Fused Spinopelvic Angle in Adult Spinal Deformity with Pure Sagittal Malalignment: A Postoperative Threshold Associated with Proximal Junctional Kyphosis

Background: Determining the appropriate magnitude of lumbar lordosis (LL) correction in adult spinal deformity (ASD) remains controversial. The fused spinopelvic angle (FSPA), a posture-independent parameter, may provide a geometric measure of fused-construct orientation associated with proximal junctional kyphosis (PJK) risk. Methods: This retrospective single-center study included 258 patients aged ≥65 years with ASD and pure sagittal malalignment associated with lumbar degenerative kyphosis/drop body syndrome who underwent long-segment fixation from T10 to the sacrum with sacropelvic fixation. Patients were divided into non-PJK (n = 135) and PJK (n = 123) groups. Adjusted nested logistic regression models, receiver operating characteristic (ROC) analysis, and DeLong comparison were used to evaluate the association of FSPA with PJK and its incremental value beyond postoperative pelvic incidence–lumbar lordosis (PI-LL). Results: Postoperative FSPA was lower in the PJK group (p < 0.001) and remained independently associated with PJK in the fully adjusted model including postoperative PI-LL (adjusted OR = 0.910 per degree; 95% CI, 0.866–0.957; p < 0.001). FSPA showed greater discriminatory ability than postoperative PI-LL (AUC, 0.694 vs. 0.596; difference, 0.099; 95% CI, 0.035–0.162; DeLong p = 0.002). Adding FSPA to the clinical model containing postoperative PI-LL significantly improved model fit (likelihood-ratio p < 0.001). The cohort-derived FSPA threshold of 2.38° yielded 64.2% sensitivity and 63.7% specificity. Conclusions: Higher postoperative FSPA was independently associated with lower odds of radiographic PJK and provided additional predictive information beyond postoperative PI-LL. The identified threshold of 2.38° may serve as a preliminary reference for postoperative alignment in this selected cohort.