title: A new bone surrogate model for testing interbody device subsidence.
abstract: METHODS
  An in vitro biomechanical study investigating interbody device subsidence measures in synthetic vertebrae, polyurethane foam blocks, and human cadaveric vertebrae.
  
  
  OBJECTIVE
  To compare subsidence measures of bone surrogates with human vertebrae for interbody devices varying in size/placement.
  
  
  BACKGROUND
  Bone surrogates are alternatives when human cadaveric vertebrae are unavailable. Synthetic vertebrae modeling cortices, endplates, and cancellous bone have been developed as an alternative to polyurethane foam blocks for testing interbody device subsidence.
  
  
  METHODS
  Indentors placed on the endplates of synthetic vertebrae, foam blocks, and human vertebrae were subjected to uniaxial compression. Subsidence, measured with custom-made extensometers, was evaluated for an indentor seated either centrally or peripherally on the endplate. Failure force and indentation stiffness were determined from force-displacement curves.
  
  
  RESULTS
  Subsidence measures in human vertebrae varied with indentor placement: failure forces were higher and indentors subsided less with peripheral placement. Subsidence measures in foam blocks were insensitive to indentor size/placement; they were similar to human vertebrae for centrally placed but not for peripherally placed indentors. Although subsidence measures in synthetic vertebrae were sensitive to indentor size/placement, failure force and indentation stiffness were overestimated, and subsidence underestimated, for both centrally placed and peripherally placed indentors.
  
  
  CONCLUSIONS
  The synthetic endplate correctly represented the human endplate geometry, and thus, failure force, stiffness, and subsidence in synthetic vertebrae were sensitive to indentor size/placement. However, the endplate was overly strong and thus synthetic vertebrae did not accurately model indentor subsidence in human cadaveric vertebrae. Foam blocks captured subsidence measures more accurately than synthetic vertebrae for centrally placed indentors, but because of their uniform density were not sufficiently robust to capture changes generated from different indentor sizes/placements. The current bone surrogates are not accurate enough in terms of material property distribution to completely model subsidence in human cadaveric vertebrae.
authors.affiliation: Department of Mechanical Engineering, University of Wisconsin-Madison, WI 53706, USA. aau@cae.wisc.edu
authors.forename: Anthony G
authors.initials: AG
authors.lastname: Au
authors.forename: Ameet K
authors.initials: AK
authors.lastname: Aiyangar
authors.forename: Paul A
authors.initials: PA
authors.lastname: Anderson
authors.forename: Heidi-Lynn
authors.initials: HL
authors.lastname: Ploeg
mesh_terms: D001696:Biomechanical Phenomena; D002102:Cadaver; D006801:Humans; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D008422:Materials Testing; D008875:Middle Aged; D008954:Models, Biological; D011140:Polyurethanes / Q000737:chemistry*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D053831:Surgical Fixation Devices / Q000592:standards*; D013904:Thoracic Vertebrae / Q000601:surgery*
chemical_list: D011140:Polyurethanes; C028279:polyurethane foam