Introduction of a New Radiographic Parameter to Predict Proximal Junctional Kyphosis in Adult Spinal Deformity: UIVPTA (Uppermost Instrumented Vertebra-Pelvic Tilt Angle)

Objective: To introduce a new sagittal parameter, uppermost instrumented vertebra-pelvic tilt angle (UIVPTA), and to determine the effects on the proximal junctional kyphosis (PJK) development in adult spinal deformity (ASD) surgery. Methods: Patients ≥ 60 years with ASD who underwent low thoracic spine to pelvis fusion with a minimum of 2-years of follow-up were included in this study. Two groups were created according to PJK development. Various clinical and radiographic factors were compared between PJK and non-PJK groups to identify the risk factors for PJK. Cutoff value of UIVPTA for PJK development was calculated using receiver operating characteristic curve according to different pelvic incidence groups. Linear regression analysis was performed to identify factors to affect UIVPTA. Results: One hundred fifity-one patients were included in this study. There were 135 female patients (89.4%). Mean age was 70.5 years. PJK developed in 65 patients (43.0%). Multivariate analysis showed that overcorrection relative to age-adjusted pelvic incidence (PI) minus lumbar lordosis (LL) (PI–LL) target and lower UIVPTA were independent risk factors for PJK. The cutoff value of UIVPTA for PJK development was calculated as 4.0° in patients with PI less than 45°, 9.5° in patients with PI between 45° and 60°, and 13.0° in patients with PI greater than 60°. Linear regression analysis showed that UIVPTA was positively affected by postoperative values of LL (coefficient = 0.505), PI–LL (coefficient = 0.674), and pelvic tilt (coefficient = 0.286). Conclusion: Optimal correction within the age-adjusted PI–LL combined with keeping UIVPTA within optimal range is suggested for the prevention of PJK.