title: Comparative Analysis of Effect of Density, Insertion Angle and Reinsertion on Pull-Out Strength of Single and Two Pedicle Screw Constructs Using Synthetic Bone Model.
abstract: METHODS
  Biomechanical study.
  
  
  OBJECTIVE
  To determine the effect of density, insertion angle and reinsertion on pull-out strength of pedicle screw in single and two screw-rod configurations.
  
  
  BACKGROUND
  Pedicle screw pull-out studies have involved single screw construct, whereas two screws and rod constructs are always used in spine fusions. Extrapolation of results using the single screw construct may lead to using expensive implants or increasing the fusion levels specifically in osteoporotic bones.
  
  
  METHODS
  Single screw and two screw pull-out strength tests were carried out according to American Society for Testing and Materials F 543-07 on foam models to test the effect of density, insertion angle and reinsertion using poly axial pedicle screws.
  
  
  RESULTS
  Bone density was the most significant factor deciding the pull-out strength in both single and two screw constructs. The difference in pull-out strength between single screw and two screw configurations in extremely osteoporotic bone model (80 kg/m(3)) was 78%, whereas in the normal bone model it was 48%. Axial pull-out value was highest for the single screw configuration; in the two screw configuration the highest pull-out strength was at 10°-15°. There was an 18% reduction in pull-out strength due to reinsertion in single screw configuration. The reinsertion effect was insignificant in the two screw configuration.
  
  
  CONCLUSIONS
  A significant difference in response of various factors on holding power of pedicle screw between single and two-screw configurations is evident. The percentage increase in pull-out strength between single and two screw constructs is higher for osteoporotic bone when compared to normal bone. Reinsertion has no significant effect on pull-out strength in the two screw rod configuration.
authors.affiliation: Spinal Disorders Surgery Unit, Department of Orthopedics, Christian Medical College, Vellore, India.
authors.forename: Venkatesh
authors.initials: V
authors.lastname: Krishnan
authors.affiliation: Department of Biotechnology, Indian Institute of Technology Madras, Chennai, India.
authors.forename: Vicky
authors.initials: V
authors.lastname: Varghese
authors.affiliation: Department of Engineering Design, Indian Institute of Technology Madras, Chennai, India.
authors.forename: Gurunathan Saravana
authors.initials: GS
authors.lastname: Kumar
keywords: Osteoporosis; Pedicle screw; Pullout test; Screw fixation; Spine fusion