title: Biomechanical analysis of the C2 intralaminar fixation technique using a cross-link and offset connector for an unstable atlantoaxial joint.
abstract: BACKGROUND
  C2 intralaminar screws offer the advantage of avoiding the vertebral artery; however, biomechanical studies have demonstrated inferiority of C2 intralaminar screw fixation compared with C2 intrapedicular fixation in the presence of an odontoid fracture. Addition of a transverse cross-link may improve stability afforded by the lamina screws but will require the use of offset connectors to complete the construct.
  
  
  OBJECTIVE
  The aims of this project were to evaluate whether transverse cross-links can add adequate stability to atlantoaxial constructs using C1 lateral mass and C2 intralaminar screw fixation. The secondary objective was to determine the biomechanical contribution of the C2 offset connectors.
  
  
  METHODS
  In vitro human cadaveric biomechanical study.
  
  
  METHODS
  Ten cadaveric specimens were obtained and instrumented with C1 lateral mass, C2 pedicle, and C2 intralaminar screws. After intact spine testing, each C1-C2 construct was nondestructively evaluated under axial rotation (AR), flexion extension (FE), and lateral bending (LB). Intralaminar fixation was tested with and without offset connectors, which allowed for cross-link addition to the construct. After normal state evaluation, the odontoid was resected and analyses were repeated.
  
  
  RESULTS
  Postreconstruction range of motion in AR, FE, and LB showed no significant differences between the four fixation constructs in the stable specimens. Transpedicular fixation at C2 proved superior to intralaminar techniques without a cross-link in AR and LB after destabilization with an odontoidectomy. The addition of a cross-link to the intralaminar construct improved segmental AR and LB stability to the level afforded by the transpedicular fixation. Offset connectors appeared to marginally weaken the intralaminar fixation, but the findings were not significant.
  
  
  CONCLUSIONS
  Coupled with an offset connector and a cross-link, C2 intralaminar screws offer similar segmental stability to intrapedicular fixation in the presence of an unstable dens fracture. Lateral offset connectors at C2 do not significantly compromise stability of C1 lateral mass-C2 intralaminar fixation.
authors.affiliation: Department of Orthopaedic Surgery and Rehabilitation, Walter Reed National Military Medical Center, 8901 Wisconsin Ave, Bethesda, MD 20889, USA. armyspine@yahoo.com
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Lehman
authors.forename: Anton E
authors.initials: AE
authors.lastname: Dmitriev
authors.forename: Kevin W
authors.initials: KW
authors.lastname: Wilson
mesh_terms: D001268:Atlanto-Axial Joint / Q000502:physiology / Q000601:surgery*; D001696:Biomechanical Phenomena / Q000502:physiology*; D001863:Bone Screws; D006801:Humans; D007593:Joint Instability / Q000601:surgery*; D016059:Range of Motion, Articular / Q000502:physiology*; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*