title: Biomechanical effects of cage positions and facet fixation on initial stability of the anterior lumbar interbody fusion motion segment.
abstract: METHODS
  An in vitro biomechanical study using porcine lumbar segments as specimens.
  
  
  OBJECTIVE
  To evaluate the effects of interbody cage support and endplate strength on the stability of instrumented segments.
  
  
  BACKGROUND
  The anterior lumbar interbody fusion (ALIF) cage is widely used to restore disc height and support the anterior column. Transpedicle or posterior spinal fusion or facet screw fixation (FSF) can improve the stability of the vertebra-instrumented segments. The cage position can affect the anterior support and initial stability of the ALIF region, but there is no consistent data on its biomechanical effects on ALIF and ALIF/FSF segments.
  
  
  METHODS
  Nine variations of 3 instrumentation modes (intact, ALIF, ALIF/FSF) and 3 cage positions (type I, anterolateral; type II, mediolateral; and type III, posteromedial) are tested under 5 lumbar motions. The range of motion and axial displacement are used as comparison indices for the different variations.
  
  
  RESULTS
  The cage placement serves as support for the intervertebral loads while the posterior fixation behaves as lever to further enhance the anterior support. At the endplate-cage interfaces, the endplate strength directly affects the cage subsidence. Type III exhibits higher stability for standing due to the greater strength of the endplate in the posterior region. Otherwise, type I consistently has higher stability for all other types of motion.
  
  
  CONCLUSIONS
  The initial stability of the ALIF region is affected by the moment arm and the mechanical strength of the engaged endplates. Type I has greater moment arm and provides more efficient support to the instrumented segments. Endplate strength provides an ability to withstand lumbar loads and suppress the cage subsidence. Bone quality at the endplate-cage interfaces must therefore be cautiously evaluated preoperatively.
  
  
  METHODS
  N/A.
authors.affiliation: *Department of Neurological Surgery, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan; and †Graduate Institute of Biomedical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan.
authors.forename: Dueng-Yuan
authors.initials: DY
authors.lastname: Hueng
authors.forename: Tzu-Tsao
authors.initials: TT
authors.lastname: Chung
authors.forename: Wen-Hsien
authors.initials: WH
authors.lastname: Chuang
authors.forename: Chi-Pin
authors.initials: CP
authors.lastname: Hsu
authors.forename: Kuan-Nien
authors.initials: KN
authors.lastname: Chou
authors.forename: Shang-Chih
authors.initials: SC
authors.lastname: Lin
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology; D055959:Intervertebral Disc Degeneration / Q000503:physiopathology / Q000601:surgery*; D007593:Joint Instability / Q000503:physiopathology / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000502:physiology / Q000601:surgery*; D008297:Male; D008422:Materials Testing / Q000379:methods; D009068:Movement / Q000502:physiology; D065289:Pedicle Screws; D016059:Range of Motion, Articular / Q000502:physiology; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D013552:Swine; D059145:Total Disc Replacement / Q000295:instrumentation / Q000379:methods*; D016474:Weight-Bearing / Q000502:physiology; D021801:Zygapophyseal Joint / Q000502:physiology / Q000601:surgery*