Bone mass and ageing

Bone can be divided into two kinds of tissue, cortical and trabecular bone. The skeleton comprises approximately 80% cortical bone, mainly in peripheral bones, and 20% trabecular bone, mainly in the axial skeleton. Bone density increases with skeletal growth to a peak in late adolescence or early adulthood. Bone loss subsequently occurs with ageing in both sexes, and in females accelerated loss occurs at the menopause. The risk of osteoporotic fracture in later life is the result of peak bone mass achieved at skeletal maturity and subsequent age-related and postmenopausal bone loss. Peak bone mass is largely genetically determined but is also influenced by environmental factors such as dietary calcium and physical activity. Bone loss with ageing occurs at different rates and different times in different skeletal sites. Femoral neck bone loss probably occurs in a linear fashion throughout life from early adulthood but may be accelerated at the menopause. Spinal bone loss may commence before the menopause but is rapidly increased in the immediate postmenopausal years. Bone strength is directly related to bone density, but the loading force is also relevant to risk of fracture.