Progression of Thoracic Kyphosis After Lower Thoracic Fusion in Lumbar Flatback Deformity: Analysis of Risk Factors and Clinical Consequences

Study Design: Retrospective cohort study. Objectives: To investigate the risk factors and clinical impact of progression of thoracic kyphosis (PTK) in who underwent lower thoracic fusion for lumbar flatback deformity (LFBD). Methods: The study included 170 patients (mean age 70.2 years; 90.6% female) who underwent fusion from the lower thoracic spine (T9 or T10) for LFBD with a minimum follow-up of 2 years. PTK was defined as a final thoracic kyphosis (TK) ≥ 40° and ΔTK ≥10°. Multivariate logistic regression was performed to identify independent risk factors for PTK. Receiver operating characteristic (ROC) curve analysis was used to determine predictive cutoff values. Results: PTK developed in 55 patients (32.4%) during a mean follow-up of 35.8 months. Patients with PTK exhibited significantly higher final TK (48.2° vs 27.3°), worse sagittal alignment, and lower clinical outcome scores compared to those without PTK. Multivariate analysis identified preoperative TK (odds ratio [OR] = 1.120, P = 0.007) and age (OR = 1.094; P = 0.041) as independent risk factors for PTK. ROC analysis determined a preoperative TK cutoff value of 10.0° (area under the curve [AUC] = 0.834) and an age cutoff of 69.5 years (AUC = 0.661). Conclusions: In this study, PTK developed in 32.4% of patients following fusion to the lower thoracic spine for LFBD. Advanced age (>70 years) and high baseline TK (>10°) were significant risk factors for PTK. Extending fusion to a more cephalad thoracic spine should be considered for patients at high risk of PTK development.