Finite Element Analysis-Based Evaluation of the Patient-Specific Spinal Rods for a Reduced Risk of Adjacent Segment Disease

Adjacent Segment Disease (ASD) is a postoperative drawback of spinal fusion surgery which yields an increase in the range of motion in the adjacent spinal level. Therefore, the main aim of this study is to investigate the optimum mechanical properties of the spinal rod allowing a reduced rigidity in the spinal fixation level for decreasing the displacement of the adjacent segment. In this study, the spinal fixation system was modelled and attached to L3-L4 level. The elasticity modulus of the rods and the follower load were parametrically defined in order to investigate their optimum values under physiological loading conditions of extension. The maximum displacement value determined for the upper adjacent intervertebral disc was defined as the output parameter. Thereafter, the biomechanical response of the spinal bone-implant complex was simulated using Finite Element Analysis (FEA). Using the parametric FEA results, a polynomial mathematical model was constructed and Response Surface Method (RSM) was used to plot the relationship between input and output parameters. According to the results of the study, the optimum elasticity modulus of the rods and the suggested follower load have been determined as 80.8 GPa and 303.84 N, respectively. The maximum principal strain values obtained in the pedicle screws were 746 µℇ, 1563 µℇ, 3037 µℇ and 2937 µℇ, respectively. However, since the results are strongly associated with anatomical and biomechanical differences, the proposed patient-specific approach may enhance the accuracy for a more successful spinal fusion surgery operation in terms of minimizing the risk of ASD.