title: Posterolateral Disc Prolapse in Flexion Initiated by Lateral Inner Annular Failure: An Investigation of the Herniation Pathway.
abstract: METHODS
  Structural investigation of mechanically induced herniations in ovine lumbar motion segments.
  
  
  OBJECTIVE
  This new study addresses the question of whether there are regions other than the posterior and posterolateral aspects that are implicated in the initiation of disc disruption and herniation.
  
  
  BACKGROUND
  Flexion in combination with compressive loading will induce disc herniations in healthy motion segments in vitro. Although it is widely accepted that the posterior and posterolateral regions of the disc are the primary sites of herniation much less is known as to whether other regions of the disc might be involved in the herniation process.
  
  
  METHODS
  Healthy ovine lumbar motion segments (n = 14) were flexed 10° and compressed at a rate of 40 mm/min up to point of failure. The discs were macroscopically analyzed using progressive transverse sectioning to obtain a more global picture of internal disc disruption and herniation.
  
  
  RESULTS
  A high prevalence of disruption in the lateral annulus was found associated with circumferential tracking of nucleus between the annular layers toward the posterolateral and posterior regions. In all tests this lateral disruption did not cause any discernible external change in the lateral disc periphery after the removal of load. After imposing the predetermined flexion the applied compression also induced a forward anterior shear of the superior vertebra of approximately equal magnitude to the axial compressive displacement.
  
  
  CONCLUSIONS
  The vulnerability of the lateral annulus to disruption is thought to arise from the overloading of its differentially recruited oblique/counteroblique fiber sets, this in turn generated by anterior shear developed in the flexed, compressed motion segment. This lateral annular disruption, followed by circumferential tracking of nuclear material and resulting in either contained or uncontained extrusions in the posterior or posterolateral annulus, highlights the complexity of the herniation process.
  
  
  METHODS
  N/A.
authors.affiliation: Experimental Tissue Mechanics Laboratory, Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand.
authors.forename: Vonne M
authors.initials: VM
authors.lastname: van Heeswijk
authors.affiliation: Experimental Tissue Mechanics Laboratory, Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand.
authors.forename: Ashvin
authors.initials: A
authors.lastname: Thambyah
authors.affiliation: Department of Orthopaedic Surgery, Auckland City Hospital, Auckland, New Zealand.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Robertson
authors.affiliation: Experimental Tissue Mechanics Laboratory, Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand.
authors.forename: Neil D
authors.initials: ND
authors.lastname: Broom
mesh_terms: D000818:Animals; D000070616:Annulus Fibrosus / Q000000981:diagnostic imaging*; D001696:Biomechanical Phenomena / Q000502:physiology; D007403:Intervertebral Disc / Q000000981:diagnostic imaging*; D055959:Intervertebral Disc Degeneration / Q000000981:diagnostic imaging / Q000209:etiology; D007405:Intervertebral Disc Displacement / Q000000981:diagnostic imaging* / Q000209:etiology; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging*; D016059:Range of Motion, Articular / Q000502:physiology; D012756:Sheep; D013314:Stress, Mechanical