected and unaffected bone (r=0.8 and 0.56, respectively), and with QCT trabecular values (r=0.72 and 0.48, respectively). There was no significant difference in the slopes for the correlations in affected or unaffected bone. Cortical QCT values are underestimated in Paget's disease compared with physical measurements of density, owing to the computer algorithm used. High DXA values may alert to the possibility of Paget's disease, especially if the value deviates from the expected normal sequence in lumbar vertebrae. Osteoporotic vertebrae may be overlooked if the average value of bone mineral density is taken in the lumbar spine without reviewing each vertebra.