title: Computed tomography-based classification of axis vertebra: choice of screw placement.
abstract: OBJECTIVE
  The purpose of the study was to: (1) introduce a new CT-based parameter: free facet area and provide its normative data; (2) standardize the method of measuring isthmus width and height of the axis vertebra; (3) propose a new grading system to predict the difficulty in inserting transarticular and C2 pedicle screws.
  
  
  METHODS
  Spiral CT scans of 47 adult dry axis vertebrae were studied. The methods of measuring isthmus width, isthmus height and free facet area are described.
  
  
  RESULTS
  The mean isthmus width was 5.04 mm on the right side and 5.42 mm on the left side. The mean isthmus height was 5.21 mm on the right side and 5.45 mm on the left side. Mean free facet area was 61.23 % on the right side and 70.18 % on the left side. A novel grading system is proposed on the basis of these three parameters. As per this grading system, 40.4 % of the sides were found to be difficult for transarticular and 24.5 % sides for C2 pedicle screw insertion (total score 2, 3, 4). A Management protocol is suggested on the basis of the grading system.
  
  
  CONCLUSIONS
  Inserting a transarticular screw was more frequently difficult as compared to pedicle screw. A new CT-based parameter (free facet area) and an efficient grading have been proposed to help surgeons choose the appropriate screw options, appreciate the complex anatomy of this region and compare data across various studies.
authors.affiliation: Department of Neurosurgery, National Institute of Mental Health and Neurosciences (NIMHANS), Hosur Road, Bangalore, 560 029, Karnataka, India, pruthi_nupur@yahoo.co.in.
authors.forename: Nupur
authors.initials: N
authors.lastname: Pruthi
authors.forename: Rose
authors.initials: R
authors.lastname: Dawn
authors.forename: Yogitha
authors.initials: Y
authors.lastname: Ravindranath
authors.forename: Tanmoy Kumar
authors.initials: TK
authors.lastname: Maiti
authors.forename: Roopa
authors.initials: R
authors.lastname: Ravindranath
authors.forename: Mariamma
authors.initials: M
authors.lastname: Philip
mesh_terms: D000328:Adult; D001368:Axis, Cervical Vertebra / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D005260:Female; D006801:Humans; D007405:Intervertebral Disc Displacement / Q000000981:diagnostic imaging / Q000601:surgery; D008297:Male; D065289:Pedicle Screws; D036542:Tomography, Spiral Computed