P95. Lower Hounsfield units (HU) of the upper instrumented vertebrae (UIV) may contribute to proximal junctional kyphosis (PJK) in adult spinal deformity (ASD) surgery

Abstract

BACKGROUND CONTEXT HU on computed tomography (CT) has been associated with bone mineral density (BMD). We wished to correlate PJK after ASD surgery and HU as measured by CT. PURPOSE To evaluate if HU at the UIV, UIV+1 and UIV+2 was correlated with PJK. STUDY DESIGN/SETTING A retrospective study. PATIENT SAMPLE Single center with ASD patients undergoing >6 levels of fusions from the lower thoracic spine to the sacrum with minimum 2-year follow-up. OUTCOME MEASURES Radiographic spinopelvic parameters were measured, including pelvic incidence (PI), pelvic tilt (PT), lumbar lordosis (LL), PI-LL, sacral slope (SS), T1 pelvic angle (TPA), T5-T12 thoracic kyphosis (TK), sagittal vertical axis (SVA), and PJK rates. PJK was defined as 10° kyphosis between UIV and UIV+2 or as 10° more kyphosis at UIV and UIV+2 than preop. METHODS A retrospective study ASD patients ≥6 fusion levels from the LT spine to S1 with minimum of 2-year follow-up was performed. Demographic and spinopelvic parameters (PI-LL, SVA, TPA, PT) were measured. HU values were measured on preop CT at UIV, UIV+1, and UIV+2 four separate times at the: mid-sagittal plane, mid-body axial, axial near the superior and inferior endplates. RESULTS Fifty-four patients (mean age, 64.95 years; 20 males) met inclusion criteria, with 30 patients with PJK and 24 patients without, and mean follow-up was 2.79 years. There was no difference in age, BMI, sex, and follow-up in the two groups. There was no statistical difference between groups with regard to PI, SS, PT, LL, PI-LL, TK, TPA and SVA (p > 0.05). Mean preoperative HUs at UIV, UIV+1 and UIV+2 were 119.83, 123.53 and 129.03 in PJK patients, and 154.88, 158.50 and 161.58 in patients without PJK (p = 0.006, 0.009, 0.012). Postop PT and TPA were higher in PJK patients than in those without (p =0.003 and0.017). However, there was no statistical difference between groups with regard to PI, SS, LL, PI-LL, TK and SVA postoperatively (p > 0.05). A receiver operating characteristic (ROC) curve for preoperative HU at UIV as a predictor for PJK was established, with an AUC of 0.726 (95% CI: 0.592-0.86). The optimal HU cut-off value determined by the Youden index is 104.5HU (sensitivity, 0.875; specificity, 0.533). CONCLUSIONS PJK is a multifactorial issue, and lower HU values at the UIV, UIV+1 and UIV+2 in patients undergoing LT to the sacrum fusions for ASD may contribute to PJK. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.