Vertebral Size, Bone Density, and Strength in Men and Women Matched for Age and Areal Spine BMD

Abstract

To explore the possible mechanisms underlying sex-specific differences in skeletal fragility that may be obscured by two-dimensional areal bone mineral density (aBMD) measures, we compared quantitative computed tomography (QCT)-based vertebral bone measures among pairs of men and women from the Framingham Heart Study Multidetector Computed Tomography Study who were matched for age and spine aBMD. Measurements included vertebral body cross-sectional area (CSA, cm2), trabecular volumetric BMD (Tb.vBMD, g/cm3), integral volumetric BMD (Int.vBMD, g/cm3), estimated vertebral compressive loading and strength (Newtons) at L3, the factor-of-risk (load-to-strength ratio), and vertebral fracture prevalence. We identified 981 male-female pairs (1:1 matching) matched on age (± 1 year) and QCT-derived aBMD of L3 (± 1%), with an average age of 51 years (range 34 to 81 years). Matched for aBMD and age, men had 20% larger vertebral CSA, lower Int.vBMD (−8%) and Tb.vBMD (−9%), 10% greater vertebral compressive strength, 24% greater vertebral compressive loading, and 12% greater factor-of-risk than women (p < 0.0001 for all), as well as higher prevalence of vertebral fracture. After adjusting for height and weight, the differences in CSA and volumetric bone mineral density (vBMD) between men and women were attenuated but remained significant, whereas compressive strength was no longer different. In conclusion, vertebral size, morphology, and density differ significantly between men and women matched for age and spine aBMD, suggesting that men and women attain the same aBMD by different mechanisms. These results provide novel information regarding sex-specific differences in mechanisms that underlie vertebral fragility.

Vertebral Strength and Estimated Fracture Risk Across the BMI Spectrum in Women

“…Structural rigidity depends on bone size and elastic modulus, (16) which was estimated from a previously published relationship: Elastic modulus = −34.7 + 3230 × Int.vBMD (34) …”

“…According to previously published methods, (16,36,37) in the biomechanical model, the body was divided into sections, and each section’s length, weight, and center of mass position were estimated, and the muscle forces required to maintain static equilibrium during each activity were calculated. For each activity, the applied load to the L4 vertebral body was determined by totaling all forces (from body weight and muscle loading) in the axial direction of L4.…”

Fracture Risk Prediction by Non-BMD DXA Measures: the 2015 ISCD Official Positions Part 1: Hip Geometry

“…The size and shape of the femur (hip geometry) is critical to the mechanical strength of the hip and its resistance to fracture under various loading conditions. For example, at the same volumetric density, male bones are stronger because they are larger (79). Hip geometry can be estimated from 2-dimensional DXA images.…”

Effects of Leisure-Time Physical Activity on Vertebral Dimensions in the Northern Finland Birth Cohort 1966

“…It often leads to non-traumatic vertebral fractures with consequent morbidity and has negative effects on the quality of life12. A systematic review suggested that small vertebral size is an independent risk factor for vertebral fractures3.…”

“…Causing changes in these factors, age affects vertebral strength to a large extent8910. Men are known to have larger vertebral cross-sectional area (CSA), both absolute and relative to body size, as well as greater compressive strength than women111. Regardless of gender, bone mass reaches its maximum value between the ages of 20 and 30 years, decreasing thereafter10.…”