The effect of vertebral collapse on spinal bone mineral density measurements in osteoporosis.

Bone mineral density (BMD) of the lumbar spine (L1-L4) was measured using dual-energy x-ray absorptiometry (DXA) in 57 postmenopausal women with spinal osteoporosis aged 50-82 years (average age 64). For each vertebra between L1 and L4, the BMD was compared to age-matched normal population values and a Z score obtained. Twenty three patients had between one and three collapsed vertebrae from L1 to L4. The average Z score for fractured vertebrae was -1.62 but for uncollapsed vertebrae was -2.26 (P less than 0.001). The difference in Z scores obtained for collapsed and uncollapsed vertebrae was greater where only one fracture was present (0.803) compared to two (0.577) or three fractures (0.245). The average increase in density for a fractures vertebra was 0.070 g/cm2. For L1 vertebral fractures alone the average increase in BMD was 0.096 g/cm2, L2 vertebral fractures 0.041 g/cm2, L3 fractures 0.029 g/cm2 and L4 fractures 0.062 g/cm2. It is concluded that as vertebral collapse is not alway detected using DXA and usually causes a rise in BMD, spinal x-rays are necessary to avoid misinterpretation of a falsely elevated BMD in osteoporotic patients, particularly in longitudinal studies or when monitoring therapy, where small changes in BMD are important.