title: Computed tomography-based determination of a safe trajectory for placement of transarticular facet screws in the subaxial cervical spine.
abstract: OBJECTIVE
  Placement of transarticular facet screws is one option for stabilization of the subaxial cervical spine. Small clinical series and biomechanical data support their role as a substitute for other posterior stabilization techniques; however, the application of transarticular facet screws in the subaxial cervical spine has not been widely adopted, possibly because of surgeon unfamiliarity with the trajectory. In this study, the authors' objective is to define insertion points and angles of safe trajectory for transarticular facet screw placement in the subaxial cervical spine.
  
  
  METHODS
  Thirty fine-cut CT scans of cervical spines were reconstructed in the multiplanar mode and evaluated for safe transarticular screw placement in the subaxial cervical spine (C2-3, C3-4, C4-5, C5-6, C6-7). As in placement of lateral mass screws, the vertebral artery and exiting nerve root were bypassed posterolaterally. The entry point was set 1 mm medial and 1 mm caudal to the center of the lateral mass. From this entry point, the sagittal angulation was set to traverse the facet joint plane approximately perpendicularly. For the axial angulation, the exit point was set posterolaterally to the transverse process. After ideal insertion angles and screw lengths were identified, the trajectory was simulated on CT scans of 20 different cervical spines to confirm safe screw placement.
  
  
  RESULTS
  The mean optimal mediolateral insertion angles (± SD) were as follows: 23° ± 5° at C2-3; 24° ± 4° at C3-4; 25° ± 5° at C4-5; 25° ± 4° at C5-6; and 33° ± 6° at C6-7. The mean sagittal insertion angles measured to the sagittal projection of the facet joint space were as follows: 77° ± 10° at C2-3; 77° ± 10° at C3-4; 80° ± 11° at C4-5; 81°± 8° at C5-6; and 100° ± 11° at C6-7. The mean trajectory lengths were 15 ± 2 mm at C2-3; 14 ± 1 mm at C3-4; 15 ± 1 mm at C4-5; 16 ± 2 mm at C5-6; and 23 ± 4 mm at C6-7. Simulation of these insertion angles on 20 different cervical spine CTs yielded a safe trajectory in 85%-95% of spines for C2-3, C3-4, C4-5, C5-6, and C6-7.
  
  
  CONCLUSIONS
  The calculated optimal insertion angles and lengths for each level may guide the safe placement of subaxial cervical transfacet screws.
authors.affiliation: Department of Neurosurgery, Clinical Neurosciences Center, University of Utah, Salt Lake City, UT, USA.
authors.forename: Gregory F
authors.initials: GF
authors.lastname: Jost
authors.forename: Erica F
authors.initials: EF
authors.lastname: Bisson
authors.forename: Meic H
authors.initials: MH
authors.lastname: Schmidt
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001863:Bone Screws; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000293:injuries / Q000601:surgery*; D005260:Female; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008297:Male; D008875:Middle Aged; D013122:Spinal Diseases / Q000000981:diagnostic imaging / Q000601:surgery*; D013123:Spinal Fusion / Q000379:methods*; D013124:Spinal Injuries / Q000601:surgery*; D014057:Tomography, X-Ray Computed / Q000379:methods*; D055815:Young Adult