Introduction: Intraoperative contouring of rods is a common procedure for surgeons in order to match the desired curvature of the spine, but may lead to premature weakening of the overall rigid construct. The current study investigates the stress in a screw and rod construct of different rod geometries and materials after simulated cyclic loading.
Methods: A finite element model simulating instrumented pre-contoured rods (3.5mmD, 40mmL and 157° curvature) was subjected to axial cyclic loading according to ASTM F1717 methodology. The curvature of the rods was modeled based on three bending techniques: in situ, French 3-point, and French 1-point (Figure 1). Two types of rod materials were compared: titanium alloy (T) and cobalt chrome (CC). The screws were modeled as titanium only. Elastic and pure plasticity material laws were used. A 100N compression and 10N tensile load was simulated for 50 cycles. Analysis was performed using ABAQUS (Dassault Systèmes, Providence, RI).
Results: The rod stress was negligible compared to the stress in the screws. At 50 cycles, the percent volume deformation in the screw for in situ, French 3-point, and French 1-point rod geometries was 0.37%, 0.38%, and 0.34%, respectively, for T rods. Similarly, with CC rods the percent volume deformation in the screw for in situ, French 3-point, and French 1-point rod geometries was 0.31%, 0.31%, and 0.25%, respectively. (Figure 2)
Conclusions: Both in situ and French 3-point rod geometries had similar percent volume deformation in the screws. French 1-point rod geometry resulted in the least deformation within the screws. Constructs with cobalt chrome rods reduced the magnitude of deformation in the screws compared to titanium rod constructs, thus possibly prolonging the lifespan of overall construct. Mechanical tests are needed to verify the FEA analysis.
Patient Care: Guide surgeons in choosing the rod material and bending technique to prevent construct failure
Learning Objectives: French 1-point rods geometry of cobalt chrome material may provide the least amount of stress on the screws and thus prolong the lifespan of the construct.