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

Area of Science:

• Spine surgery
• Orthopedic surgery
• Biomechanical analysis

Background:

• The T4-L1 Hip Axis framework defines normative sagittal alignment relative to the pelvis.
• Deviations from this axis may link to mechanical complications in adult spinal deformity (ASD) surgery.
• The influence of error direction and fusion length on outcomes requires further investigation.

Purpose of the Study:

• To assess the predictive value of T4-L1 Hip Axis Error (HAE) and L1PA Error (L1PAE) for mechanical failure post-ASD surgery.
• To determine how error directionality and fusion length impact surgical outcomes.

Main Methods:

• Retrospective cohort study of 271 ASD patients undergoing fusion from L2 to the pelvis (2016-2024).
• Measurement of T4-L1 Hip Axis (T4PA) and L1PA from postoperative radiographs.
• Calculation of alignment errors (HAE, L1PAE) and use of logistic regression to identify predictors of mechanical failure.

Main Results:

• Hip Axis Error (HAE) significantly predicted mechanical failure (OR=1.20 per degree), unlike L1PAE.
• HAE remained predictive irrespective of L1PA achievement and was associated with failure in short fusions.
• Positive HAE (anterior T4 alignment) indicated the highest risk; higher BMI and short constructs independently predicted increased HAE.

Conclusions:

• Hip Axis Error (HAE) is a reliable, direction-sensitive predictor of mechanical failure in ASD surgery.
• HAE's predictive ability exceeds L1PAE and is significant across various fusion lengths.
• Routine intraoperative measurement and minimization of HAE are recommended to decrease postoperative mechanical complications.