The Impact of L1PA Error and T4PA-L1PA Mismatch on Mechanical Failure in Spinal Deformity Surgery: Direction Matters

Introduction: Mechanical failure following spinal deformity surgery is a significant concern. The T4-L1 hip axis framework, encompassing L1 Pelvic Angle (L1PA) error and T4 Pelvic Angle-L1PA mismatch (T4PA error), has been proposed to guide regional and global sagittal alignment correction. However, all errors are not identical, and the direction of the error (either positive or negative) can play a role in the mechanisms of failure. This study investigates the relationship between the directionality of these alignment errors and mechanical failure.

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: Positive T4PA error is the only variable significantly associated with mechanical failure. Both positive and negative L1PA errors, as well as negative T4PA error, do not show a significant relationship with mechanical failure.

Conclusion : Our findings indicate that positive T4PA error is significantly associated with mechanical failure, suggesting that the direction of mal-alignment error plays a crucial role in patient outcomes following spinal deformity surgery. Positive T4PA error, which may represent a global under-correction in relation to the pelvis, could potentially introduce undue strain on adjacent segments or lead to an imbalance that predisposes patients to failure modes such as proximal junctional kyphosis (PJK) or rod fractures.