title: "Surprise" Loading in Flexion Increases the Risk of Disc Herniation Due to Annulus-Endplate Junction Failure: A Mechanical and Microstructural Investigation.
abstract: METHODS
  Microstructural investigation of compression-induced herniation of the flexed lumbar disc.
  
  
  OBJECTIVE
  To provide a microstructural analysis of the mechanisms of annular wall failure in healthy discs subjected to flexion and a rate of compression comparable with the maximum rate at which the muscles of the spinal column can generate a force.
  
  
  BACKGROUND
  Clinical evidence indicates the involvement of the endplate in herniation. It is known that both an elevated rate of compression and a flexed posture are necessary to cause disc failure either within the midspan of the annulus or at the annular-endplate interface. However, the question of what effect a sudden or "surprise" loading might have on the mode of failure is, as yet, unanswered.
  
  
  METHODS
  Twenty-four healthy mature ovine lumbar motion segments were compressed to failure in high physiological flexion (10º). This occurred over approximately 5 mm of crosshead displacement in 0.75 seconds that resulted in a displacement rate of 400 mm/min (defined as a "surprise" rate) and was intended to simulate the maximum rate at which the muscles of the spinal column can generate a force. The damaged discs were then analyzed microstructurally.
  
  
  RESULTS
  Fifty-eight percent of discs suffered annular-endplate junction rupture, 25% suffered midspan annular rupture, and the balance of 17% endplate fracture. Microstructural analysis indicated that annular rupture initiated at the endplate apical ridge in the mid-to-outer region of the annulus in both annular-endplate and midspan annulus rupture.
  
  
  CONCLUSIONS
  Motion segments subjected to a "surprise" loading rate are likely to fail via some form of annular rupture. Failure under such sudden loading occurs mostly via rupture of the annular-endplate junction and is thought to arise from a rate-induced mechanostructural imbalance between the annulus and the endplate.
  
  
  METHODS
  N/A.
authors: *Experimental Tissue Mechanics Laboratory, Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand; and †Department of Orthopaedic Surgery, Auckland City Hospital, Auckland, New Zealand.
authors: Kelly R
authors: KR
authors: Wade
authors: Peter A
authors: PA
authors: Robertson
authors: Ashvin
authors: A
authors: Thambyah
authors: Neil D
authors: ND
authors: Broom
mesh_terms: D000818:Animals; D064131:Back Muscles / Q000473:pathology / Q000503:physiopathology*; D001696:Biomechanical Phenomena; D005260:Female; D007403:Intervertebral Disc / Q000473:pathology / Q000503:physiopathology*; D007405:Intervertebral Disc Displacement / Q000209:etiology* / Q000473:pathology / Q000503:physiopathology; D008159:Lumbar Vertebrae / Q000473:pathology / Q000503:physiopathology*; D018570:Risk Assessment; D012307:Risk Factors; D012756:Sheep; D013314:Stress, Mechanical; D013997:Time Factors; D016474:Weight-Bearing