ANTEROPOSTERIOR AND LATERAL SPINAL DXA FOR THE ASSESSMENT OF VERTEBRAL BODY STRENGTH - COMPARISON WITH HIP AND FOREARM MEASUREMENT

Spinal bone mineral density (BMD) is traditionally measured by dual-en ergy X-ray absorptiometry (DXA) in the anteroposterior (AP) projection which includes both the vertebral body and the posterior elements in the measurement. The posterior elements, however, contribute little to the compressive strength of the spine. It has therefore been suggeste d that spinal BMD measured in the lateral projection, including only t he vertebral body in the measurement, might be more appropriate for th e prediction of fracture risk. To date little clinical evidence has be en presented to support this assumption. To address the issue, we meas ured vertebral, hip and forearm BMD in situ in 14 human cadavers and r emeasured BMD in vitro in excised vertebrae. Lateral spinal measuremen ts were performed in the decubitus position. Fracture force and other biomechanical measures were determined for 32 vertebrae in a mechanica l testing machine and compared with BMD values in situ and in vitro. C orrelations of BMD with vertebral fracture force were r = 0.48/0.51 (i n situ/in vitro) for the AP spinal measurements, r = 0.45/0.71 (in sit u/in vitro) for the lateral spinal measurements, and r = 0.64 and r = 0.53 for total hip and forearm measurements in situ, respectively. Thu s, despite an apparent diagnostic advantage in vitro, lateral spinal B MD measurement was not superior to AP measurement when performed in si tu. This observation corresponds well with previous clinical findings and is probably due to the larger accuracy error in the lateral than i n the AP projection resulting from a lower ratio of bone to soft tissu e. The high correlation between hip BMD and vertebral fracture force s uggests that hip measurement may prove as useful for vertebral fractur e risk assessment as spinal measurement in any projection, especially in the elderly with a high prevalence of degenerative changes in the s pine.