Impact of four-rod instrumentation and interbody cages on pseudarthrosis and rod breakage in adult spinal deformity surgery with pelvic fixation and Schwab grade 2 osteotomies: a combined finite element model and clinical data analysis

Abstract

OBJECTIVE The objective of this study was to evaluate the impact of four-rod (4R) constructs and interbody cages (IBCs) on pseudarthrosis and rod breakage (PA/RB) in patients with adult spinal deformity (ASD) who had undergone surgery with pelvic fixation and Schwab grade 2 osteotomies, using a combined finite element model (FEM) and clinical data analysis. METHODS A validated FEM simulated Schwab grade 2 osteotomies at the L4-5 level in two-rod and 4R configurations, with or without IBCs at L4-5 and L5-S1. Rod strain and range of motion were calculated under a 7.5-Nm moment. Clinical analysis was conducted on ASD patients with pelvic fixation and Schwab grade 2 osteotomies and >= 2 years of follow-up. Patients were classified into 2 groups depending on the presence or absence of PA/RB. Demographic, surgical, radiographic, and patient-reported outcome measure (PROM) data were compared. RESULTS The FEM analysis revealed maximal rod strain of 399 MPa at the osteotomy site in flexion. The 4R constructs and IBCs reduced strain to 114 MPa at L4-5 and 80 MPa at L5-S1. Among the 213 patients included in the study, PA/RB occurred in 61 (28.6%). Multivariate analysis revealed the use of 4R constructs (OR 0.331, 95% CI 0.16-0.71, p = 0.004) and IBCs (OR 0.46, 95% CI 0.23-0.94, p = 0.033) as protective factors. Patients with PA/RB experienced more unplanned reinterventions, worse scores on PROMs, and greater loss of sagittal alignment at 2 years postoperatively. CONCLUSIONS Constructs with 4Rs and IBCs in ASD surgeries with pelvic fixation and Schwab grade 2 osteotomies significantly reduced rod strain and decreased the risk of PA/RB, leading to better scores on PROMs and decreasing unplanned reinterventions and loss of alignment. https://thejns.org/doi/abs/10.3171/2025.9.SPINE25604