Variation in Mechanical Properties of 5.5 mm Cobalt Chromium and Titanium Spinal Rods Across Suppliers

Cobalt chromium (CoCr) and titanium (Ti) alloys are popular spinal rod choices due to biocompatibility, less imaging artifact than stainless steel, and greater corrosion resistance. This study assesses the mechanical behavior in bending of commercially available 5.5 mm diameter CoCr and Ti spinal rods in a four-point bending configuration. For each supplier, rod segments were loaded to 20 mm of deflection with continuous load and displacement recording. Rod mechanical properties were calculated. Suppliers were compared by ANOVA, with post hoc pairwise comparisons. For CoCr rods, flexural modulus and flexural stiffness varied across suppliers with maximum differences of 18.0 GPa and 47.0 N/mm, respectively, representing 11% and 15% variation. Range for CoCr rod yield load was 780-1004 N (29% variation), yield displacement was 2.3-2.8 mm, elastic displacement was 2.2-2.7 mm, and maximum load was 1791-2181 N, representing 22% variation. For Ti rods, flexural modulus and flexural stiffness varied across suppliers with maximum differences of 7.3 GPa and 24.3 N/mm, respectively, representing 9% and 14% variation. Range for Ti rod yield load was 1054-1207 N (14% variation), yield displacement was 5.9-7.0 mm, elastic displacement was 5.8-6.9 mm, and maximum load was 1515-1678 N, representing 11% variation. There is substantial variation in mechanical behavior in bending of CoCr and Ti rods (5.5 mm diameter) amongst suppliers, suggesting that they are not mechanically equivalent. Surgeons must ensure that the mechanical properties of a particular rod meet the requirements of the surgical correction required.