title: Primary stability of pedicle screws depends on the screw positioning and alignment.
abstract: BACKGROUND
  There is no universal consensus regarding the biomechanical aspects and relevance on the primary stability of misplaced pedicle screws.
  
  
  OBJECTIVE
  The study is aimed to the determination of the correlation between axial pullout forces of pedicle screws with the possible screw misplacement, including mild and severe cortical violations.
  
  
  METHODS
  Eighty-eight monoaxial pedicle screws were implanted into 44 porcine lumbar vertebral bodies, paying attention on trying to obtain a wide range of placement accuracy. After screw implantation, all specimens underwent a spiral computed tomography scan, and the screw placements were graded following the scales of Laine et al. and Abul Kasim et al. Axial pullout tests were then performed on a servohydraulic material testing system.
  
  
  RESULTS
  Decreasing pullout forces were determined for screws implanted with increasing cortical violation. A smaller influence of cortical violations in the medial direction with respect to the lateral direction was observed. Screws implanted with a large cortical violation and misplacement in the craniocaudal direction were found to be significantly less stable than screws having comparable cortical violation but in a centered sagittal position.
  
  
  CONCLUSIONS
  These results provide adjunctive criteria to evaluate more accurately the fate of a spine instrumentation. Particular care should be placed in the screw evaluation regarding the craniocaudal positioning and alignment.
authors.affiliation: Department of Neurosurgery, Humanitas Clinical and Cancer Research Center, Via Manzoni 56, 20089 Rozzano (MI), Italy. Electronic address: f.costa@fastwebmail.it.
authors.forename: Francesco
authors.initials: F
authors.lastname: Costa
authors.forename: Tomaso
authors.initials: T
authors.lastname: Villa
authors.forename: Federica
authors.initials: F
authors.lastname: Anasetti
authors.forename: Massimo
authors.initials: M
authors.lastname: Tomei
authors.forename: Alessandro
authors.initials: A
authors.lastname: Ortolina
authors.forename: Andrea
authors.initials: A
authors.lastname: Cardia
authors.forename: Luigi
authors.initials: L
authors.lastname: La Barbera
authors.forename: Maurizio
authors.initials: M
authors.lastname: Fornari
authors.forename: Fabio
authors.initials: F
authors.lastname: Galbusera
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000009:adverse effects*; D008159:Lumbar Vertebrae / Q000601:surgery*; D013123:Spinal Fusion / Q000009:adverse effects* / Q000295:instrumentation*; D034421:Sus scrofa
keywords: Pedicle screw; Porcine; Pullout; Screw misplacement; Stability