title: The influence of static axial torque in combined loading on intervertebral joint failure mechanics using a porcine model.
abstract: BACKGROUND
  The spine is routinely subjected to repetitive combined loading, including axial torque. Repetitive flexion-extension motions with low magnitude compressive forces have been shown to be an effective mechanism for causing disc herniations. The addition of axial torque to the efficacy of failure mechanisms, such as disc herniation, need to be quantified. The purpose of this study was to determine the role of static axial torque on the failure mechanics of the intervertebral joint under repetitive combined loading.
  
  
  METHODS
  Repetitive flexion-extension motions combined with 1472 N of compression were applied to two groups of nine porcine motion segments. Five Nm of axial torque was applied to one group. Load-displacement behaviour was quantified, and planar radiography was used to document tracking of the nucleus pulposus and to identify fractures.
  
  
  RESULTS
  The occurrence of facet fractures was found to be higher (P=0.028) in the axial torque group (7/9), compared to the no axial torque group (2/9). More hysteresis energy was lost up to 3000 cycles of loading in the axial torque group (P<0.014). The flexion-extension cycle stiffness was not different between the two groups until 4000 cycles of loading, after which the axial torque group stiffness increased (P=0.016). The percentage of specimens that herniated after 3000 cycles of loading was significantly larger (P=0.049) for the axial torque group (71%) compared to the no axial torque group (29%).
  
  
  CONCLUSIONS
  Small magnitudes of static axial torque alter the failure mechanics of the intervertebral disc and vertebrae in combined loading situations. Axial torque appears to accelerate the susceptibility for injury to the intervertebral joint complex. This suggests tasks involving axial torque with other types of loading, apart from axial twist motion, should be monitored to assess exposure and injury risk.
authors.affiliation: Faculty of Applied Health Sciences, Department of Kinesiology, University of Waterloo, Waterloo, Ont., Canada N2L 3G1.
authors.forename: Janessa D M
authors.initials: JD
authors.lastname: Drake
authors.forename: Crystal D
authors.initials: CD
authors.lastname: Aultman
authors.forename: Stuart M
authors.initials: SM
authors.lastname: McGill
authors.forename: Jack P
authors.initials: JP
authors.lastname: Callaghan
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000379:methods; D002574:Cervical Vertebrae / Q000293:injuries* / Q000503:physiopathology*; D004195:Disease Models, Animal; D004548:Elasticity; D050724:Fractures, Cartilage / Q000503:physiopathology*; D006547:Hernia / Q000503:physiopathology; D066298:In Vitro Techniques; D007405:Intervertebral Disc Displacement / Q000503:physiopathology*; D016059:Range of Motion, Articular; D016103:Spinal Fractures / Q000503:physiopathology*; D013314:Stress, Mechanical; D013552:Swine; D019415:Torque; D016474:Weight-Bearing