Superior displacement of the inflexion point does not predict proximal junctional kyphosis after fusion for adult spinal deformity.

Background: Proximal junctional kyphosis (PJK) is a common mechanical complication following long-segment fusion for adult spinal deformity (ASD). Although alignment goals have been extensively studied, the role of the sagittal balance inflection point remains underexplored. Displacement of this transition has been theorized to alter postoperative biomechanics, elevating stress at the proximal junction and predisposing patients to PJK. This study aimed to determine whether superior displacement of the inflection point is associated with PJK after fusion for those with ASD. Methods: A single-center retrospective cohort study of ASD patients undergoing posterior fusion from L2 or above to the pelvis (2017-2024). Patients required complete pre- and postoperative standing scoliosis radiographs and ≥12 months follow-up. The thoracolumbar inflection point and spinopelvic parameters were measured using Surgimap. Superior displacement was defined as a ≥1-level cranial shift relative to the preoperative position. PJK was defined as a >10° increase in the Cobb angle between the upper instrumented vertebra (UIV) and UIV + 1. Logistic regression evaluated associations between displacement and PJK with multivariable adjustment, interaction testing, and assessment of model discrimination. Results: Among 250 patients (PJK incidence 34.8%), rates were similar between those with and without superior inflection-point displacement >1 level (35.8% Conclusions: Superior displacement of the thoracolumbar sagittal inflection point was not associated with radiographic PJK following long-segment fusion for ASD. Although frailty and prior fusion were associated with PJK in partially adjusted analyses, no covariate retained independent significance in the fully adjusted model. These findings suggest that static inflection-point displacement is not a reliable standalone marker for PJK risk stratification.