The role of biological fusion and anterior column support in a long lumbopelvic spinal fixation and its effect on the s1 screw-an in silico biomechanics analysis

Study Design. Finite element analysis. Objective. The aim of this study was to determine the role of biological fusion and anterior column support in a long lumbopelvic spinal fixation. Summary of Background Data. Retrospective studies have shown that adding anterior column support is not sensitive to construct failure, highlighting that posterior fusion quality may be a more important factor. Methods. Finite element models were created to match the average spinal-pelvic parameters of two patient cohorts reported in the literature: Major failure and nonfailure. A moment load was applied at the T10 superior endplate to simulate gravimetric loading in a standing position. Effects of three factors on the biomechanical behavior of a fused spine were evaluated: Sagittal alignment; posterior fusion versus no fusion; and anterior support at L4-S1 versus no anterior support. Results. Sagittal balance of the major failure group was positively correlated with 15% higher translation, 14% higher rotation, and 16% higher stress than in the nonfailure group. Simulated posterior fusion-only decreased motion by 32% and 29%, and alleviated rod stress by 15% and 5% and S1 screw stress by 26% and 35%, respectively, in major failure and nonfailure groups. The addition of anterior fusion without posterior fusion did not help with rod stress alleviation but dramatically decreased S1 screw stress (by 57% and 41%), respectively. With both posterior fusion and anterior support, screw stress at the S1 was decreased by additional 30% and 6%, respectively. Conclusion. The spinopelvic parameters of the major failure group produced increased gravity load, resulting in increased stresses in comparison to the nonfailure group. Simulated posterior "solid" fusion in the lumbar region helped reduce stresses in both major failure and nonfailure patients. Anterior column support was an important factor in reducing S1 screw stress, with or without posterior fusion, and should be considered for patients with poor alignment.