title: Posterior C2 instrumentation: accuracy and complications associated with four techniques.
abstract: METHODS
  Retrospective review of a single tertiary care spine database to identify patients with C2 instrumentation between January 2001 and September 2008.
  
  
  OBJECTIVE
  (1) Evaluate a large series of posterior C2 screws to determine accuracy by computed tomography (CT) scan, (2) assess dimensions of "safe bony windows" with CT, and (3) assess perioperative complication rate related to errant screw placement.
  
  
  BACKGROUND
  The variable C2 anatomy can make instrumentation challenging and prone to potentially severe complications. New techniques have expanded available options.
  
  
  METHODS
  Clinical data were obtained from the medical record. Radiographic analyses included preoperative and postoperative CT scans to quantify the patients' bone and to classify accuracy of instrumentation. Screws were graded using the following definitions:
  
  
  RESULTS
  Seven hundred and thirty-six screws in 383 patients were identified. Fifty-five patients were excluded because of lack of data leaving 328 patients (188 male patients, 140 female patients) with 633 screws. Three hundred and thirty-nine pedicle, 154 transarticular, 63 laminar, and 77 short pars screws were placed, and of the 509 screws with postoperative CT scans, accuracy rates (Types I and II) were 98.8%, 98.5%, 100%, and 94.6%, respectively. Eight were unacceptably placed: two medially and six encroaching on the vertebral artery foramen. One patient had a vertebral artery occlusion and another had a dissection. There were no neurologic injuries. Mean CT measurements of pedicle height, axial width, and laminar width were 8.1, 5.8, and 5.7 mm respectively, with males having significantly larger pedicle height (P<0.001), pedicle width (P<0.001), and laminar width (P<0.022).
  
  
  CONCLUSIONS
  We show a lower than previously reported incidence of complications associated with posterior C2 screw placement. The multiple techniques of posterior C2 fixation available allow for flexibility in determining ideal technique.
authors.affiliation: Department of Orthopaedics and Sports Medicine, Harborview Medical Center/University of Washington Medical Center, Seattle, WA, USA. rbransfo@uw.edu
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Bransford
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Russo
authors.forename: Mark
authors.initials: M
authors.lastname: Freeborn
authors.forename: Quynh T
authors.initials: QT
authors.lastname: Nguyen
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Lee
authors.forename: Jens R
authors.initials: JR
authors.lastname: Chapman
authors.forename: Carlo
authors.initials: C
authors.lastname: Bellabarba
mesh_terms: D001863:Bone Screws; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D005260:Female; D006801:Humans; D008297:Male; D011183:Postoperative Complications / Q000209:etiology; D015203:Reproducibility of Results; D012189:Retrospective Studies; D013123:Spinal Fusion / Q000009:adverse effects / Q000295:instrumentation* / Q000379:methods*; D014057:Tomography, X-Ray Computed