Biomechanical determinants of annular suture repair in an idealized post-discectomy annular defect: a finite element study

Background A residual annular defect after lumbar discectomy can allow recurrent nucleus pulposus (NP) displacement. Annular suturing may improve containment, but the roles of suture number, axial spring stiffness, placement, and edge apposition are unclear. Methods A public human L4-L5 finite element model was used to compare an intact disc, an unrepaired posterolateral full-thickness annular defect, open-gap repairs with one to three sutures, and graded three-suture partial-apposition repairs. The NP was retained, so the model represents an idealized post-discectomy defect. A 1-MPa expansive nucleus-boundary pressure preload was followed by axial compression to 2,000 N, with or without a 7.5-N m flexion moment. Primary outcomes were peak contained NP protrusion and the positive-only area under the protrusion-load curve (AUC). Rim stress and intradiscal pressure were secondary outcomes. Placement, suture number, axial spring stiffness, pretension, defect size, preload, and mesh density were varied in sensitivity analyses. Results The defect increased peak protrusion from 0.107 to 0.358 mm under pure compression and from 0.183 to 0.474 mm under flexion plus compression. Every primary load-targeted repair reduced protrusion. Open-gap repairs restored 29%–32% of the defect-to-intact AUC difference at 1 MPa and about 90% at 0.1 MPa. Among untensioned single sutures, only those crossing the deep, load-opening part of the defect produced large reductions. Suture number and axial spring stiffness changed peak protrusion by at most 1.0%. As the mean residual gap decreased from 1.69 to 0.80 mm, the AUC fell from 235 to 102 mm N under pure compression. Rim stress remained above intact in every repair. The ordering of repairs was robust, but the size of improvement depended on preload, protrusion definition, and mesh density. Conclusion In this idealized model, mechanically engaged suturing improved containment. Edge apposition and placement mattered more than suture number or axial spring stiffness. Improved containment did not normalize annular stress. Closing the protrusion pathway and restoring annular load transfer are therefore distinct goals.