Correlation of thoracic and lumbar vertebral failure loads with in situ vs. ex situ dual energy X-ray absorptiometry

In this study we explore the hypothesis that estimates of failure loads in the thoracic spins by lumbar dual energy X-ray absorptiometry (DXA) are com promised of skeletal heterogeneity throughout the spine and artifacts of sp inal DXA. We studied the correlation between mechanical failure loads of th oracic and lumbar vertebrae, and that of in situ vs, ex situ lumbar DXA wit h thoracic and lumbar fracture loads, respectively. One hundred and ninetee n subjects (76 female, age 82 +/- 9 yr; 43 male, age 77 +/- 11 yr) were exa mined under in situ conditions (anterior-posterior direction), the scans be ing repeated ex situ (lateral projection) in 68 cases. The failure loads of thoracic vertebrae (T) 6 and 10, and lumbar vertebra (L) 3 were determined in axial compression, using a functional 3-segment unit. The correlation b etween thoracic failure loads (T6 vs. T10) was significantly (p < 0.01) hig her (r = 0.85) than those between thoracic and lumbar vertebrae (r = 0.68 a nd 0.61, respectively). Lateral ex situ DXA displayed a significantly highe r correlation (p < 0.05) with lumbar vertebral fracture loads than in situ anterior-posterior DXA (r = 0.85 vs. 0.71), but the correlation of thoracic failure loads with lateral ex situ lumbar DXA was similar to that obtained in situ in anterior-posterior direction (r = 0.69 vs, 0.69 for T10, and r = 0.61 vs. 0.65 for T6). The correlation between fracture loads of differen t spinal segments, and between DXA and failure loads was not significantly different between men and women. The results demonstrate a substantial hete rogeneity of mechanical competence throughout the spine in elderly individu als. Because of the high incidence of fractures in the thoracic spine, thes e findings suggest that, clinically, lateral DXA involves no relevant advan tage over anterior-posterior measurements of the lumbar spine. (C) 2001 Els evier Science Ltd. All rights reserved.