Evaluation of S2 alar and traditional S1 pedicle fixation for severe lumbar spondylolisthesis in different bone mineral densities: a finite element analysis

Abstract

Purpose: Little is known about the biomechanical performance of L5-S2 alar internal fixation constructs after posterior lumbar interbody fusion. This study aimed to compare the biomechanical effect of L5-S1 internal fixation and L5-S2 alar internal fixation on severe lumbar spondylolisthesis. Methods: A normal male volunteer without a history of spinal disease was selected, lumbar CT data was collected. An intact L5-S2 three-dimensional finite element model was created by Mimics and Geomagic.Two kinds of fixation methods were reconstructed including (1) the L5-S1 screw fixation model and (2) the L5-S2 alar fixation model. The inverse repair was performed using Geomagic software, the internal fixation device was drawn using Creo software, and the model parameters were set and analyzed using ANSYS Workbench software. Results: The validity of the intact model shows that the ROM of the model is similar to that of a reported cadaveric study. The average stress of the L5-S2 alar internal fixation device was 86.9-111% higher(P＜0.001) than that of the L5-S1 fixation device when the bone of the S1 screw path reached the yield threshold. The maximal stress of the S1 screw in the L5-S2 alar fixation was significantly lower (P＜0.001) than it in the L5-S1 fixation when the stress exceeds the S1 bone yield threshold. When the S1 screw path bone yielded, the maximal deformation value of the S1 screw path was similar in both models (P>0.05), while the average deformation value of the S1 screw path in L5-S2 alar fixation was significantly higher (P＜0.01) than it in L5-S1 fixation. Conclusion: Extending fixation to the S2 wing can significantly improve internal fixation device stability and reduce the risk of intraoperative and postoperative fractures while avoiding injury to the sacroiliac joint, reducing the difficulty of surgery and the risk of injury to surrounding tissues. It is a reasonable plan for the treatment of moderate and severe lumbar spondylolisthesis with osteoporosis.