Bone mineral density and quality in older people : a study in relation to exercise, and fracture occurrence, and the assessment of mechanical properties

Abstract

The study examined bone mineral content (BMC) and density (BMD) in older men and women. The purpose was to relate BMD to physical exercise, to examine the association between BMD and the risk of fractures and to assess the biomechanical properties of bone. Bone measurements were performed at the calcaneus by ¹²⁵I-photon absorption in 160 men and 324 women aged 75 and 80 and 108 women aged 50-60. In addition, the elastic wave propagation technique and computerized tomography were developed for studying the mechanical properties of the tibia in two groups of 78-year-old women (n=37) with high and low calcaneus BMD. Higher calcaneus BMC and BMD values were obtained for the men than women and for the younger than older age groups among the women. Body weight correlated positively with BMC and BMD in the older age groups, especially in the women. High BMD was associated with vigorous physical activity in the middle-aged women and with moderate physical activity in the elderly men and women. In 75-year-old men and women, BMD also correlated with previous physical activity and smoking history. Both retrospective and prospective fracture cases among the 75- and 80-year-olds had lower BMC and BMD than non-fracture subjects. With increased BMC and BMD values the probability of fractures showed a similar decrease in both sexes. The low calcaneus BMD group also had lower tibia BMD and moments of inertia. The analyses on bone mass distribution showed that the low BMD group had lost bone mainly from the endosteal surface of the tibia shaft, especially in the direction of greatest strength. Moreover, elastic wave propagation measurements indicated harder and less elastic bone in the low BMD group. The results show that calcaneus BMC and BMD are adequate predictors of fractures in the elderly. Combining elastic wave velocity with cross-sectional geometry provides a good estimate of bone mechanical properties in studying bone fragility and assessing the resistance of long bones to bending.