title: A safe screw trajectory for atlantoaxial transarticular fixation achieved using an aiming device.
abstract: METHODS
  A retrospective evaluation and characterization of the trajectory of atlantoaxial transarticular screws inserted using an aiming device.
  
  
  OBJECTIVE
  To confirm that the screws were inserted through the safest trajectory, which is through the most dorsal and medial part of the isthmus of C2, and to characterize the trajectory on lateral radiograms by comparison with historical controls.
  
  
  BACKGROUND
  Posterior atlantoaxial transarticular screw fixation entails the potential risk of vertebral artery (VA) injury, which may be lethal. Although much literature recommends that the screws should be inserted aiming at the anterior arch of C1, the authors considered that the safest screw path is via the most dorsal and medial part of the isthmus regardless of the C1 anterior arch, and have used an original aiming device to achieve this trajectory.
  
  
  METHODS
  Forty-three consecutive patients who submitted to atlantoaxial transarticular screw fixation using the aiming device were evaluated for screw position using computed tomography (CT) and lateral radiogram. The medialization index (the distance between the screw and the cortex of the spinal canal of C2 on axial CT) and the dorsalization index (the thickness of the bone remaining dorsal to the screw at the isthmus of C2 on sagittal reconstruction CT) were measured. Further, three parameters on the lateral radiograms of these patients were compared with those in the literature and those of our previous cases performed without the aiming device.
  
  
  RESULTS
  Neither VA injury nor violation of the spinal canal was encountered, although 12 high-riding VAs were included in this series. The mean medialization index was 0.21 mm, and the indexes of 86.3% of the screws were zero. The mean dorsalization index was 0.36 mm, and the indexes of 76.8% of the screws were zero. These results demonstrated that most of the screws were inserted as aimed, proving the usefulness of the aiming device. The trajectory of these screws on lateral radiograms was characterized by significantly less bone thickness dorsal to the screw at the isthmus compared with the two control groups. As a result, more screws were pointed above the anterior arch of C1.
  
  
  CONCLUSIONS
  The atlantoaxial transarticular screw was inserted safely as aimed by using the aiming device. The trajectory was characterized by less bone thickness dorsal to the screw on lateral radiogram, which should be a new intraoperative landmark for screw insertion, in place of the anterior arch of C1.
authors.affiliation: Department of Orthopaedic Surgery, Kyoto University Graduate School of Medicine, Kyoto, Japan. neo@kuhp.kyoto-u.ac.jp
authors.forename: Masashi
authors.initials: M
authors.lastname: Neo
authors.forename: Takeshi
authors.initials: T
authors.lastname: Sakamoto
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Fujibayashi
authors.forename: Takashi
authors.initials: T
authors.lastname: Nakamura
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001268:Atlanto-Axial Joint / Q000000981:diagnostic imaging / Q000601:surgery*; D001863:Bone Screws; D005260:Female; D006801:Humans; D016268:Internal Fixators; D007593:Joint Instability / Q000601:surgery; D008297:Male; D008875:Middle Aged; D009983:Orthopedic Equipment; D010003:Osteoarthritis / Q000601:surgery; D011183:Postoperative Complications / Q000517:prevention & control*; D012189:Retrospective Studies; D013123:Spinal Fusion / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods*; D014057:Tomography, X-Ray Computed; D014711:Vertebral Artery / Q000033:anatomy & histology / Q000000981:diagnostic imaging