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S Boonen, J Aerssens and J Dequeker

Introduction

Osteoporosis is a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture (Peck et al. 1993). Type I (postmenopausal) osteoporosis is characterized by a disproportionate postmenopausal trabecular bone loss in a subset of women and complicated by vertebral compression fractures. Type II (senile) osteoporosis, on the other hand, is associated with a proportionate age-related loss of both cortical and trabecular bone, in men as well as in women, ultimately leading to hip fractures (Riggs & Melton 1983). Fractures of the proximal femur in the elderly represent a global issue, associated with significant morbidity and mortality (Cummings et al. 1985, Maggi et al. 1991, Mander & Lindahl 1993, Melton 1993). The cumulative incidence of hip fractures reaches 33% in women and 17% in men by 90 years of age (Melton et al. 1992). However

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S Boonen, J Aerssens and J Dequeker

Introduction

Age is the most important empirical determinant of bone mass. Factors associated with ageing account for the slow phase of bone loss, which begins in cortical bone by the age of 40 and continues throughout life at a rate of about 0·6% per year. In trabecular bone, it may begin even earlier and continues at a rate of about 0·7% per year (Burger et al. 1994, Greenspan et al. 1994, May et al. 1994). Although this loss probably reflects the aggregate effects of several processes, bone mass and bone strength depend, in part, upon calcium balance. When the amount of calcium absorbed from the diet is insufficient, calcium must be withdrawn from bone, which contains 99% of the total body stores. In both sexes, active intestinal transport of calcium decreases with ageing, particularly after the age of 70. This decrease is partially accounted for by an age-related deficiency of

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J Aerssens, S Boonen, J Joly and J Dequeker

Skeletal site-related differences in trabecular bone composition have been studied in autopsy samples from 63 individuals (age range 23-92 years). From each individual, bone samples were excised from the iliac crest, lumbar spine, femoral neck, and calcaneus. Samples were analyzed for their content of ash, calcium, collagen, extractable proteins, osteocalcin, and IGF-I. Significant differences were found between the skeletal sites, the lumbar spine being the least mineralized site and the femur the most. The femur and lumbar spine had a higher osteocalcin and IGF-I content compared with the other skeletal sites, suggesting a higher bone turnover rate. The intercorrelations between the anatomical sites were low for minerals and collagen but high for osteocalcin and IGF-I. The latter might indicate that the presence of these proteins in the bone matrix is mainly controlled by endocrine mechanisms which may influence the osteoblast function. Finally, regression analysis showed a significant age-related decrease of skeletal IGF-I at all sites examined. This finding supports the hypothesis of an IGF-I-mediated pathogenesis of senile osteoporosis. In summary, our data imply that a global assessment of skeletal function and bone quality, based upon analyses at one anatomical site, should be applied with caution.