title: The Mechanical Effect of Rod Contouring on Rod-Screw System Strength in Spine Fixation.
abstract: OBJECTIVE
  Rod-screw fixation systems are widely used for spinal instrumentation. Although many biomechanical studies on rod-screw systems have been carried out, but the effects of rod contouring on the construct strength is still not very well defined in the literature. This work examines the mechanical impact of straight, 20° kyphotic, and 20° lordotic rod contouring on rod-screw fixation systems, by forming a corpectomy model.
  
  
  METHODS
  The corpectomy groups were prepared using ultra-high molecular weight polyethylene samples. Non-destructive loads were applied during flexion/extension and torsion testing. Spine-loading conditions were simulated by load subjections of 100 N with a velocity of 5 mm min(-1), to ensure 8.4-Nm moment. For torsional loading, the corpectomy models were subjected to rotational displacement of 0.5° s(-1) to an end point of 5.0°, in a torsion testing machine.
  
  
  RESULTS
  Under both flexion and extension loading conditions the stiffness values for the lordotic rod-screw system were the highest. Under torsional loading conditions, the lordotic rod-screw system exhibited the highest torsional rigidity.
  
  
  CONCLUSIONS
  We concluded that the lordotic rod-screw system was the most rigid among the systems tested and the risk of rod and screw failure is much higher in the kyphotic rod-screw systems. Further biomechanical studies should be attempted to compare between different rod kyphotic angles to minimize the kyphotic rod failure rate and to offer a more stable and rigid rod-screw construct models for surgical application in the kyphotic vertebrae.
authors.affiliation: Department of Orthopaedics and Traumatology, Ilyas Cokay Catalca Hospital, Catalca, Istanbul, Turkey.
authors.forename: Nihat
authors.initials: N
authors.lastname: Acar
authors.affiliation: Department of Orthopaedics and Traumatology, Faculty of Medicine, Dokuz Eylul University, Balcova, Izmir, Turkey.
authors.forename: Ahmet
authors.initials: A
authors.lastname: Karakasli
authors.affiliation: Department of Orthopedics and Traumatology, Sifa Hospital, Konak, Izmir, Turkey.
authors.forename: Ahmet A
authors.initials: AA
authors.lastname: Karaarslan
authors.affiliation: Department of Computer Engineering, Faculty of Engineering, Dokuz Eylul University, Tinaztepe, Izmir, Turkey.
authors.forename: Mehmet Hilal
authors.initials: MH
authors.lastname: Ozcanhan
authors.affiliation: Department of Biomechanics, Health Science Institute, Dokuz Eylul University, Balcova, Izmir, Turkey.
authors.forename: Fatih
authors.initials: F
authors.lastname: Ertem
authors.affiliation: Department of Orthopaedics and Traumatology, Faculty of Medicine, Dokuz Eylul University, Balcova, Izmir, Turkey.
authors.forename: Mehmet
authors.initials: M
authors.lastname: Erduran
keywords: Biomechanics; Rod contouring; Rod failure