title: Influence of an auxiliary facet system on intervertebral discs and adjacent facet joints.
abstract: BACKGROUND
  Facet supplementation stabilizes after facetectomy and undercutting laminectomy. It is indicated in degenerative spondylolisthesis with moderate disc degeneration and dynamic stenosis.
  
  
  OBJECTIVE
  To determine the influence of an auxiliary facet system (AFS) on the instrumented disc, adjacent levels' discs, and facet joints and to compare it with fusion.
  
  
  METHODS
  Finite element study.
  
  
  METHODS
  L3-L4, L4-L5, and L5-S1 were studied using a validated finite element model with prescribed displacements for an intact spine, lesion by facetectomy and undercutting laminectomy, AFS, and fusion at L4-L5. The distribution of segmental range of motion (ROM) and applied moments, von Mises stress at the annulus, and facet joint contact forces were calculated with rotations in all planes. Institutional support for implant evaluation and modeling was received by Clariance.
  
  
  RESULTS
  In flexion-extension and lateral bending, fusion decreased L4-L5 ROM and increased adjacent levels' ROM. Range of motion was similarly distributed with intact lesion and AFS. In axial rotation, L4-L5 ROM represented 33% with intact, 55% after lesion, 25% with AFS, and 21% with fusion. Fusion increased annulus stress at adjacent levels in flexion-extension and lateral bending, but decreased stress at L4-L5 compared with AFS. In axial rotation, von Mises stress was similar with fusion and AFS. Facet loading increased in extension and lateral bending with fusion. It was comparable for fusion and AFS in axial rotation.
  
  
  CONCLUSIONS
  This study suggests that the AFS stabilizes L4-L5 in axial rotation after facetectomy and undercutting laminectomy as fusion does. This is because of the cross-link that generates an increased annulus stress in axial rotation at adjacent levels. With imposed displacements, without in vivo compensation of the hips, the solicitation at adjacent levels' discs and facet joints is higher with fusion compared with AFS. Fusion decreases intradiscal stress at the instrumented level.
authors.affiliation: Laboratoire de Biomécanique, Arts et Métiers ParisTech, Paris, France; Service de Chirurgie du Rachis, Hôpitaux Universitaires de Strasbourg, Fédération de Médecine Translationnelle, Université de Strasbourg, 1, Place de l'hôpital, B.P. 426, 67091 Strasbourg Cedex, France. Electronic address: yann.philippe.charles@chru-strasbourg.fr.
authors.forename: Yann Philippe
authors.initials: YP
authors.lastname: Charles
authors.forename: Lucas Venancio Pires Carvalho
authors.initials: LV
authors.lastname: Lima
authors.forename: Sylvain
authors.initials: S
authors.lastname: Persohn
authors.forename: Philippe
authors.initials: P
authors.lastname: Rouch
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Steib
authors.forename: Wafa
authors.initials: W
authors.lastname: Skalli
mesh_terms: D001696:Biomechanical Phenomena; D020342:Finite Element Analysis; D006801:Humans; D016268:Internal Fixators; D007403:Intervertebral Disc / Q000601:surgery*; D016059:Range of Motion, Articular / Q000502:physiology; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D021801:Zygapophyseal Joint / Q000502:physiology / Q000601:surgery*
keywords: Annulus stress; Facet joint contact forces; Facet supplementation; Finite elements; Segmental kinematics