title: A multiscale structural investigation of the annulus-endplate anchorage system and its mechanisms of failure.
abstract: BACKGROUND
  The annulus-endplate anchorage system performs a critical role in the disc, creating a strong structural link between the compliant annulus and the rigid vertebrae. Endplate failure is thought to be associated with disc herniation, a recent study indicating that this failure mode occurs more frequently than annular rupture.
  
  
  OBJECTIVE
  The aim was to investigate the structural principles governing annulus-endplate anchorage and the basis of its strength and mechanisms of failure.
  
  
  METHODS
  Loading experiments were performed on ovine lumbar motion segments designed to induce annulus-endplate failure, followed by macro- to micro- to fibril-level structural analyses.
  
  
  METHODS
  The study was funded by a doctoral scholarship from our institution. Samples were loaded to failure in three modes: torsion using intact motion segments, in-plane tension of the anterior annulus-endplate along one of the oblique fiber angles, and axial tension of the anterior annulus-endplate. The anterior region was chosen for its ease of access. Decalcification was used to investigate the mechanical influence of the mineralized component. Structural analysis was conducted on both the intact and failed samples using differential interference contrast optical microscopy and scanning electron microscopy.
  
  
  RESULTS
  Two main modes of anchorage failure were observed--failure at the tidemark or at the cement line. Samples subjected to axial tension contained more tidemark failures compared with those subjected to torsion and in-plane tension. Samples decalcified before testing frequently contained damage at the cement line, this being more extensive than in fresh samples. Analysis of the intact samples at their anchorage sites revealed that annular subbundle fibrils penetrate beyond the cement line to a limited depth and appear to merge with those in the vertebral and cartilaginous endplates.
  
  
  CONCLUSIONS
  Annulus-endplate anchorage is more vulnerable to failure in axial tension compared with both torsion and in-plane tension and is probably due to acute fiber bending at the soft-hard interface of the tidemark. This finding is consistent with evidence showing that flexion, which induces a similar pattern of axial tension, increases the risk of herniation involving endplate failure. The study also highlights the important strengthening role of calcification at this junction and provides new evidence of a fibril-based form of structural integration across the cement line.
authors.affiliation: Experimental Tissue Mechanics Laboratory, Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand.
authors.forename: Samantha A
authors.initials: SA
authors.lastname: Rodrigues
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: Experimental Tissue Mechanics Laboratory, Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand. Electronic address: nd.broom@auckland.ac.nz.
authors.forename: Neil D
authors.initials: ND
authors.lastname: Broom
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002113:Calcification, Physiologic; D007403:Intervertebral Disc / Q000473:pathology / Q000503:physiopathology*; D007405:Intervertebral Disc Displacement; D008159:Lumbar Vertebrae / Q000473:pathology / Q000503:physiopathology*; D008854:Microscopy, Electron; D019736:Prostheses and Implants; D011475:Prosthesis Failure; D019919:Prosthesis Implantation; D016059:Range of Motion, Articular; D012756:Sheep
keywords: Acute fiber bending; Annulus-endplate anchorage; Failure modes; Macro, micro and fibril-level structural analysis; Relevance to herniation; Role of flexion in soft-hard junction failure