title: Height and torsional stiffness are most sensitive to annular injury in large animal intervertebral discs.
abstract: BACKGROUND
  Acute annulus fibrosus injury has been identified as a contributing factor to intervertebral disc (IVD) degeneration. Injuries as small as those resulting from needle injection result in localized mechanical disruption via fiber breakage, but it is unknown whether these injuries initiate degeneration locally or through changes in the mechanical behavior of the entire disc. However, in vitro biomechanical studies of injury are limited to a single type of injury or measurements in only one or two degrees of freedom.
  
  
  OBJECTIVE
  The aim of this study is to provide a comprehensive assessment of the joint level mechanical response to IVD injuries of various sizes in a large animal model. We hypothesize that annular injuries will affect disc mechanics differently depending on size, location, and mode of loading. We further hypothesize that a large injury to one side of the disc will induce a bending moment reaction under axial compression, which may decrease spinal column stability.
  
  
  METHODS
  A comprehensive biomechanical study investigating effects of small and large injuries on IVD pressurization and six-degree-of-freedom stiffness behaviors using bovine motion segments.
  
  
  METHODS
  Bovine caudal motion segments were subjected to a series of annular injuries ranging from 21-gauge needle puncture to 10-mm scalpel incisions and evaluated before and after injury with both mechanical testing under multiple degrees of freedom (axial compression, flexion-extension, lateral bending, and torsion) and nucleus pulposus (NP) fluid pressurization tests.
  
  
  RESULTS
  Mechanical tests showed that axial torsional stiffness and disc height under resting compressive load were the parameters most sensitive to large annular injury. Bending and compressive stiffnesses, as well as bending moments induced by axial compression, were not significantly changed by scalpel incisions. Additionally, large injuries resulted in altered relaxation behavior after NP pressurization indicative of increases in both radial bulge compliance and fluid flow rates.
  
  
  CONCLUSIONS
  These findings suggest that loss of disc height, torsional stiffness, and NP fluid pressurization are the immediate results of acute annular injury and are therefore those properties that IVD repair strategies must strive to restore or maintain. The lack of change in bending stiffness and moment under compression suggests that acute annular tears alone are not sufficient to induce off-axis motion and instability.
authors.affiliation: Department of Molecular Physiology and Biophysics, The University of Vermont, 149 Beaumont Ave, Burlington, VT 05405, USA.
authors.forename: Arthur J
authors.initials: AJ
authors.lastname: Michalek
authors.forename: James C
authors.initials: JC
authors.lastname: Iatridis
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002417:Cattle; D019245:Compressive Strength; D007403:Intervertebral Disc / Q000293:injuries*; D055959:Intervertebral Disc Degeneration / Q000209:etiology*; D016059:Range of Motion, Articular; D013314:Stress, Mechanical