Risk Factors for Rod Fracture at ≥L4-5 Levels Following Long-Segment Fusion for Adult Spinal Deformity: Results from Segment-Based Analysis

Background/Objectives: Given the different biomechanical properties and surgical techniques between the L5-S1 and ≥L4-5 levels, it is necessary to explore RF risk factors at ≥L4-5 levels separately from the lumbosacral junction. This study aims to investigate the risk factors for rod fracture (RF) occurring at ≥L4-5 levels following adult spinal deformity (ASD) surgery. RF occurrence was assessed at the segment level. Methods: Patients who underwent ≥ 5-level fusion, including the sacrum or pelvis, with a minimum follow-up of 2 years were included in this study. Presumed risk factors in terms of patient, surgical, and radiographic variables were compared between the non-RF and RF groups at the segment level. Multivariate logistic regression analysis was performed to identify independent risk factors for RF at ≥L4-5 levels. Results: A total of 318 patients (mean age, 69.3 years; 88.4% female) were included, and 1082 segments were evaluated. During the mean follow-up duration of 47.4 months, RF developed in 45 (14.2%) patients for 51 (4.7%) segments. In multivariate logistic regression analysis, several risk factors were identified, as follows: the use of perioperative teriparatide (odds ratio [OR] = 0.26, p = 0.012), operated levels (L2-3 and L3-4 vs. L4-5 level [OR = 0.45, p = 0.022; OR = 0.16, p < 0.001, respectively]), fusion methods (posterior fusion and anterior column realignment vs. posterior lumbar interbody fusion [OR = 8.04, p < 0.001; OR = 5.37, p = 0.002, respectively]), pedicle subtraction osteotomy (PSO) (OR = 3.14, p = 0.020), and number of rods (four-rod configuration vs. dual-rod fixation [OR = 0.34, p = 0.044]). Conclusions: In this study, the factors related to RF at ≥L4-5 levels included the perioperative use of teriparatide, operated levels, fusion methods, performance of PSO, and rod configuration. Considering that surgical procedures vary by each segment, our findings may help establish segment-specific preventive strategies to reduce RF at ≥L4-5 levels.