title: Biomechanical implications of extending occipitocervical instrumentation to include the subaxial spine.
abstract: BACKGROUND
  No clear biomechanical data exist regarding where to place the caudal end of a screw-rod occipitocervical instrumentation construct.
  
  
  OBJECTIVE
  This study examines whether range of motion (ROM) from the occiput to C2 is altered by subaxial extension of occipitocervical instrumentation constructs.
  
  
  METHODS
  Cadaver specimens underwent intact biomechanical testing followed by destabilization via an odontoid osteotomy. Subsequent biomechanical testing was performed of four occipitocervical constructs: occipital plate + C2 pars screws (construct 1), occipital plate + C2 pars screws + C4 lateral mass screws (construct 2), occipital plate + C1-C2 transarticular screws (construct 3), and occipital plate + C1-C2 transarticular screws + C4 lateral mass screws (construct 4).
  
  
  RESULTS
  All constructs significantly reduced occiput-C2 ROM in all loading modes compared with the intact cervical spine, with one exception (construct 1, lateral bending). No significant ROM differences were noted when C4 lateral mass screws (construct 4) were added to construct 3. Addition of C4 lateral mass screws (construct 2) to construct 1 decreased the ROM in the flexion mode only. No significant ROM differences were seen between construct 2 and construct 3 in any loading mode.
  
  
  CONCLUSIONS
  The addition of subaxial instrumentation to occipitocervical instrumentation constructs in this study decreased occiput-C2 ROM only when the construct was anchored by C2 pars screws and only in flexion. Screws that cross the C1 to C2 articulation provide stable fixation when combined with an occipital plate, and the addition of subaxial instrumentation to this construct for stabilizing the occipitocervical junction does not significantly decrease ROM.
authors.affiliation: Department of Neurosurgery, The Medical College of Wisconsin, Milwaukee, Wisconsin 53226, USA.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Martin
authors.forename: Harlan J
authors.initials: HJ
authors.lastname: Bruner
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Wolfla
authors.forename: Narayan
authors.initials: N
authors.lastname: Yoganandan
mesh_terms: D001269:Atlanto-Occipital Joint / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000592:standards; D002574:Cervical Vertebrae / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery*; D006801:Humans; D016268:Internal Fixators / Q000592:standards*; D007593:Joint Instability / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D009777:Occipital Bone / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery*; D009809:Odontoid Process / Q000473:pathology / Q000503:physiopathology / Q000601:surgery; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D013314:Stress, Mechanical