Sexual dimorphism in vertebral fragility is more the result of gender differences in age-related bone gain than bone loss

Spine fractures usually occur less commonly in men than in women. To identi fy the structural basis for this gender difference in vertebral fragility, we studied 1013 healthy subjects (327 men and 686 women) and 76 patients wi th spine fractures (26 men and 50 women). Bone mineral content (BMC), cross -sectional area (CSA), and volumetric bone mineral density (vBMD) of the th ird lumbar vertebral body (L3) were measured by posteroanterior (PA) and la teral scanning using dual-energy X-ray absorptiometry (DXA). In this cross- sectional study, the diminution in peak vertebral body BMC from young adult hood to old age was less in men than in women (6% vs. 27%). This diminution was the net result of two opposing changes occurring concurrently througho ut adult life: the removal of bone adjacent to marrow on the inner (endoste al) surface by bone resorption and the deposition of bone on the outer (per iosteal) surface by bone formation. For L3, we estimated that men resorbed 3.7 g and deposited 3.1 g, producing a net loss of 0.6 g from young adultho od to old age and women resorbed 3.1 g and deposited only 1.2 g, producing a net loss of 1.9 g. Thus, based on our indirect estimates of periosteal ga in and endosteal loss across life, the observed net diminution in BMC durin g aging was less in men than women because absolute periosteal bone formati on was greater in men than women (3.1 g vs. 1.2 g) not because absolute bon e resorption was less in men. On the contrary, the absolute amount of bone resorbed was greater in men than women (3.7 g vs. 3.1 g). Periosteal bone f ormation also increased vertebral body CSA 3-fold more in men than in women , distributing loads onto a larger CSA, so that the load imposed per unit C SA decreased twice as much in men than in women (13% vs. 5%). In men and wo men with spine fractures, CSA and vBMD were reduced relative to age-matched controls. However, vBMD was no different to the adjusted vBMD in age-match ed controls derived assuming controls had no periosteal bone formation duri ng aging. Thus, large amounts of bone are resorbed in men as well as in wom en, accounting for the age-related increase in spine fractures in both gend ers. Periosteal bone formation increases CSA and offsets bone loss in both genders but more greatly in men, accounting for the lower incidence of spin e fractures in men than in women. We speculate that reduced periosteal bone formation, during growth or aging, may be in part responsible for both red uced vertebral size and reduced vBMD in men and women with spine fractures. Sexual dimorphism in vertebral fragility is more the result of gender diff erences in age-related bone gain than age-related bone loss.