Global sagittal mismatch, not isolated L1 pelvic angle achievement, is associated with mechanical failure after adult spinal deformity surgery.

Abstract

Mechanical failure remains a common complication following adult spinal deformity (ASD) surgery. Although postoperative alignment targets such as the ideal L1 pelvic angle (L1PA) have been proposed to reduce failure risk, the relative importance of achieving local alignment targets versus restoring global sagittal harmony remains unclear. To evaluate the associations between postoperative L1PA metrics, global T4-L1PA mismatch, and mechanical failure following ASD surgery. Retrospective analysis of a prospective, multicenter ASD registry. Adult patients undergoing long-segment fusion from the lower thoracic spine to L2 extending to the pelvis between 2019 and 2023 with minimum 2-year follow-up. Mechanical failure, defined as proximal junctional kyphosis/failure, rod fracture, implant failure, or pseudarthrosis requiring revision. Ideal L1PA was defined as 0.5×pelvic incidence-19°, with achievement defined as absolute deviation ≤2° on first standing postoperative radiograph. Global alignment was assessed using signed and absolute T4-L1PA mismatch. Univariable comparisons and multivariable logistic regression models adjusted for age, sex, and body mass index (BMI) were performed using R. A total of 108 patients were included, of whom 33 (30.6%) experienced mechanical failure. In multivariable analyses, neither signed nor absolute deviation from ideal L1PA was independently associated with failure. In contrast, greater absolute T4-L1PA mismatch was independently associated with mechanical failure (OR 1.10 per degree; 95% CI 1.01-1.19; p = 0.04), whereas signed mismatch was not. When both variables were included in the same model, T4-L1PA mismatch remained significant (OR 1.10; 95% CI 1.01-1.20; p = 0.03), while ideal L1PA achievement did not. Higher BMI was also independently associated with failure (OR 1.06; CI 1.00-1.13; p = 0.04). While achieving ideal L1PA goal did not mitigate the risk of mechanical failure following adult spinal deformity surgery, greater global sagittal mismatch, as measured by absolute T4-L1PA deviation, was independently associated with increased failure risk. These findings suggest that restoration of global sagittal harmony, rather than isolated local alignment targets, is critical for mechanical durability after ASD surgery.