title: Safe zone for C1 lateral mass screws: anatomic and radiological study.
abstract: OBJECTIVE
  To evaluate the possible complications of overpenetrated C1 lateral mass screws and to identify and define a "safe zone" area anterior to the C1 vertebra.
  
  
  METHODS
  The study was performed on 10 cadavers and 50 random patients who had undergone computed tomographic scanning with contrast medium of the neck for other purposes. Atlas lateral mass screw trajectories were plotted, and the safe zone for screw placement anterior to the atlas vertebra was determined for each trajectory.
  
  
  RESULTS
  The trajectory of the internal carotid artery was measured from its medial wall. The trajectory of the internal carotid artery according to the ideal entrance point of the screw was 11.55 +/- 4.55 degrees (range, 2-22 degrees) in the cadavers and 9.78 +/- 4.55 degrees (range, -5 to 22 degrees) bilaterally in the patients. At 15 degrees (ideal screw trajectory), the thickness of the rectus capitis anterior muscle and longus capitis muscle was 6.69 +/- 0.83 mm (range, 5.32-7.92 mm) in the cadavers and 7.29 +/- 1.90 mm (range, 0.50-13.63 mm) bilaterally in the patients. The smallest distance from the internal carotid artery to the anterior cortex of the C1 vertebra was calculated as 4.33 +/- 2.03 mm (range, 1.15-8.40 mm) bilaterally in the cadavers and 5.07 +/- 1.72 mm (range, 2.15-8.91 mm) bilaterally in radiological specimens.
  
  
  CONCLUSIONS
  The internal carotid artery trajectory is lateral to the ideal entrance point of C1 lateral mass screws. The medial angulation of a screw placed in the lateral mass of C1 seemed to increase the margin of safety for the internal carotid artery. The rectus capitis anterior and longus capitis muscles may be thought of as a safe zone area for C1 lateral mass screws. At more than 25 degrees of medial angulation, the risk of perforation of the oropharyngeal wall increases.
authors.affiliation: 2nd Neurosurgery Clinic, Ankara Diskapi Training and Research Hospital, Ministry of Health, Ankara, Turkey.
authors.forename: Serkan
authors.initials: S
authors.lastname: Simsek
authors.forename: Kazim
authors.initials: K
authors.lastname: Yigitkanli
authors.forename: Ulku C
authors.initials: UC
authors.lastname: Turba
authors.forename: Ayhan
authors.initials: A
authors.lastname: Comert
authors.forename: Hakan
authors.initials: H
authors.lastname: Seçkin
authors.forename: Ibrahim
authors.initials: I
authors.lastname: Tekdemir
authors.forename: Alaittin
authors.initials: A
authors.lastname: Elhan
mesh_terms: D000368:Aged; D001268:Atlanto-Axial Joint / Q000000981:diagnostic imaging* / Q000601:surgery*; D001863:Bone Screws / Q000009:adverse effects; D002102:Cadaver; D002343:Carotid Artery, Internal / Q000601:surgery*; D002540:Cerebral Cortex / Q000601:surgery; D002574:Cervical Vertebrae / Q000601:surgery; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009334:Neck Muscles / Q000473:pathology / Q000601:surgery; D013123:Spinal Fusion / Q000009:adverse effects / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods