title: Appropriate size and angulation for axis screw placement.
abstract: BACKGROUND
  A screw larger than 4.0-4.5-mm-diameter screw has now become the preferred sizefor providing maximum anchorage during atlantoaxial segmental fixation and transarticular screw fixation. At present, there are no studies available of Axis (C2) morphology related to screw placement specifically in Thai patients, a situation that might result in surgical complications.
  
  
  OBJECTIVE
  The purpose ofthis study was to determine the typical width, height and angulations in both C2 pars interarticularis and C2 pedicle in Thai population.
  
  
  METHODS
  A radiographic-based study was conducted in 54 Thai patients aged over 20 from July 2011 to January 2012 in Chiang Mai University Hospital. C2 parameters including the height, width, medial angulation and superior angulation of the pars interarticularis and the pedicle were measured by using a CT scan. Allparameters were measured using the ONIS 2.3 program.
  
  
  RESULTS
  The C2 pedicle in Thais was found to have a mean width of 5.47 mm (range 3.28-6.81 mm), a height 7.54 mm (5.9-9.54), a superior angulation of 27.54° (range 20. 65°-33.95°), and a medial angulation of 38.95° (range 28.07°-52.85°). C2 Pars interarticularis had a mean width of 7.72 mm (range 5.93-10.61 mm), a height of 4.47 mm (range 2.33-6.3 mm), a superior angulation of 49.85° (range 41.89°-58.65°), and a medial angulation of 6.76° (range 1.18°-13.5°).
  
  
  CONCLUSIONS
  Because the mean height of pars interarticularis is 4.47 mm, atlantoaxial arthrodesis should not use 4.5-mm-diameter screw, especially transarticular screws, due to the possibility of vascular and nerve damage.
authors.forename: Torphong
authors.initials: T
authors.lastname: Bunmaprasert
authors.forename: Narote
authors.initials: N
authors.lastname: Treenarong
authors.forename: Aekkarith
authors.initials: A
authors.lastname: Khamkhad
mesh_terms: D000328:Adult; D044466:Asian People; D001268:Atlanto-Axial Joint / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000601:surgery; D001368:Axis, Cervical Vertebra / Q000033:anatomy & histology* / Q000000981:diagnostic imaging / Q000601:surgery; D001863:Bone Screws; D005260:Female; D006801:Humans; D008297:Male; D012016:Reference Values; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D013785:Thailand; D014057:Tomography, X-Ray Computed; D055815:Young Adult