title: Load transfer characteristics between posterior spinal implants and the lumbar spine under anterior shear loading: an in vitro investigation.
abstract: METHODS
  A biomechanical human cadaveric study.
  
  
  OBJECTIVE
  To determine the percentage of shear force supported by posterior lumbar spinal devices of varying stiffnesses under anterior shear loading in a degenerative spondylolisthesis model.
  
  
  BACKGROUND
  Clinical studies have demonstrated beneficial results of posterior arthrodesis for the treatment of degenerative spinal conditions with instability. Novel spinal implants are designed to correct and maintain spinal alignment, share load with the spine, and minimize adjacent level stresses. The optimal stiffness of these spinal systems is unknown. To our knowledge, low-stiffness posterior instrumentation has not been tested under an anterior shear force, a highly relevant force to be neutralized in the clinical case of degenerative spondylolisthesis.
  
  
  METHODS
  The effects of implant stiffness and specimen condition on implant load and intervertebral motion were assessed in a biomechanical study. Fifteen human cadaveric lumbar functional spinal units were tested under a static 300 N axial compression force and a cyclic anterior shear force (5-250 N). Implants (high-stiffness [HSI]: ø 5.5-mm titanium, medium-stiffness [MSI]: ø 6.35 × 7.2-mm oblong PEEK, and low-stiffness [LSI]: ø 5.5-mm round PEEK) instrumented with strain gauges were used to calculate loads and were tested in each of 3 specimen conditions simulating degenerative changes: intact, facet instability, and disc instability. Intervertebral motions were measured with a motion capture system.
  
  
  RESULTS
  As predicted, implants supported a significantly greater shear force as the specimen was progressively destabilized. Mean implant loads as a percent of the applied shear force in order of increasing specimen destabilization for the HSI were 43%, 67%, and 76%; mean implant loads for the MSI were 32%, 56%, and 77%; and mean implant loads for the LSI were 18%, 35%, and 50%. Anterior translations increased with decreasing implant stiffness and increasing specimen destabilization.
  
  
  CONCLUSIONS
  Implant shear stiffness significantly affected the load sharing between the implant and the natural spine in anterior shear ex vivo. Low-stiffness implants transferred significantly greater loads to the spine. This study supports the importance of load-sharing behavior when designing new implants.
authors.affiliation: Department of Mechanical Engineering, University of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: Angela D
authors.initials: AD
authors.lastname: Melnyk
authors.forename: Tian Lin
authors.initials: TL
authors.lastname: Wen
authors.forename: Stephen
authors.initials: S
authors.lastname: Kingwell
authors.forename: Jason D
authors.initials: JD
authors.lastname: Chak
authors.forename: Vaneet
authors.initials: V
authors.lastname: Singh
authors.forename: Peter A
authors.initials: PA
authors.lastname: Cripton
authors.forename: Charles G
authors.initials: CG
authors.lastname: Fisher
authors.forename: Marcel F
authors.initials: MF
authors.lastname: Dvorak
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Oxland
mesh_terms: D000368:Aged; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D002102:Cadaver; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000503:physiopathology* / Q000601:surgery; D008297:Male; D019637:Orthopedic Procedures / Q000295:instrumentation / Q000379:methods; D019736:Prostheses and Implants; D013168:Spondylolisthesis / Q000503:physiopathology* / Q000601:surgery; D016474:Weight-Bearing / Q000502:physiology*