title: Pullout analysis of a lumbar plate with varying screw orientations: experimental and computational analyses.
abstract: METHODS
  Experimental and finite element analysis of anterior lumbar interbody fixation (ALIF) plate pullout.
  
  
  OBJECTIVE
  The objective of this study was to determine the effect of screw angle and orientation on ALIF plate pullout strength.
  
  
  BACKGROUND
  It has been thought that angling the screws in an ALIF plate leads to better fixation strength; however, a few studies are published on this question, which produced conflicting results.
  
  
  METHODS
  Using custom guides, screws were configured in 9 different orientations to affix ALIF plates to polyurethane foam blocks. Pullout tests were performed at a rate of 1 mm/min. In addition, finite element analyses were performed on a 2-dimensional screw-block model to gain insight into the internal stress during pullout.
  
  
  RESULTS
  The pullout load was the greatest, with screws positioned 12° outward sagittaly and 6° inward coronally (936 ± 72 N). This orientation was statistically greater than the orientation with the lowest pullout load (812 ± 45 N, P < 0.05); however, no group was statistically different than placing the screws straight in (868 ± 86 N, P > 0.05). Finite elements analysis showed some gain in pullout strength at 12° followed by some loss at greater angles. As the screw insertion angle increased, stress levels elevated within the block even in the regions away from the screw.
  
  
  CONCLUSIONS
  Significant difference was found between certain screw-angle configurations; however, when compared with simply placing the screws straight in, the difference was never more than 8%. This implies that there is greater freedom in the angle and placement of screws than previously thought. Our results show that there is little change in fixation strength when placing the screw in a different direction.
authors.affiliation: Department of Orthopaedic Surgery, Orthopaedic Biomechanics Laboratory, Loma Linda University, Loma Linda, CA 92354, USA.
authors.forename: David
authors.initials: D
authors.lastname: Rios
authors.forename: Wyzscx Merfil
authors.initials: WM
authors.lastname: Patacxil
authors.forename: Daniel Kyle
authors.initials: DK
authors.lastname: Palmer
authors.forename: Paul A
authors.initials: PA
authors.lastname: Williams
authors.forename: Wayne K
authors.initials: WK
authors.lastname: Cheng
authors.forename: Serkan
authors.initials: S
authors.lastname: Inceoğlu
mesh_terms: D001696:Biomechanical Phenomena; D001860:Bone Plates; D001863:Bone Screws; D003198:Computer Simulation; D055119:Elastic Modulus; D020342:Finite Element Analysis; D017276:Friction; D008159:Lumbar Vertebrae / Q000601:surgery*; D008422:Materials Testing; D008962:Models, Theoretical; D011140:Polyurethanes; D011474:Prosthesis Design; D012399:Rotation; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical
chemical_list: D011140:Polyurethanes; C028279:polyurethane foam