ASSOCIATION BETWEEN ROD AND FUSION MASS FRACTURES IN ADULT SPINAL DEFORMITY: A RETROSPECTIVE COHORT STUDY

Rod fractures are a major issue in adult spinal deformity (ASD) surgery, which can have deleterious implications for patients. While pseudarthrosis is the primary failure mode, fractures of the fusion mass can lead to rod fractures. This study aimed to assess whether fusion mass fractures in a solid posterior fusion mass are a distinct cause of rod fractures. A retrospective review of 200 consecutive ASD cases with thoracic-to-pelvis fusion was conducted, including full chart reviews and post-rod fracture CT scans. Collected demographics included age, BMI, and neurodegenerative disorders. Fusion mass fractures were defined as transverse fractures outside the facet joint area, while pseudarthrosis was identified by mass defects along the facet joint orientation. Disc height and interbody cage presence were recorded. Continuous variables were summarized with mean and standard deviation (SD), and categorical variables using counts (N) and percentages (%). Four trained observers reviewed radiographs and CT scans to determine rod fracture etiology by consensus opinion. The study included 161 patients, with 20 requiring revision for rod fractures. Fusion mass fractures were observed in 7 patients, while 13 had pseudarthrosis. Transverse rod fractures were common in patients with disc heights >10 mm at the fracture site (6/7), without interbody cages. In contrast, 92.3% of pseudarthrosis cases had disc heights < 1 0 mm, with 23.1% containing cages. No correlation was found between fracture type and age, sex, or BMI. Cobalt chrome rods were more common in transverse fractures. Fusion mass fractures primarily occurred at L2-L4 (71%), while pseudarthrosis mainly involved L3-S1 (77%). All patients had single-stage posterior surgeries with two-rod constructs. This study shows fusion mass fractures, distinct from pseudarthrosis, accounted for 35% of rod fracture cases in ASD patients. Notably, 6/7 occurred at levels with disc heights >10 mm without interbody cages. This suggests that motion in the anterior column can occur despite solid posterior fusion. As such, adding anterior support and multi-rod constructs may help prevent these fractures.