Mechanical Failures as Predicted by Achieving Local Versus Global T4-L1 Hip Axis Goals: A Single-center Experience

Study Design. – Retrospective cohort study. Objective. – To evaluate the predictive value of T4-L1 Hip Axis Error (HAE) and L1PA Error (L1PAE) on mechanical failure following adult spinal deformity (ASD) surgery, and to assess how error directionality and fusion length influence outcomes. Summary of Background Data. – The T4-L1 Hip Axis is a novel framework for defining normative sagittal alignment by aligning thoracic and lumbar curvatures relative to the pelvis. Prior studies suggest that deviation from this axis may contribute to mechanical complications following ASD surgery. However, the impact of directionality, construct length, and specific risk factors remains underexplored. Methods. – A retrospective review was conducted of 271 ASD patients who underwent fusion from L2 or above to the pelvis between 2016 and 2024. T4PA and L1PA were measured from six-week postoperative standing radiographs. Alignment errors (HAE, L1PAE) were calculated based on the target threshold derived from prior normative studies. Multivariate logistic regression identified predictors of mechanical failures. Subgroup analyses were stratified by fusion length and error direction. Results. – HAE was a significant predictor of mechanical failure (OR=1.20 per degree, P<0.001), whereas L1PAE was not. HAE remained predictive regardless of whether patients achieved L1PA targets. In short fusions, both the signed and absolute value of HAE were associated with mechanical failure. Positive HAE (anterior T4 alignment) conferred the highest risk. Higher BMI and short constructs were independent predictors of increased HAE. Conclusions. – HAE is a robust, direction-sensitive predictor of mechanical failure in ASD surgery. Its predictive power surpasses L1PAE and remains significant in both long and short segment fusions. HAE should be routinely measured and minimized intraoperatively to reduce postoperative mechanical complications.