Influence of Rod Contouring on Rod Strength and Stiffness in Spine Surgery

Abstract

For spinal fusion or the correction of spine deformity at multiple levels, intraoperative rod contouring is required to realign the spine. A French bender is the most common contouring tool used. There are several reports on the mechanical properties of various rods with manufactured straight rod; however, few reports describe the changes in a rod's mechanical properties after rod contouring. The authors investigated the influences of rod contouring on rod strength and stiffness. A 3-point bending test was conducted. Each 18-cm rod was loaded at a rate of 10 mm/min with a load applicator. Three different rod diameters (5.5, 6.0, and 6.35 mm) and 2 types of materials (titanium [Ti] alloy and cobalt-chromium [CoCr] alloy) were assessed. Different rod curvatures were evaluated: (1) a no-preparation rod of 0° (control); (2) a 0° rod bent at one point to make tangential angles of 10° and then bent back from the opposite side; (3) a bent rod with tangential angles of 20°; and (4) a 40° bent rod. The yield strength in all types of rod materials and diameter decreased after rod contouring using a French bender. The extent of decrease depended on the degree of bend. The bending stiffness of each rod also decreased. The CoCr rod showed higher bending stiffness than the Ti rod of the same diameter. Rod contouring using a French bender reduced the yield strength and stiffness in all types of rods. Decrease of yield strength correlated to the degree of bend. In the comparison of 20° and 40° contoured rods, the 6.0-mm CoCr rod showed the highest reduction force. This study found that rod contouring procedures reduced rod yield strength and stiffness.