Comparative Mechanical Properties of Commonly Used Spinal Rods

Abstract

Introduction Pedicle-screw-rod constructs are used for a wide range of applications with strongly varying demands on construct stiffness. Rod stiffness is an important factor determining construct performance. A wide range of rod diameters and materials is available from manufacturers. No catalog has so far been published that would permit surgeons to objectively compare rods based on their specific properties. The goal of our study was to independently and objectively compare the biomechanical properties of spinal rods. Material and Methods Testing was according to ASTM norm F2193. We measured the bending stiffness, bending ultimate moment, bending yield moment, and elastic recoil. Over 200 tests were performed on 63 rods. Results We found decreasing material stiffness from cobalt–chrome (CoCr) and stainless steel (SS) via titanium (Ti) to PEEK. Surprising differences of up to 10% were found between Ti rods from different manufacturers. Differences between materials were most pronounced with small diameters: a 4.5 mm CoCr rod has about twice the stiffness of a 4.5 mm Ti rod, but the difference is only 15% with 6.0 mm rods. CoCr rods provided the highest elastic modulus with 4.5 mm diameter CoCr rods being as stiff as 5.5 mm diameter Ti rods and 5.5 mm CoCr rods being stiffer than 6.35 mm Ti rods. The stiffness of PEEK rods was only 4% that of Ti rods of the same diameter with the exception of carbon fiber-reinforced PEEK, which was close to titanium. Conclusion Our results show surprising variability between the popular 5.5 mm Ti rods from different manufacturers. With smaller diameters, the choice of material is increasingly important and may sometimes allow for using smaller implants while maintaining construct stiffness. Ultimate moment and yield moment determine the load at which a construct can be expected to plastically deform in vivo and what amount of force is required to contour the rod before implantation. The elastic recoil after plastic deformation makes exact contouring more difficult, which makes certain rods less comfortable to use in certain applications. We found considerable differences in the diameters/materials tested for all these parameters.