title: Biomechanics of C-7 transfacet screw fixation.
abstract: OBJECTIVE
  The small diameter of the pedicle can make C-7 pedicle screw insertion dangerous. Although transfacet screws have been studied biomechanically when used in pinning joints, they have not been well studied when used as part of a C7-T1 screw/rod construct. The authors therefore compared C7-T1 fixation using a C-7 transfacet screw/T-1 pedicle screw construct with a construct composed of pedicle screws at both levels.
  
  
  METHODS
  Each rigid posterior screw/rod construct was placed in 7 human cadaveric C6-T2 specimens (14 total). Specimens were tested in normal condition, after 2-column instability, and once fixated. Nondestructive, nonconstraining pure moments (maximum 1.5 Nm) were applied to induce flexion, extension, lateral bending, and axial rotation while recording 3D motion optoelectronically. The entire construct was then loaded to failure by dorsal linear force.
  
  
  RESULTS
  There was no significant difference in angular range of motion between the 2 instrumented groups during any loading mode (p > 0.11, nonpaired t-tests). Both constructs reduced motion to < 2 degrees in any direction and allowed significantly less motion than in the normal condition. The C-7 facet screw/T-1 pedicle screw construct allowed a small but significantly greater lax zone than the pedicle screw/rod construct during lateral bending, and it failed under significantly less load than the pedicle screw/rod construct (p < 0.001).
  
  
  CONCLUSIONS
  When C-7 transfacet screws are connected to T-1 pedicle screws, they provide equivalent stability of constructs formed by pedicle screws at both levels. Although less resistant to failure, the transfacet screw construct should be a viable alternative in patients with healthy bone.
authors.affiliation: Department of Neurological Surgery, Indiana University College of Medicine, Indianapolis, Indiana, USA.
authors.forename: Eric M
authors.initials: EM
authors.lastname: Horn
authors.forename: Phillip M
authors.initials: PM
authors.lastname: Reyes
authors.forename: Seungwon
authors.initials: S
authors.lastname: Baek
authors.forename: Mehmet
authors.initials: M
authors.lastname: Senoglu
authors.forename: Nicholas
authors.initials: N
authors.lastname: Theodore
authors.forename: Volker K H
authors.initials: VK
authors.lastname: Sonntag
authors.forename: Neil R
authors.initials: NR
authors.lastname: Crawford
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena / Q000502:physiology; D001863:Bone Screws; D002102:Cadaver; D002574:Cervical Vertebrae / Q000601:surgery*; D019544:Equipment Failure Analysis; D005260:Female; D005592:Fracture Fixation; D006801:Humans; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology; D021801:Zygapophyseal Joint / Q000503:physiopathology* / Q000601:surgery*