The benefit of hormone replacement therapy on bone mass is greater at the vertebral body than posterior processes or proximal femur

Abstract

The aim of this study was to determine whether the higher vertebral bone mass in women receiving hormone replacement therapy (HRT) is confined 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, is less than at the spine. Bone mass (g) of the third lumbar vertebra (total, vertebral body and posterior processes, measured by lateral scanning), and bone mineral density (g/cm2) of the femoral neck, Ward's triangle, and trochanter were measured using dual X-ray absorptiometry 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 controls, and 42 premenopausal controls aged 20 to 40 years. Relative to untreated postmenopausal controls, total bone mass of the third lumbar vertebra (body plus posterior processes) by postero-anterior (PA) scanning 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-matched controls (z score 0.4 +/- 0.1 SD or 11.2 +/- 3.4%, p < 0.01). This difference was confined to the vertebral body (z score 0.6 +/- 0.1 SD, p < 0.001), which was 17.1 +/- 3.3% higher than in age-matched controls (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 vertebral body in HRT-treated women, relative to premenopausal controls, was half the deficit at the vertebral body in untreated postmenopausal women (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 vertebral body in the HRT treated group was half the deficit at their posterior 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 with age at the posterior processes (r = -0.31, p < 0.01), but not at the vertebral body (r = -0.21, p = 0.07). Bone mass diminished significantly with age at the vertebral body and posterior processes in untreated women (r = -0.55, p < 0.001; r = -0.45, p < 0.001, respectively). Bone density (g/cm2) diminished at all femoral sites with advancing age in HRT-treated women. A protective effect was seen at the femoral neck 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 conclusion, the protective effect of HRT against bone loss at the vertebral body, the site of fracture in osteoporosis, may be underestimated by PA scanning. The greater benefit at the vertebral body, and more modest effect at the proximal femur, suggests that HRT may be a more effective means of reducing the risk of spine than hip fractures.