The increase in bone fragility after menopause results from reduced periosteal bone formation and increased endocortical resorption. Women with highest remodeling had greatest loss of bone mass and estimated bone strength, whereas those with low remodeling lost less bone and maintained estimated bone strength.

Introduction:​ Bone loss from the inner (endocortical) surface contributes to bone fragility, whereas deposition of bone on the outer (periosteal) surface is believed to be an adaptive response to maintain resistance to bending.

Materials and Methods:​ To test this hypothesis, changes in bone mass and estimated indices of bone geometry and strength of the one-third distal radius, bone turnover markers, and fracture incidence were measured annually in 821 women 30–89 years of age for 7.1 ± 2.5 years. The analyses were made in 151 premenopausal women, 33 perimenopausal women, 279 postmenopausal women, and 72 postmenopausal women receiving hormone replacement therapy (HRT).

Results:​ In premenopausal women, periosteal apposition increased the radius width, partly offsetting endocortical resorption;​ therefore, the estimated cortical thickness decreased. Outward displacement of the thinner cortex maintained bone mass and cortical area and increased estimated bending strength. Estimated endocortical resorption accelerated during perimenopause, whereas periosteal apposition decreased. Further cortical thinning occurred, but estimated bending strength was maintained by modest outward cortical displacement. Endocortical resorption accelerated further during the postmenopausal years, whereas periosteal apposition declined further;​ cortices thinned, but because outward displacement was minimal, estimated cortical area and bending strength now decreased. Women with highest remodeling had the greatest loss of bone mass and strength. Women with low remodeling lost less bone and maintained estimated bone strength. In HRT-treated women, loss of bone strength was partly prevented. These structural indices predicted incident fractures;​ a 1 SD lower section modulus doubled fracture risk.

Conclusions:​ Periosteal apposition does not increase after menopause to compensate for bone loss;​ it decreases. Bone fragility of osteoporosis is a consequence of reduced periosteal bone formation and increased endocortical resorption. Understanding the mechanisms of the age-related decline in periosteal apposition will identify new therapeutic targets. On the basis of our results, it may be speculated that the stimulation of periosteal apposition will increase bone width and improve skeletal strength.