title: Biomechanical comparison of occipitoatlantal screw fixation techniques.
abstract: OBJECTIVE
  The stability provided by 3 occipitoatlantal fixation techniques (occiput [Oc]-C1 transarticular screws, occipital keel screws rigidly interconnected with C-1 lateral mass screws, and suboccipital/sublaminar wired contoured rod) were compared.
  
  
  METHODS
  Seven human cadaveric specimens received transarticular screws and 7 received occipital keel-C1 lateral mass screws. All specimens later underwent contoured rod fixation. All conditions were studied with and without placement of a structural graft wired between the skull base and C-1 lamina. Specimens were loaded quasistatically using pure moments to induce flexion, extension, lateral bending, and axial rotation while recording segmental motion optoelectronically. Flexibility was measured immediately postoperatively and after 10,000 cycles of fatigue.
  
  
  RESULTS
  Application of Oc-C1 transarticular screws, with a wired graft, reduced the mean range of motion (ROM) to 3% of normal. Occipital keel-C1 lateral mass screws (also with graft) offered less stability than transarticular screws during extension and lateral bending (p < 0.02), reducing ROM to 17% of normal. The wired contoured rod reduced motion to 31% of normal, providing significantly less stability than either screw fixation technique. Fatigue increased motion in constructs fitted with transarticular screws, keel screws/lateral mass screw constructs, and contoured wired rods, by means of 19, 5, and 26%, respectively. In all constructs, adding a structural graft significantly improved stability, but the extent depended on the loading direction.
  
  
  CONCLUSIONS
  Assuming the presence of mild C1-2 instability, Oc-C1 transarticular screws and occipital keel-C1 lateral mass screws are approximately equivalent in performance for occipitoatlantal stabilization in promoting fusion. A posteriorly wired contoured rod is less likely to provide a good fusion environment because of less stabilizing potential and a greater likelihood of loosening with fatigue.
authors.affiliation: Division of Neurological Surgery, Phoenix, Arizona, USA.
authors.forename: Nicholas C
authors.initials: NC
authors.lastname: Bambakidis
authors.forename: Iman
authors.initials: I
authors.lastname: Feiz-Erfan
authors.forename: Eric M
authors.initials: EM
authors.lastname: Horn
authors.forename: L Fernando
authors.initials: LF
authors.lastname: Gonzalez
authors.forename: Seungwon
authors.initials: S
authors.lastname: Baek
authors.forename: K Zafer
authors.initials: KZ
authors.lastname: Yüksel
authors.forename: Anna G U
authors.initials: AG
authors.lastname: Brantley
authors.forename: Volker K H
authors.initials: VK
authors.lastname: Sonntag
authors.forename: Neil R
authors.initials: NR
authors.lastname: Crawford
mesh_terms: D001269:Atlanto-Occipital Joint / Q000503:physiopathology* / Q000601:surgery*; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D005260:Female; D006801:Humans; D004204:Joint Dislocations / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular / Q000502:physiology*; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*