title: Age- and sex-specific differences in the factor of risk for vertebral fracture: a population-based study using QCT.
abstract: We used QCT scans obtained in 687 men and women, 21-97 years of age, to estimate the factor of risk for vertebral fracture, Phi(vert), defined as the ratio of spinal loading to vertebral strength. With age, vertebral strength declined and Phi(vert) increased significantly more in women than men. Age- and sex-specific differences in Phi(vert) closely resembled previously reported vertebral fracture incidence.
  
  
  BACKGROUND
  Despite the high prevalence of vertebral fractures, little is known about the interaction between spinal loading and vertebral fragility.
  
  
  METHODS
  We assessed the ratio of spinal loading to vertebral strength (i.e., the factor of risk, Phi(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-L3 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, Phi(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, Phi(vert) increased with age, but significantly more so in women than men (p < 0.001). For bending and lifting, Phi(vert-bending) exceeded 1.0 in 30% of women and 12% of men > or =50 years of age, values that are similar to the reported frequency of vertebral fracture.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Orthopedic Biomechanics Laboratory, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215, USA. mbouxsei@bidmc.harvard.edu
authors.forename: Mary L
authors.initials: ML
authors.lastname: Bouxsein
authors.forename: L Joseph
authors.initials: LJ
authors.lastname: Melton
authors.forename: B Lawrence
authors.initials: BL
authors.lastname: Riggs
authors.forename: John
authors.initials: J
authors.lastname: Muller
authors.forename: Elizabeth J
authors.initials: EJ
authors.lastname: Atkinson
authors.forename: Ann L
authors.initials: AL
authors.lastname: Oberg
authors.forename: Richard A
authors.initials: RA
authors.lastname: Robb
authors.forename: Jon J
authors.initials: JJ
authors.lastname: Camp
authors.forename: Peggy A
authors.initials: PA
authors.lastname: Rouleau
authors.forename: Cynthia H
authors.initials: CH
authors.lastname: McCollough
authors.forename: Sundeep
authors.initials: S
authors.lastname: Khosla
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D015519:Bone Density; D019245:Compressive Strength; D005260:Female; D050723:Fractures, Bone / Q000453:epidemiology / Q000209:etiology; D006801:Humans; D017770:Lifting / Q000009:adverse effects; D008297:Male; D008875:Middle Aged; D011859:Radiography; D012307:Risk Factors; D012727:Sex Characteristics; D016103:Spinal Fractures / Q000453:epidemiology*; D013124:Spinal Injuries / Q000453:epidemiology*; D013131:Spine / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D013718:Tensile Strength; D015898:Tomography Scanners, X-Ray Computed; D016474:Weight-Bearing