Age- and sex-specific differences in the factor of risk for vertebral fracture: A population-based study using QCT

We used QCT scans obtained in 687 men and women, 21-97 years of age, to estimate the factor of risk for vertebral fracture, Φ_vert, defined as the ratio of spinal loading to vertebral strength. With age, vertebral strength declined and Φ_vert increased significantly more in women than men. Age- and sex-specific differences in Φ_vert, closely resembled previously reported vertebral fracture incidence. Introduction: Despite the high prevalence of vertebral fractures, little is known about the interaction between spinal loading and vertebral fragility. Materials and Methods: We assessed the ratio of spinal loading to vertebral strength (i.e., the factor of risk, Φ_vert) in an age- and sex-stratified population-based sample of 700 women and men 21-97 years of age. We measured volumetric BMD (vBMD, mg/cm³) and cross-sectional area (CSA, cm²) of the midvertebral bodies of L₁-L ₃ using QCT and computed vertebral compressive strength from these data using engineering beam theory. A biomechanical model of the trunk was used to estimate compressive forces applied to the L₃ vertebral body during standing, bending forward, and bending forward while lifting 10 kg. The factor of risk for fracture, Φ_vert, was computed as the ratio of spinal compressive force to vertebral strength for each activity. Results: Men had a higher vertebral strength at all ages, largely because of their greater CSA. Whereas both sexes exhibited a marked decline in vertebral compressive strength with age (p < 0.001), the decline was greater in women than men (-43% versus -31%, p = 0.008). Compressive forces on L₃ were greater in men than women, because of their greater body weight and height. For both sexes, forces during bending and lifting were 8-fold higher than those experienced during upright standing. For all activities, Φ_vert increased with age, but significantly more so in women than men (p < 0.001). For bending and lifting, Φ_vert-bending exceeded 1.0 in 30% of women and 12% of men ≥50 years of age, values that are similar to the reported frequency of vertebral fracture. Conclusion: These findings illustrate potential mechanisms underlying vertebral fractures and provide strong rationale for further evaluation of this QCT-based biomechanical approach for assessment of fracture risk.