Thoracic Spine Degeneration in Adult Spinal Deformity Patients: Impact on Baseline Alignment, Reciprocal Changes, and Proximal Junctional Kyphosis

Purpose To evaluate the impact of thoracic spine degeneration in adult spinal deformity (ASD) patients on radiographic outcomes. Methods Primary ASD patients undergoing thoracolumbar fusion with T9-L1 upper instrumented vertebra (UIV) and S1/ilium lower instrumented vertebra were included. Thoracic spine degeneration was assessed radiographically using Kellgren-Lawrence (KL) grading and segmented into T1-T5, T5-T9, and T9-L1 arcs per Lafage criteria. Arc degeneration was defined as ≥ 2 levels with KL grade 3 + in an arc and thoracic spine degeneration as ≥ 1 degenerated arc. Proximal zone degeneration was KL grade 3 + in the two levels above the UIV and distal zone degeneration was KL grade 3 + in unfused thoracic levels outside the proximal zone. Patients with no degenerated levels served as controls. Results Among 272 patients (mean age 65.1years, 74% female, mean Charlson Comorbidity Index 1.9), 19, 81, and 100 patients had T1-T5, T5-T9, and T9-L1 arc degeneration, respectively, corresponding to 151 degenerated and 72 control patients. Degenerated patients exhibited higher thoracic kyphosis (36.1° vs 27.6°), reduced expected thoracic compensation via hypokyphosis (-0.7° vs 8.8°), and higher lumbar lordosis (38.6° vs 32.3°) (p < 0.05). At six weeks, degenerated patients had smaller reciprocal kyphotic changes (1.6° vs 5.8°). At two years, they had higher proximal junctional kyphosis (PJK)-Normative rates (38% vs 24%) (p < 0.05). Proximal zone sub-analysis showed that two-year PJK-Normative rates (43% vs 32% vs 24%) decreased incrementally from proximal to distal zone degeneration to controls (p < 0.05). Conclusion Over half of ASD patients exhibited thoracic spine degeneration, predominantly in the lower arcs (T5-L1). This was associated with reduced thoracic hypokyphosis, smaller six-week reciprocal kyphotic changes, and higher two-year PJK rates. Proximal zone degeneration predicted junctional failure. Surgical planning should consider thoracic degeneration and compensatory mechanisms, with careful UIV selection to ensure sustained correction. Level of Evidence : IV