During aging, men lose less bone than women because they gain more periosteal bone, not because they resorb less endosteal bone

Bone size is greater in young men than in women but the load imposed is als o greater. Therefore, the load per unit area (stress) on the vertebral body is the same in men and women. Fractures are uncommon in young adulthood be cause volumetric bone mineral density (vBMD) is normal so that the stress i mposed on the bone is well below its strength in both sexes. The gender dif ference in bone fragility emerges during aging because the biomechanical re lationship between the stress on bone and its strength changes in men and w omen. Periosteal apposition occurs in both genders during aging but more in men than in women so that vertebral cross-sectional area (CSA) increases m ore in men than women. Consequently, the stress on bone decreases more in m en than women. In addition, periosteal apposition offsets endosteal bone lo ss more in men than in women so that net bone loss is less in men than wome n. Thus, the stress on the bone decreases more and the strength of the bone decreases less in men than in women. The ratio of the stress on bone to it s strength can be quantitated and is below unity in young men and women. Du ring aging this ratio increases to approach or exceed unity, the value at w hich fracture risk is high because the stress on bone is similar to its str ength. As the stress decreases more and strength decreases less in men, the ratio increases less in men than in women so that in old age only 2-3% of men but 20% of women have this stress to strength ratio exceeding unity. Me n and women with spine fractures have the same vBMD and same stress to stre ngth ratio which is no different from unity. Fewer men than women are at ri sk for fracture because fewer men than women have these structural determin ants of bone strength below a level at which the loads exceed the bone's ab ility to tolerate them.