THE BENEFIT OF HORMONE REPLACEMENT THERAPY ON BONE MASS IS GREATER ATTHE VERTEBRAL BODY THAN POSTERIOR PROCESSES OR PROXIMAL FEMUR

The aim of this study was to determine whether the higher vertebral bo ne mass in women receiving hormone replacement therapy (HRT) is confin ed to the trabecular rich vertebral body rather than the predominantly cortical posterior processes, and to determine whether the protective effect of HRT at the proximal femur, a predominantly cortical site, i s less than at the spine. Bone mass (g) of the third lumbar vertebra ( total, vertebral body and posterior processes, measured by lateral sca nning), and bone mineral density (g/cm(2)) of the femoral neck, Ward's triangle, and trochanter were measured using dual X-ray absorptiometr y in a cross-sectional study of 71 women receiving HRT for 5.7 +/- 0.4 years (mean +/- SEM), ranging from 1 to 21 years, 69 age-matched cont rols, and 42 premenopausal controls aged 20 to 40 years. Relative to u ntreated postmenopausal controls, total bone mass of the third lumbar vertebra (body plus posterior processes) by postero-anterior (PA) scan ning was 0.4 +/- 0.1 SD or 9.6 +/- 3.0% higher in HRT treated women (p < 0.01). By lateral scanning, total bone mass was higher than age-mat ched controls (z score 0.4 +/- 0.1 SD or 11.2 +/- 3.4%, p < 0.01). Thi s difference was confined to the vertebral body (z score 0.6 +/- 0.1 S D, p < 0.001), which was 17.1 +/- 3.3% higher than in age-matched cont rols (p < 0.001). Bone mass of the posterior processes was no higher [ z score 0.1 +/- 0.1, not significant (NS)]. The deficit at the vertebr al body in HRT-treated women, relative to premenopausal controls, was half the deficit at the vertebral body in untreated postmenopausal wom en (t score -0.7 +/- 0.1 vs. -1.4 +/- 0.1 SD, respectively; p < 0.001) but no less at the posterior processes (t score -1.6 +/- 0.2 vs. -1.9 +/- 0.2 SD, respectively; NS). Similarly, the deficit in the vertebra l body in the HRT treated group was half the deficit at their posterio r processes (t score -0.7 +/- 0.1 SD vs. -1.6 +/- 0.2, respectively; p < 0.001). In HRT-treated women, bone mass diminished significantly wi th age at the posterior processes (r = -0.31, p < 0.01), but not at th e vertebral body (r = -0.21, p = 0.07). Bone mass diminished significa ntly with age at the vertebral body and posterior processes in untreat ed women (r = -0.55, p < 0.001; r = -0.45, p < 0.001, respectively). B one density (g/cm(2)) diminished at all femoral sites with advancing a ge in HRT-treated women. A protective effect was seen at the femoral n eck and Ward's triangle, but not trochanter (z score 0.2 +/- 0.1, p = 0.06; 0.3 +/- 0.1, p < 0.05; 0.0 +/- 0.1, NS, respectively). In conclu sion, the protective effect of HRT against bone loss at the vertebral body, the site of fracture in osteoporosis, may be underestimated by P A scanning. The greater benefit at the vertebral body, and more modest effect at the proximal femur, suggests that HRT may be a more effecti ve means of reducing the risk of spine than hip fractures. (C) 1997 by Elsevier Science Inc. All rights reserved.