Effect of Low Plasticity Burnishing On the Fatigue Strength of Spinal Rods

Various types of spinal devices are fatigue tested in the laboratory to document their fatigue loading behavior in order to satisfy minimum implant design criteria. The ASTM test methods include protocols for the fatigue testing of spinal constructs, components, and interconnections or sub-assemblies. A research project utilizing a cantilever fatigue testing procedure was undertaken to determine the effect of low plasticity burnishing (LPB) on the fatigue strength of lumbar spinal rods which contained flexible segments.

Semi-finished lumbar rods were machined from implant quality TAV ELI round bar and subjected to LPB at Lambda Research. The LPB process includes a spherical socket to support a smooth free-rolling ball that is pressed and rolled along the surface which deforms the workpiece surface into a state of compression. A CNC positioning tool is moved in a controlled pattern to obtain maximum compression with minimum cold working. LPB rods and non-LPB control rods machined from the same lot of raw material were fabricated into semi-rigid lumbar spine rods, nitric acid passivated, and laser etched to meet prevailing product drawings.

Fully reversed cantilever fatigue testing at 5 Hz was performed in DI water at 37 degrees C. Special fixtures were built to ensure that a controlled offset was maintained when the sample was fixtured between the rigid holder and the cantilever headpiece. The staircase method was used by starting at a pre-determined load, decreasing the previous load by 20% if failure occurred, or testing 10% above the previous load if runout was achieved. This testing strategy established comparative fatigue curves for the LPB and control rods. Runout was defined as 5 million cycles. Loads versus number of fatigue cycles were plotted and ANOVA determined statistical significance between the two sets of data. Fracture surfaces were examined to compare fatigue fracture morphologies.