A Patient-specific Approach to Alignment and Proximal Junctional Kyphosis Risk Assessment in Adult Spinal Deformity Surgery: Development and Validation of a Predictive Tool

Study Design: This was a single-institution, retrospective cohort study. Objective: We aimed to develop a predictive model for proximal junctional kyphosis (PJK) severity that considers multiple preoperative variables and modifiable surgical alignment. Summary of Background Data: PJK is a common complication following adult deformity surgery. Current alignment targets account for age and pelvic incidence but not other risk factors. Materials and Methods: This is a single-institution, retrospective cohort study of adult deformity patients with a minimum 2-year follow-up undergoing instrumented fusion between 2009 and 2018. A proportional odds regression model was fit to estimate PJK probability and Hart-International Spine Study Group (ISSG) PJK severity score. Predictors included preoperative Charlson Comorbidity Index, vertebral Hounsfield Units near the upper instrumented vertebrae, pelvic incidence, T1-pelvic angle, and postoperative L1-L4 and L4-S1 lordosis. Predictor effects were assessed using adjusted odds ratios and a nomogram constructed for estimating PJK probability. Bootstrap resampling was used for internal validation. Results: Of 145 patients, 47 (32%) developed PJK. The median PJK severity score was 6 (interquartile range, 4-7.5). After adjusting for predictors, Charlson Comorbidity Index, Hounsfield Units, preoperative T1-pelvic angle, and postoperative L1-L4 and L4-S1 lordosis were significantly associated with PJK severity (P<0.05). After adjusting for potential overfitting, the model showed acceptable discrimination [C-statistic (area under the curve)=0.75] and accuracy (Brier score=0.10). Conclusions: We developed a model to predict PJK probability, adjusted for preoperative alignment, comorbidity burden, vertebral bone density, and modifiable postoperative L1-L4 and L4-S1 lordosis. This approach may help surgeons assess the patient-specific risk of developing PJK and provide a framework for future predictive models assessing PJK risk after adult deformity surgery. Level of Evidence: Level III.