title: Effect of physiological loads on cortical and traditional pedicle screw fixation.
abstract: METHODS
  Human cadaveric biomechanical study.
  
  
  OBJECTIVE
  To determine the fixation strength of laterally directed, cortical pedicle screws under physiological loads.
  
  
  BACKGROUND
  Lateral trajectory cortical pedicle screws have been described as a means of obtaining improved fixation while minimizing soft-tissue dissection during lumbar instrumentation. Biomechanical data have demonstrated equivalent strength in a quasi-static model; however, no biomechanical information is available comparing the fixation of cortical with traditional pedicle screws under cyclic physiological loads.
  
  
  METHODS
  Seventeen vertebral levels (T11-L5) underwent quantitative computed tomography. On 1 side, a laterally directed, cortical pedicle screw was inserted with a traditional, medially directed pedicle screw placed on the contralateral side. With the specimen constrained in a testing apparatus, each screw underwent cyclic craniocaudal toggling under incrementally increasing physiological loads until 2 mm of head displacement occurred. Next, uniaxial pullout of each toggled screw was performed. The number of craniocaudal toggle cycles and load (N) required to achieve pedicle screw movement as well as axial pullout resistance (N) were compared between the 2 techniques.
  
  
  RESULTS
  The mean trabecular bone mineral density of the specimens was 202 K2HPO4 mg/cm. Cortical pedicle screws demonstrated significantly improved resistance to toggle testing, requiring 184 cycles to reach 2 mm of displacement compared with 102 cycles for the traditional pedicle screws (P=0.002). The force necessary to displace the screws was also significantly greater for the cortical versus the traditional screws (398 N vs. 300 N, P=0.004). There was no statistical difference in axial pullout strength between the previously toggled cortical and traditional pedicle screws (1722 N vs. 1741 N, P=0.837).
  
  
  CONCLUSIONS
  Laterally directed cortical pedicle screws have superior resistance to craniocaudal toggling compared with traditional pedicle screws.
  
  
  METHODS
  N/A.
authors.affiliation: *Department of Orthopaedic Surgery and Rehabilitation, Loyola University Medical Center, Maywood, IL †Department of Orthopaedic Surgery, Northwestern University Feinberg School of Medicine, Chicago, IL ‡Loyola University Stritch School of Medicine, Maywood, IL; and §Orthopaedic Musculoskeletal Biomechanics Laboratory, Edward Hines, Jr. VA Hospital, Hines, IL.
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Baluch
authors.forename: Alpesh A
authors.initials: AA
authors.lastname: Patel
authors.forename: Brett
authors.initials: B
authors.lastname: Lullo
authors.forename: Robert M
authors.initials: RM
authors.lastname: Havey
authors.forename: Leonard I
authors.initials: LI
authors.lastname: Voronov
authors.forename: Ngoc-Lam
authors.initials: NL
authors.lastname: Nguyen
authors.forename: Gerard
authors.initials: G
authors.lastname: Carandang
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Ghanayem
authors.forename: Avinash G
authors.initials: AG
authors.lastname: Patwardhan
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D006801:Humans; D008297:Male; D008875:Middle Aged; D011475:Prosthesis Failure; D019919:Prosthesis Implantation / Q000379:methods*; D013131:Spine / Q000000981:diagnostic imaging / Q000502:physiology / Q000601:surgery*; D013314:Stress, Mechanical; D014057:Tomography, X-Ray Computed; D016474:Weight-Bearing / Q000502:physiology*