title: Biomechanical comparison of sagittal-parallel versus non-parallel pedicle screw placement.
abstract: BACKGROUND
  While convergent placement of pedicle screws in the axial plane is known to be more advantageous biomechanically, surgeons intuitively aim toward a parallel placement of screws in the sagittal plane. It is however not clear whether parallel placement of screws in the sagittal plane is biomechanically superior to a non-parallel construct. The hypothesis of this study is that sagittal non-parallel pedicle screws do not have an inferior initial pull-out strength compared to parallel placed screws.
  
  
  METHODS
  The established lumbar calf spine model was used for determination of pull-out strength in parallel and non-parallel intersegmental pedicle screw constructs. Each of six lumbar calf spines (L1-L6) was divided into three levels: L1/L2, L3/L4 and L5/L6. Each segment was randomly instrumented with pedicle screws (6/45 mm) with either the standard technique of sagittal parallel or non-parallel screw placement, respectively, under fluoroscopic control. CT was used to verify the intrapedicular positioning of all screws. The maximum pull-out forces and type of failure were registered and compared between the groups.
  
  
  RESULTS
  The pull-out forces were 5,394 N (range 4,221 N to 8,342 N) for the sagittal non-parallel screws and 5,263 N (range 3,589 N to 7,554 N) for the sagittal-parallel screws (p = 0.838). Interlevel comparisons also showed no statistically significant differences between the groups with no relevant difference in failure mode.
  
  
  CONCLUSIONS
  Non-parallel pedicle screws in the sagittal plane have at least equal initial fixation strength compared to parallel pedicle screws in the setting of the here performed cadaveric calf spine experiments.
authors.affiliation: Orthopaedic Surgery, Balgrist University Hospital, University of Zürich, Forchstrasse 340, 8008, Zürich, Switzerland, mazda.farshad@balgrist.ch.
authors.forename: Mazda
authors.initials: M
authors.lastname: Farshad
authors.forename: Nadja A
authors.initials: NA
authors.lastname: Farshad-Amacker
authors.forename: Elias
authors.initials: E
authors.lastname: Bachmann
authors.forename: Jess G
authors.initials: JG
authors.lastname: Snedeker
authors.forename: Samuel L
authors.initials: SL
authors.lastname: Schmid
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002417:Cattle; D005471:Fluoroscopy; D008159:Lumbar Vertebrae / Q000503:physiopathology / Q000601:surgery*; D019637:Orthopedic Procedures / Q000379:methods*; D065289:Pedicle Screws; D025321:Surgery, Computer-Assisted