OBJECTIVE: To determine whether a residual positive T4-L1PA mismatch predicts mechanical failure after lower thoracic fusion, and to identify factors associated with failure to achieve the target.
SUMMARY OF BACKGROUND DATA: The T4-L1-Hip axis defines sagittal alignment using two vertebral pelvic angles that are fixed or directly modifiable in surgery. When fusion stops in the lower thoracic region, T4 remains above the construct and outside the surgeon's control.
METHODS: Patients who underwent fusion from T9-L1 to the pelvis between 2015 and 2024 with two-year follow-up were reviewed. Sixteen with a mismatch below -4° were excluded, leaving 357 patients. A mismatch within ±4° was defined as matched and one above +4° as a residual positive mismatch. Proximal junctional kyphosis (PJK), rod fracture, and revision surgery were assessed by Firth penalized logistic regression with covariates specified a priori.
RESULTS: A residual positive mismatch was present in 140 patients (39.2%). PJK occurred in 35.0%, rod fracture in 9.2%, and revision in 15.4%. A residual positive mismatch was associated with PJK (odds ratio [OR] 1.83, P=0.011) and revision (OR 2.40, P=0.004) but not rod fracture (OR 1.19, P=0.639). A hyperlordotic lordosis distribution index (LDI) was its strongest determinant for a residual positive T4-L1PA mismatch (OR 5.40, P<0.001). The L1PA offset showed no association (P=0.150); 14 of 19 patients (73.7%) who met the L1PA target with a hyperlordotic LDI still had a residual mismatch. L1 tilt increased across the LDI categories whereas T4 tilt did not.
CONCLUSIONS: A residual positive T4-L1PA mismatch after lower thoracic fusion was associated with PJK and revision surgery, but not with rod fracture. A hyperlordotic distribution of lordosis, rather than the magnitude of correction, was associated with this residual mismatch.
STUDY DESIGN: Retrospective cohort study.
LEVEL OF EVIDENCE: 3.
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