Abstract
BACKGROUND: Limited lumbar fusion surgery for adult spinal deformity (ASD) increases the risk of proximal junctional failure (PJF) at the thoracolumbar junction due to preserved mobility in this region. The majority of the extant research on PJF focuses on cases where the upper instrumented vertebra (UIV) is in the thoracic spine, whereas the aim of this study was to evaluate the correlation between Hounsfield Unit (HU) values around the thoracolumbar junction and the incidence of PJF following limited lumbar fusion. METHODS: A retrospective review identified patients aged ≥ 40 years who underwent fusion surgery spanning ≥ 3 levels with a UIV in the upper lumbar spine (L1-L3) and had a follow-up of at least two years. Demographic data, surgical factors, and spinopelvic parameters were analyzed. HU values were measured at the UIV, UIV + 1, and lower instrumented vertebra and were assessed for their association with PJF. RESULTS: Of 50 patients, 46 were included after excluding 4 who required reoperation for distal junctional failure. The mean age was 65.9 years, with a mean follow-up of 6.3 years. PJF was observed in 17 patients (36.9%). While most demographic and surgical factors were similar between groups, age was significantly associated with PJF (p = 0.024). Preoperative parameters significantly differed, including PT (p = 0.011), PI-LL (p = 0.010), SVA (p = 0.008), and HU at UIV/UIV + 1 (p = 0.006). Postoperative differences included PT (p = 0.007), PI-LL (p = 0.005), proximal junctional angle (p = 0.021), SVA (p = 0.021), and the global alignment and proportion (GAP) score (p = 0.001). Logistic regression identified low HU at UIV/UIV + 1 as the only independent PJF risk factor (OR: 0.975, 95% CI: 0.950-0.996, p = 0.016), with a cutoff value of 97.8 HU (AUC = 0.745, p = 0.022). PJF rates were 73.3%, 26.7%, and 12.5% for patients with HU < 98, 98-134, and > 134, respectively. CONCLUSION: Preoperative low HU values at UIV/UIV + 1 independently predict PJF risk. HU assessment via preoperative CT imaging offers a critical tool for surgical planning in ASD limited lumbar fusion cases. CLINICAL TRIAL NUMBER: Not applicable.