Biomechanical Study of Minimally Invasive Nonfusion Surgery for Treatment of Disc Herniation Associated with Adjacent Segment Disease: A Finite Element Analysis

- OBJECTIVE: We explored the biomechanical changes of 2 conventional minimally invasive nonfusion surgical methods for treating disc herniation in adjacent segment disease using 3-dimensional finite element analysis.- METHODS: A model comprising L3 to the sacrum was validated and used to establish an L4-L5 fusion model, and an adjacent segment disease (ASD) model was developed by modifying the material properties of the intervertebral discs. The ASD model was used to simulate 2 conventional minimally invasive nonfusion surgical methods, which resulted in the creation of 2 postoperative models (M1 and M2). The range of motion and the equivalent stress for each model were recorded under 6 different working conditions. The data are descriptive and were analyzed comparatively u nder a normal load.- RESULTS: Compared with the ASD group, the range of motion of the adjacent segment in the M1 and M2 groups remained unaffected. However, significant Von-Mises stress changes were found in the annulus fibrosus and n ucleus pulposus (NP), especially during extension, ipsi- lateral bending, and rotation. Stress in the NP also shifted toward the surgical incision in the annulus fibrosus during these movements. The maximum Von-Mises stress in the NP of the cephalic segment increased more than did that of the caudal segment. -CONCLUSIONS: Minimal nonfusion surgery for ASD might not affect adjacent segment stability significantly. Nonetheless, it can lead to segmental degeneration dete-rioration and postoperative recurrence. The cephalic segment is affected more than the caudal segment. Therefore, consideration of disc degeneration and appro-priate selection of surgical methods for ASD are crucial.