Medical Student Washington University in St. Louis Washington University in St. Louis School of Medicine St Louis, MO, US
Introduction: Mechanical failure following spinal deformity surgery is a significant concern, particularly in long fusions. The T4-L1 hip axis framework, encompassing L1 Pelvic Angle (L1PA) error and T4PA-L1PA mismatch (T4PA mismatch), has been proposed to guide sagittal alignment correction. This study investigates the relationship between these alignment errors and mechanical failure, including stratification by short and long fusion cohorts.
Methods: We retrospectively reviewed 116 thoracolumbar fusion patients at an academic tertiary care center between 2015 and 2024 with at least 6 months of follow-up. Mechanical failure was defined as Cobb Angle UIV/UIV+2 > 30° or any reoperation within two years for pseudoarthrosis, proximal junctional kyphosis (PJK), or rod fracture. L1PA error and T4PA-L1PA mismatch were analyzed, including their absolute values, in relation to mechanical failure using logistic regression. Stratified analyses were performed for short and long fusion subgroups.
Results: In the full cohort, T4PA-L1PA mismatch (p = 0.011) and its absolute value (p = 0.033) were significant predictors of mechanical failure, while L1PA error was not. In the short fusion cohort (n = 50), none of the alignment errors were significant predictors. In the long fusion cohort (n = 66), both T4PA-L1PA mismatch (p = 0.013) and absolute value of the mismatch (p = 0.013) were significantly associated with mechanical failure, underscoring the importance of minimizing sagittal mismatch in long fusion surgeries.
Conclusion : T4PA-L1PA mismatch is a significant predictor of mechanical failure, particularly in long fusion surgeries, while L1PA error appears to play a less critical role. These findings suggest that the T4-L1 hip axis framework may not apply as strongly to short fusions and highlights the need to focus on minimizing mismatch in long fusion procedures to reduce mechanical complications.
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