Post-menopausal vertebral osteoporosis: can dual energy X-ray absorptiometry forearm bone density substitute for axial measurements?

This study compared measurements of BMD using dual energy X-ray absorptiometry (DXA) at three sites in the nondominant forearm (ultradistal, distal one-third and a mid-region between these two), the lumbar spine (L1-L4) and the proximal femur (femoral neck, trochanter and Ward's triangle) for the evaluation of vertebral osteoporosis. Studies were performed on 100 normal women aged 29-69 yr (average 52 yr) and 63 osteoporotic women age 48-75 yr (average 66 yr) using the Hologic QDR-1000. Precision values of < 1% were obtained in both normal and osteoporotic women at both forearm and axial sites. Z-scores for the osteoporotics [Z = (mean BMD normal--mean abnormal)/S.D.] were--2.02 lumbar spine, -1.96 femoral neck, -1.79 ultradistal forearm, -1.73 mid region and -1.66 distal one-third forearm. Receiver operating characteristic curves showed that the lumbar spine and femoral neck equally discriminated between normals and osteoporotics with a 95% area under each curve, significantly greater values than those for the forearm which were 87% ultradistal, 89% mid forearm and 86% distal one-third forearm. Fracture thresholds, defined as the 90th centile of BMD for osteoporotic patients, were 1 S.D. below the normal mean for lumbar spine and femoral neck but equal to the mean for ultradistal, 0.6 S.D. below mean for mid region and 0.3 S.D. below mean for distal one-third region. We conclude that for the discrimination of normals and osteoporotic women the three forearm sites are comparable. However, both lumbar spine and proximal femur BMD are superior to all forearm sites.