title: The Effect of Contouring on Fatigue Strength of Spinal Rods: Is it Okay to Re-bend and Which Materials Are Best?
abstract: METHODS
  Small-diameter spinal rods were tested in fatigue loading before and after contouring in pedicle screw constructs using dynamic testing machines.
  
  
  OBJECTIVE
  To characterize the change in fatigue performance of spinal rods resulting from contouring.
  Spine surgeons have a variety of rod materials to choose from, and selecting the best rod depends on patient characteristics and rod material properties, including fatigue performance.
  
  
  METHODS
  Four rod materials were tested, all 4.5 mm in diameter: titanium alloy (Ti), cobalt-chromium alloy (CoCr), and 2 different grades of stainless steel (SS and ultra SS). Three conditions were tested: straight (virgin rods), bent (rods bent to a radius of curvature of 100 mm), and re-bent (rods over-bent to a radius of 50 mm, then partially straightened to a 100-mm radius). Fatigue testing was conducted on unilateral vertebrectomy constructs with polyaxial screws.
  
  
  RESULTS
  In all conditions, the endurance limit of the CoCr rods was at least 25% higher than the other materials but could not be determined because screw failure precluded rod failure. In the bent condition, the endurance limits of Ti, standard SS and ultra SS were reduced between 20% and 40%. In the re-bent condition, the endurance limit of Ti, standard SS, and ultra SS increased compared with the bent condition. Changes in fatigue performance are best explained by residual rod stresses induced during contouring.
  
  
  CONCLUSIONS
  It appears safe to over-bend and then re-bend, for 1 cycle, small-diameter spinal rods made of the materials tested in this study using tube benders, and CoCr rods were clearly superior for all conditions. However, larger rods, multiple cycles of bending and re-bending, and rods bent using other instruments such as French benders were not studied and may result in different performance under the same conditions.
authors.affiliation: DePuy Synthes Spine, 325 Paramount Drive, Raynham, MA 02767, USA. Electronic address: mslivka@its.jnj.com.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Slivka
authors.affiliation: DePuy Synthes Spine, 325 Paramount Drive, Raynham, MA 02767, USA.
authors.forename: Yung K
authors.initials: YK
authors.lastname: Fan
authors.affiliation: Center for Sports Medicine and Orthopaedics, 2415 McCallie Ave, Chattanooga, TN 37404, USA.
authors.forename: Jason C
authors.initials: JC
authors.lastname: Eck
keywords: Contouring; Fatigue; Mechanical testing; Pedicle screw construct; Spinal rod