title: The effect of posterior non-fusion instrumentation on segmental shear loading of the lumbar spine.
abstract: BACKGROUND
  Lumbar stenosis and facet osteoarthritis represent indications for decompression and instrumentation. It is unclear if degenerative spondylolisthesis grade I with a remaining disc height could be an indication for non-fusion instrumentation. The purpose of this study was to determine the influence of a mobile pedicle screw based device on lumbar segmental shear loading, thus simulating the condition of spondylolisthesis.
  
  
  METHODS
  Six human cadaver specimens were tested in 3 configurations: intact L4-L5 segment, then facetectomy plus undercutting laminectomy, then instrumentation with lesion. A static axial compression of 400 N was applied to the lumbar segment and anterior displacements of L4 on L5 were measured for posterior-anterior shear forces from 0 to 200 N. The slope of the loading curve was assessed to determine shear stiffness.
  
  
  RESULTS
  Homogenous load-displacement curves were obtained for all specimens. The average intact anterior displacement was 1.2 mm. After lesion, the displacement increased by 0.6mm compared to intact (P=0.032). The instrumentation decreased the displacement by 0.5 mm compared to lesion (P=0.046). The stiffness's were: 162 N/mm for intact, 106 N/mm for lesion, 148 N/mm for instrumentation. The difference was not significant between instrumented and intact segments (P=0.591).
  
  
  CONCLUSIONS
  Facetectomy plus undercutting laminectomy decreases segmental shear stiffness and increases anterior translational L4-L5 displacement. Shear stiffness of the instrumented segment is higher with the device and anterior displacements under shear loading are similar to the intact spine. This condition could theoretically be interesting for the simulation of non-fusion instrumentation in degenerative spondylolisthesis.
authors.affiliation: Laboratoire de Biomécanique, Arts et Métiers ParisTech, 151, boulevard de l'Hôpital, 75013 Paris, France; Service de Chirurgie du Rachis, Hôpitaux Universitaires de Strasbourg, Fédération de Médecine Translationnelle (FMTS), 1, place de l'Hôpital, BP 426, 67091 Strasbourg Cedex, France. Electronic address: yann.philippe.charles@chru-strasbourg.fr.
authors.forename: Y P
authors.initials: YP
authors.lastname: Charles
authors.affiliation: Laboratoire de Biomécanique, Arts et Métiers ParisTech, 151, boulevard de l'Hôpital, 75013 Paris, France.
authors.forename: S
authors.initials: S
authors.lastname: Persohn
authors.affiliation: Laboratoire de Biomécanique, Arts et Métiers ParisTech, 151, boulevard de l'Hôpital, 75013 Paris, France.
authors.forename: P
authors.initials: P
authors.lastname: Rouch
authors.affiliation: Service de Chirurgie du Rachis, Hôpitaux Universitaires de Strasbourg, Fédération de Médecine Translationnelle (FMTS), 1, place de l'Hôpital, BP 426, 67091 Strasbourg Cedex, France.
authors.forename: J-P
authors.initials: JP
authors.lastname: Steib
authors.affiliation: Département de Santé Publique, Hôpitaux Universitaires de Strasbourg, Fédération de Médecine Translationnelle (FMTS), 1, place de l'Hôpital, BP 426, 67091 Strasbourg Cedex, France.
authors.forename: E A
authors.initials: EA
authors.lastname: Sauleau
authors.affiliation: Laboratoire de Biomécanique, Arts et Métiers ParisTech, 151, boulevard de l'Hôpital, 75013 Paris, France.
authors.forename: W
authors.initials: W
authors.lastname: Skalli
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002102:Cadaver; D005260:Female; D006801:Humans; D007796:Laminectomy; D008159:Lumbar Vertebrae / Q000503:physiopathology* / Q000601:surgery; D008297:Male; D008422:Materials Testing; D008875:Middle Aged; D019736:Prostheses and Implants; D013168:Spondylolisthesis / Q000503:physiopathology* / Q000601:surgery; D016474:Weight-Bearing / Q000502:physiology*; D021801:Zygapophyseal Joint / Q000601:surgery
keywords: Anterior shear; Degenerative spondylolisthesis; Facetectomy; Non-fusion instrumentation; Undercutting laminectomy