If the Adjustment of Insertional Pedicle Screw Positions Will Affect the Risk of Adjacent Segment Diseases Biomechanically? An in-silico Study

Background: The posterior fixation system (PFS) is extensively used in the treatment of spinal diseases. However, the fixation-induced pathological stress distribution pattern (e.g., stress concentration and motility compensation) will increase the risk of adjacent segment diseases (ASD). The accurate adjustment of insertional screw positions is possible under the guidance of the C-arm during percutaneous pedicle screw insertion. Although studies reported that the adjustment of insertional screw positions would affect the stiffness of the fusion segment, there were still no studies that clarified the issue that if the adjustment of insertional screw positions would affect the postoperative biomechanical environment and the risk of ASD. Methods: To investigate if the change of screw positions and related moment arm in adjacent segments will affect mechanical indicators related to ASD, an intact lumbosacral model has been constructed and validated based on our anteriorly constructed model, the oblique lumbar interbody fusion fixed by bilateral PFS with different insertional positions has been simulated in the L4-L5 segment. Models were computed under completely identical loading conditions, including flexion, extension, bending, and rotation. Motility parameters, stress distribution in the intervertebral disc (IVD), and zygapophyseal joints (ZJ) in both cranial and caudal sides of functional units were recorded. Results: Consist with anterior published studies, the change of fixation lengths has a more apparent biomechanical effect on the cranial than the caudal segment. Positive collections can be observed between the reduction of the moment arm in adjacent segments and the aggravation of motility compensation. Furthermore, which can also lead to stress concentration on ZJ facet cartilages. By contrast, no pronounced tendency of stress distribution on IVD can be observed with the change of moment arm. Conclusions: during LIF operations fixed by percutaneous PFS, reducing the fixation stiffness by adjusting the insertional screw positions on the sagittal plane could alleviate the biomechanical deterioration and may be an effective method to reduce the risk of ASD (adjacent segmental instability in the short term and spinal stenosis in the long term).