Thoracic spine degeneration in adult spinal deformity patients: impact on baseline alignment, reciprocal changes, and proximal junctional kyphosis.

Abstract

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.