EQUIVALENT DEFICITS IN BONE MASS OF THE VERTEBRAL BODY AND POSTERIOR PROCESSES IN WOMEN WITH VERTEBRAL FRACTURES - IMPLICATIONS REGARDING THE PATHOGENESIS OF SPINAL OSTEOPOROSIS

Reduced bone mass of the spine in women with vertebral fractures is at tributed to excessive trabecular bone loss fi om the vertebral body. H owever, the measurement obtained by posteroanterior (PA) scanning incl udes the posterior processes and the vertebral body, each comprising a bout 50% of the total vertebral mass. Thus, the deficit in bone mass b y PA scanning may be due to deficits in one or both of these structure s. We asked two questions: (1) In healthy women, is the age-related di minution in bone mass of the vertebral body greater than the diminutio n at the posterior processes? (2) In women with vertebral fractures, i s the deficit in bone mass at the vertebral body, the fracture site in spinal osteoporosis, greater than at the posterior processes? Bone ma ss of the posterior processes and vertebral body of the third lumbar v ertebra was measured by lateral scanning using dual-energy X-ray absor ptiometry (DXA). Compared with 27 premenopausal women, deficits in 27 postmenopausal women at the posterior processes and vertebral body, re spectively, were 35.9 +/- 3.7 and 25.2 +/- 4.1% (p < 0.05); t score, - 1.5 +/- 0.2 and -1.1 +/- 0.2 SD (p = 0.09). Compared with the postmeno pausal (age-matched) women, deficits in 21 women,vith vertebral fractu res at the posterior processes and vertebral body, respectively, were 22.6 +/- 4.9 and 24.5 +/- 8.3% (p = NS); Z score, -0.8 +/- 0.2 and -0. 8 +/- 0.3 (p = NS). In vivo the bone mass of the vertebral body as a p ercentage of the whole vertebra was 45.7 +/- 0.1 in premenopausal wome n, 48.9 +/- 1.9 in postmenopausal women, 51.5 +/- 1.1 in women with lo w bone mass but no fractures, 52.7 +/- 2.4 in women with vertebral fra ctures, and 51.9 +/- 2.5% in vitro, based on autopsy specimens from 19 postmenopausal women aged 65-95 years. The lo,ver spinal bone density measured using PA scanning in women with spine fractures may not be d ue to excessive or disproportionate trabecular bone loss from the vert ebral body because comparable deficits are found at the posterior proc esses. Whether these deficits are due to reduced peak bone mass, trabe cular bone loss, cortical bone loss, or varying combinations of these mechanisms remains to be established.