title: A biomechanical study of posterior spinal instrumentation using pedicle screws with and without cross-links.
abstract: OBJECTIVE
  It is not clear from the studies reported in the literature whether the addition of cross-links to bilateral pedicle screws increases the stiffness of the construct significantly to fix multilevel vertebral bodies. The current study addresses the question of how cross-links change the rotational stiffness of constructs with 6, 12, and 20 pedicle screws.
  
  
  METHODS
  Seven thoracolumbar 10-level spine segments (T4-L1), from individuals of average age 72 years (range 46-83 years), were instrumented with pedicle screws (6, 12, and 20) and tested in torsion with and without cross-links. As the cadaver specimens had signs of osteopenia, constructs consisting of 10 wood blocks 35 x 35 x 78 mm (weight x height x length) were also tested following an identical protocol as in the cadavers to prove the concept.
  
  
  RESULTS
  For both wood blocks and cadavers, the rotational stiffness increased linearly with number of screws with and without cross-links. Two cross-links increased rotational stiffness significantly for both 12 and 20 screws (nonsignificant for 6 screws) for wood block constructs. However, the rotational stiffness values of the cadaver constructs with or without cross-links failed to show any significant differences.
  
  
  CONCLUSIONS
  This work has raised the need for developing a suitable animal model for the study of multilevel vertebral bodies to simulate the fixation of scoliosis corrections.
authors: Department of Orthopaedic Surgery, Louisiana State University Health Sciences Center, Shreveport, LA, USA.
authors: M Umar
authors: MU
authors: Burney
authors: Debi P
authors: DP
authors: Mukherjee
authors: Alan L
authors: AL
authors: Ogden
authors: Errin
authors: E
authors: Robinson
authors: Richard E
authors: RE
authors: McCall
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D009984:Orthopedic Fixation Devices; D012399:Rotation; D013131:Spine / Q000503:physiopathology* / Q000601:surgery*; D014102:Torsion Abnormality