Rod Fractures After Multi-rod Constructs in Adult Spinal Deformity Patients Fused to the Sacrum/pelvis: Where Do They Occur and Why?

Introduction: Multi-rod constructs (MRC) are used in long spinal fusion operations to reinforce high stress areas such as the lumbosacral junction or 3-column osteotomy (3CO) sites. In this consecutive series of ASD patients with long segment fusions to the pelvis, we sought to: 1) Describe the use of MRCs in adult spinal deformity (ASD) surgery, 2) report rod fractures occurring at MRC sites, and 3) evaluate risk factors for rod fractures.

Methods: A single-center, retrospective cohort study was conducted of patients undergoing ASD surgery with these inclusion criteria: minimum 2-year follow-up, MRCs used, ≥10-level fusion, and fused to sacrum. The primary outcome was rod fracture. Univariate/multivariate logistic regression was performed controlling for age, kickstand rod usage, number of rods across the lumbosacral junction (LSJ), and the amount of coronal/sagittal cobb correction.

Results: Among 57 patients undergoing ASD surgery with MRCs, mean age was 60±11years. With respect to MRCs, 32(56%) patients had 3 rods, 18(32%) had 4, and 7(12%) had 5. Rods crossing the LSJ were most often 3(63%), followed by 4(25%) and 5(5%) rods. 9(16%) patients experienced rod fractures with 8(89%) patients having no more than 3 rod crossing the LSJ. A coronal correction>30mm was more often seen in patients with rod fracture(p=0.030), while an SVA correction>50mm was not significantly different(p=0.608). Multivariate logistic regression revealed that the amount of coronal correction was significantly associated with rod fracture (OR=1.03, 95%CI=1.01-1.07,p=0.044), as was achieving a coronal correction>30mm (OR=7.72, 95%CI=1.17-51.10,p=0.034), with no association between the amount of sagittal correction obtained and rod fracture.

Conclusion : This study found that greater coronal correction was associated with higher odds of rod fracture. We suggest adding at least 4 rods across the LSJ cephalad to the interbody fusions to avoid rod fractures in these high demand areas.