title: Challenging the cumulative injury model: positive effects of greater body mass on disc degeneration.
abstract: BACKGROUND
  There are differing views as to the causes of disc degeneration. One view, referred to as a cumulative or repetitive injury model, explains disc degeneration, in great part, as the result of "wear and tear" on the disc from routine daily exposures to physical loading or biomechanical forces. Perhaps paradoxically, such physical loading has been thought to be generally beneficial for other structures of the musculoskeletal system.
  
  
  OBJECTIVE
  The goal of this study was to investigate the associations of physical loading from body weight using quantitative measures of disc degeneration.
  
  
  METHODS
  This is an exposure-discordant twin study.
  
  
  METHODS
  We selected 44 pairs of healthy male monozygotic (MZ) twins with 8 kg or more discordance in body weight (mean 13 kg), with lumbar magnetic resonance imaging and bone density measurements available. The main outcome of disc degeneration was assessed through quantitatively measured disc height, and disc signal intensity and signal variation adjusted by the intrabody reference, adjacent cerebrospinal fluid signal, at L1-L4. Data on suspected constitutional and environmental risk factors were available for control of possible confounding factors.
  
  
  RESULTS
  Higher body weight was associated with 6.2% higher bone density in the lumbar spine, confirming an effective discordance (p<.0001). Disc signal variation was 5.4% higher ("better") among the heavier MZ co-twins (p=.005), but the 2.6% higher disc heights and 2.9% higher adjusted disc signals were not statistically significant.
  
  
  CONCLUSIONS
  Contrary to common beliefs, our findings suggest that cumulative or repetitive loading because of higher body mass (nearly 30 pounds on average) was not harmful to the discs. In fact, a slight delay in L1-L4 disc desiccation was observed in the heavier men, as compared with their lighter twin brothers.
authors.affiliation: The Finnish Twin Cohort Study, Department of Public Health, University of Alberta, Edmonton T6G 2G4, Canada. tapio.videman@ualberta.ca
authors.forename: Tapio
authors.initials: T
authors.lastname: Videman
authors.forename: Laura E
authors.initials: LE
authors.lastname: Gibbons
authors.forename: Jaakko
authors.initials: J
authors.lastname: Kaprio
authors.forename: Michele C
authors.initials: MC
authors.lastname: Battié
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000328:Adult; D000368:Aged; D001835:Body Weight / Q000502:physiology*; D015519:Bone Density / Q000502:physiology; D006801:Humans; D007403:Intervertebral Disc / Q000293:injuries / Q000473:pathology / Q000502:physiology*; D055959:Intervertebral Disc Degeneration / Q000235:genetics / Q000473:pathology / Q000503:physiopathology*; D008019:Life Style; D008159:Lumbar Vertebrae / Q000502:physiology; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D010809:Physical Fitness / Q000502:physiology; D014430:Twins, Monozygotic; D016474:Weight-Bearing / Q000502:physiology*