Primary stability of pedicle screws depends on the screw positioning and alignment

Spine J. 2013 Dec;13(12):1934-9. doi: 10.1016/j.spinee.2013.03.046. Epub 2013 May 14.


Background context: There is no universal consensus regarding the biomechanical aspects and relevance on the primary stability of misplaced pedicle screws.

Purpose: 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.

Keywords: Pedicle screw; Porcine; Pullout; Screw misplacement; Stability.

MeSH terms

  • Animals
  • Biomechanical Phenomena
  • Bone Screws / adverse effects*
  • Lumbar Vertebrae / surgery*
  • Spinal Fusion / adverse effects*
  • Spinal Fusion / instrumentation*
  • Sus scrofa