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

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.