title: Torsional stability of cross-link configurations: a biomechanical analysis.
abstract: BACKGROUND
  Cross-link systems have been used to augment segmental spinal instrumentation since the earliest introduction of these fixation systems. Although transverse cross-links have little impact on sagittal motion of spinal constructs, cross-linkage does affect torsional rigidity. Despite the wide variety of cross-link designs, almost all have been configured as transverse devices. The relative mechanical benefit of different cross-link configurations is not known.
  
  
  OBJECTIVE
  The purpose of this study was to compare the torsional stability of three different cross-link configurations.
  
  
  METHODS
  Biomechanical analysis of segmental instrumentation constructs using porcine spines.
  
  
  METHODS
  Thoracic porcine spines (T4 to T10) were instrumented with 6.5-mm conical pedicle screws and 7.0-mm connecting rods from T5 to T9. Terminal vertebrae were embedded in polymethylmethacrylate (PMMA) after a T7 corpectomy. Four cross-link configurations were tested in a randomized manner: Un-cross-linked Control (CONT); Transverse Rod-Rod (RR); Transverse Screw-Screw (SS); and Diagonal Screw-Screw (DX) Cross-links. The specimens were rotated to 3 Nm at a rate of 0.2 degrees/s and cycled six times with data acquisition over the final two cycles. Stiffness, rotation, and energy data were normalized to each control. A Newman-Keuls repeated measures analysis of variance was used to infer significant differences.
  
  
  RESULTS
  Diagonal cross-link configurations provided the most rigid construct. Transverse cross-links did not significantly change torsional behavior compared with the unlinked control. Rotation and energy expended were not significantly greater torsional stiffness compared with other constructs tested (p<.01).
  
  
  CONCLUSIONS
  The diagonal cross-link configuration provided increased torsional stiffness as compared with unlinked or transverse configurations. This observation should be considered in future cross-link designs.
authors.affiliation: Department of Biomedical Engineering, The Cleveland Clinic Foundation, OH 44195, USA.
authors.forename: Antonio
authors.initials: A
authors.lastname: Valdevit
authors.forename: Helen E
authors.initials: HE
authors.lastname: Kambic
authors.forename: Robert F
authors.initials: RF
authors.lastname: McLain
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D066298:In Vitro Techniques; D009984:Orthopedic Fixation Devices; D013123:Spinal Fusion / Q000379:methods*; D013552:Swine; D013904:Thoracic Vertebrae / Q000601:surgery*