title: Comparison of stiffness and failure load of two cervical spine fixation techniques in an in vitro human model.
abstract: OBJECTIVE
  Recently, an unpaired threaded cage has been introduced as a fusion device for the cervical spine. No biomechanical comparison of a stand-alone single interbody threaded cage to a standard plated Smith-Robinson construct has been reported. Accordingly, an in vitro biomechanical comparison of a single threaded cylindrical interbody fusion cage versus a plated Smith-Robinson cervical discectomy and fusion construct was conducted to establish whether a single cylindrical interbody cage in the cervical spine would perform mechanically as well as a plated structural interbody graft.
  
  
  METHODS
  Six fresh-frozen human cadaveric cervical spines were used for biomechanical testing. Flexion-extension and load-to-failure testing were performed on two single-level discectomy and interbody fusion constructs from each specimen.
  
  
  RESULTS
  Initial range of motion (ROM) was significantly greater for the specimens implanted with a cage than specimens implanted with a structural graft and plate (9.1 degrees +/- 3.7 degrees vs 5.8 degrees +/- 2.4 degrees ; P = 0.040). Initial stiffness in flexion in caged specimens was significantly less than in plated specimens (0.7 +/- 0.3 vs 0.9 +/- 0.3 Nm/ degrees ; P = 0.028). Cage specimens also failed at a significantly lower load than plated specimens (9.8 +/- 3.5 vs 15.8 +/- 4.1 Nm; P = 0.0104).
  
  
  CONCLUSIONS
  This study demonstrates that a plated Smith-Robinson cervical discectomy and fusion construct provides greater stiffness and failure load and reduced ROM across operated levels than a single interbody cage construct. Although clinical success may not directly correlate with biomechanical data, these results raise concern regarding the use of a single threaded interbody cage as a stand-alone device for cervical interbody fusion.
authors.affiliation: Orthopaedic Biomechanics Laboratory, Department of Orthopaedics, Oregon Health and Science University, Portland, Oregon 97239-3098, USA. hartro@ohsu.edu
authors.forename: Robert
authors.initials: R
authors.lastname: Hart
authors.forename: Joel
authors.initials: J
authors.lastname: Gillard
authors.forename: Shilpa
authors.initials: S
authors.lastname: Prem
authors.forename: Marie
authors.initials: M
authors.lastname: Shea
authors.forename: Scott
authors.initials: S
authors.lastname: Kitchel
mesh_terms: D001860:Bone Plates / Q000592:standards; D002102:Cadaver; D002574:Cervical Vertebrae / Q000473:pathology / Q000502:physiology* / Q000601:surgery*; D017586:Diskectomy / Q000295:instrumentation / Q000379:methods; D006801:Humans; D016268:Internal Fixators / Q000592:standards*; D011183:Postoperative Complications / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control; D016059:Range of Motion, Articular / Q000502:physiology; D013123:Spinal Fusion / Q000009:adverse effects / Q000295:instrumentation*; D013314:Stress, Mechanical; D017211:Treatment Failure; D016474:Weight-Bearing / Q000502:physiology