title: The effect of interbody cage positioning on lumbosacral vertebral endplate failure in compression.
abstract: METHODS
  A biomechanical investigation using a human cadaver, multisegmental lumbosacral spine model.
  
  
  OBJECTIVE
  To determine if 2 small, posterolaterally positioned titanium mesh interbody cages would provide superior construct strength and stiffness in compression compared to central cage placement. In addition, determine construct stiffness with interbody cages as opposed to an intact spine and assess the effect of bone mineral density (BMD).
  
  
  BACKGROUND
  Previous work has shown that the posterolateral corners of the lumbosacral endplates are stronger than the anterior and central regions. Information to suggest appropriate interbody cage positioning to avoid subsidence into adjacent vertebrae would be valuable for spine surgeons and implant designers.
  
  
  METHODS
  A total of 27 functional spinal units from L3 to S1 were dual x-ray absorptiometry scanned for BMD, instrumented with pedicle screw systems, and tested to failure in compression with titanium mesh interbody cages placed in 1 of 3 positions: 2 small posterolateral, 2 small central, or 1 large central. Analysis of covariance was conducted to compare failure load and stiffness across the different cage configurations. Repeated measures analysis of variance was used to analyze stiffness between functional spinal units with intact disc, discectomy, or interbody cages. Failure load was correlated against BMD.
  
  
  RESULTS
  Of the 3 placement patterns, 2 small titanium mesh cages in the posterolateral corners had 20% higher failure loads, although the difference was not significant (P = 0.20). Stiffness in compression for the 3 cage positions was not significantly different (P = 0.82). All intact discs with posterior instrumentation were significantly stiffer than any of the cage patterns (P = 0.0001). BMD of the vertebrae significantly correlated with failure loads (P = 0.007).
  
  
  CONCLUSIONS
  The placement of 2 small interbody cages posterolaterally tended to result in higher failure loads than central cage placement, although the results were not statistically significant. It is noteworthy that cage placement in any position resulted in a less stiff construct in compression than with an intact disc.
authors.affiliation: Department of Orthopaedics, University of British Columbia, Vancouver, Canada.
authors.forename: Robert D
authors.initials: RD
authors.lastname: Labrom
authors.forename: Juay-Seng
authors.initials: JS
authors.lastname: Tan
authors.forename: Christopher W
authors.initials: CW
authors.lastname: Reilly
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Tredwell
authors.forename: Charles G
authors.initials: CG
authors.lastname: Fisher
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Oxland
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D015519:Bone Density; D002102:Cadaver; D019245:Compressive Strength; D017586:Diskectomy; D005260:Female; D006801:Humans; D016268:Internal Fixators / Q000009:adverse effects*; D008159:Lumbar Vertebrae / Q000503:physiopathology* / Q000601:surgery*; D008297:Male; D012447:Sacrum / Q000503:physiopathology* / Q000601:surgery*; D014025:Titanium; D016474:Weight-Bearing
chemical_list: D014025:Titanium