The Effects of Rod Bending Method and Metal Type on Fatigue Strength and Corrosion in Posterolateral Lumbar Fusion

Study DesignBasic Science Study.ObjectivesTo determine the impact of rod composition and bending method on metal debris production during fatigue testing of posterolateral lumbar fusion constructs.MethodsPosterolateral lumbar fusion constructs were embedded into Ultra-High Molecular Weight Polyethylene (UHMW-PE) blocks and subject to fatigue testing, following a modified ASTM F1717-21 protocol including cycles of compression with novel axial rotation. Variations in constructs included rod bending methods of pre-bent (PB) and surgeon-bent (SB) and rod compositions of titanium alloy (Ti) or cobalt chromium alloy (CC). Constructs were wrapped in lactated ringer solution-soaked cotton, which was dissolved and analyzed for metal particulate using inductively coupled mass spectrometry (ICP-MS).ResultsMetal debris produced by surgeon-bent cobalt chromium and pre-bent cobalt chromium constructs did not have significant differences in quantity or quality. Pre-bent cobalt chromium alloy rods produced a larger amount of chromium and cobalt metal debris than pre-bent titanium alloy rods.ConclusionsWe find that cobalt chromium alloy rods produce more metal debris than titanium alloy rods. We find no evidence that rod bending method affects metal debris quality or quantity. In considering factors that impact susceptibility to corrosion and metallosis, our data supports that rod composition, but not bending method, contributes significantly to metal debris production.