title: Relationship between screw trajectory of C1 lateral mass screw and internal carotid artery.
abstract: METHODS
  Evaluation of diagnostic imaging.
  
  
  OBJECTIVE
  To comprehend anatomic relationships between the internal carotid artery (ICA) and bicortical purchase of C1 lateral mass screws from the perspective of avoiding ICA injury.
  
  
  BACKGROUND
  No studies have evaluated safety trajectory of atlantal lateral mass screw that would avoid the ICA injury in relation to its location, although previous studies have indicated concern about ICA injury by the screw tip at the anterior surface of the lateral mass of the atlas.
  
  
  METHODS
  From 149 of 177 human 3-dimensional computed tomography reconstruction images, 6 distance and 2 angle parameters related to both atlas and ICA were measured on the plane 15 degrees cephalad to the transverse plane. In addition, angle of error during screw insertion from intended trajectory was checked.
  
  
  RESULTS
  The ICA was located in front of the C1 lateral mass in 64.4% of cases and faced the lateral one third of the C1 lateral mass in 54.6% of cases. None were located in front of the medial one third of the C1 lateral mass. The maximum inward screw trajectory that would violate the ICA was 8.6 degrees . Mean angle of preoperative C1/2 rotation and angle of error from intended trajectory was about 5 degrees , respectively.
  
  
  CONCLUSIONS
  The possibility of ICA injury can be excluded by correct insertion of the screw 10 degrees inward. Although bicortical purchase with adequately medially angulated trajectory might be safe enough, we must remember the possibility to violate the ICA in bicortical purchase, because the intended screw trajectory never be assured.
authors.affiliation: Department of Orthopaedic Surgery, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan.
authors.forename: Satona
authors.initials: S
authors.lastname: Murakami
authors.forename: Jun
authors.initials: J
authors.lastname: Mizutani
authors.forename: Muneyoshi
authors.initials: M
authors.lastname: Fukuoka
authors.forename: Kenji
authors.initials: K
authors.lastname: Kato
authors.forename: Isato
authors.initials: I
authors.lastname: Sekiya
authors.forename: Hideki
authors.initials: H
authors.lastname: Okamoto
authors.forename: Kuniyoshi
authors.initials: K
authors.lastname: Abumi
authors.forename: Takanobu
authors.initials: T
authors.lastname: Otsuka
mesh_terms: D000368:Aged; D001268:Atlanto-Axial Joint / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D001863:Bone Screws; D020212:Carotid Artery Injuries / Q000000981:diagnostic imaging / Q000209:etiology / Q000517:prevention & control*; D002343:Carotid Artery, Internal / Q000000981:diagnostic imaging*; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011857:Radiographic Image Interpretation, Computer-Assisted; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000009:adverse effects / Q000295:instrumentation*; D014057:Tomography, X-Ray Computed