Pitfalls in the measurement of bone mineral density by dual energy x-ray absorptiometry

Study Design. Fifty-three human thoracic and lumbar vertebrae were analyzed by dual energy x-ray absorptiomety to obtain projectional (g/cm²) and volumetric (g/cm³) bone mineral density. Objectives. To determine the effect of volume on the measurement of bone mineral density by DEXA. Summary of Background Data. Despite the widespread use of dual energy x-ray absorptiomety to measure bone mineral density expressed as grams per projectional unit area (g/cm²), the effect of volume has been ignored in many published biomechanical studies. Methods. Projectional bone mineral density (g/cm²) of fifty-three human vertebrae was obtained by dual energy x-ray absorptiomety measurement. This was compared to bone mineral density expressed as grams per unit volume. Results. Many specimens with near equal projectional bone mineral density were demonstrated to have significantly different true densities when measured by dual energy x-ray absorptiomety in grams per unit volume. The difference in true bone mineral density for these specimens ranged from 24.1% to 139%. Conclusion. The effect of volume on bone mineral density data based on projectional areas can cause very significant perturbations of the data in biomechanical studies.