title: Biomechanical pullout strength and stability of the cervical artificial pedicle screw.
abstract: METHODS
  This study used cadaveric specimens to compare the biomechanical performance of artificial pedicle screws (APS) versus lateral mass screws (LMS).
  
  
  OBJECTIVE
  The goal of this study was to biomechanically characterize APS range-of-motion and pullout strength in surgical instances that preclude LMS insertion.
  
  
  BACKGROUND
  Posterior approaches used in instances of ventral spine tumors often necessitate complete facetectomy, thereby removing fixation points for LMS and requiring longer constructs with fewer segmental fixations to span the resected levels. Recently, APS were developed to overcome this obstacle. Although APS have been used successfully in clinical cases, they have yet to be biomechanically validated.
  
  
  METHODS
  Seven fresh-frozen cervical spine segments (C2-C7) were harvested from human donors (F = 1, M = 6; 65 +/- 5 years old, range: 50-72 years old). Nondestructive range-of-motion tests were conducted on each specimen in its intact and surgically destabilized states, and after each of 3 different APS and LMS surgical stabilizations. After nondestructive bending tests, a final pullout test of APS and LMS at the C4 level was performed for each specimen.
  
  
  RESULTS
  The pullout strength of the APS was twice as strong on average as that for the LMS (503.4 +/- 338.3 vs.- 254.3 +/- 142.3 N); this difference approached but was not statistically significant (P = 0.07). There was no significant difference in specimen stiffness between the APS- and LMS-instrumented configurations in all ranges of primary and off-axis motions (P > 0.05). However, all fixation methods increased specimen stiffness in comparison with the intact conditions (1.7-36.5 times increase; P < 0.05).
  
  
  CONCLUSIONS
  Our results demonstrate that APS provide comparable stability to LMS and can therefore be considered a viable alternative in surgical scenarios requiring the complete removal of lateral masses. Moreover, APS may provide some enhanced strength in the face of destructive pullout forces.
authors.affiliation: Columbia University College of Physicians and Surgeons, 630 West 168th Street, New York, NY 10032, USA. ahb2113@columbia.edu
authors.forename: Alison H
authors.initials: AH
authors.lastname: Barnes
authors.forename: Johnny A
authors.initials: JA
authors.lastname: Eguizabal
authors.forename: Frank L
authors.initials: FL
authors.lastname: Acosta
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Lotz
authors.forename: Jenni M
authors.initials: JM
authors.lastname: Buckley
authors.forename: Christopher P
authors.initials: CP
authors.lastname: Ames
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002574:Cervical Vertebrae / Q000601:surgery*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation*