Assistant Professor of Neurosurgery and Orthopaedics Vanderbilt University Medical Center Nashville, Tennessee, United States
Introduction: Surgeons have heavily scrutinized the upper instrumented vertebrae (UIV), including vertebrae tilt angle and screw angle, to avoid proximal junctional kyphosis/failure (PJK/F) in adult spinal deformity surgery (ASD). In a cohort of patients undergoing ASD surgery, we sought to determine which aspect of the UIV – the UIV tilt angle or UIV screw angle – was more strongly associated with: 1) PJK/F, 2) remaining mechanical complications, and 3) reoperation.
Methods: A single-institution, retrospective cohort study was undertaken for patients undergoing ASD surgery from 2011-17 with 2-year follow-up. Only patients with UIV at T7 or below were included. The two primary exposure variables were: 1) UIV tilt angle: angle between UIV inferior endplate and the horizontal, and 2) UIV screw angle: angle between UIV screws and superior endplate. The multivariate logistic regression model included: age, BMI, osteopenia, postoperative sagittal vertical axis and pelvic-incidence-lumbar-lordosis mismatch, UIV tilt angle, and UIV screw angle.
Results: A total of 117 patients underwent ASD surgery. 1) UIV tilt angle: 96(82.1%) had positive, lordotic UIV tilt angles, 6(5.1%) had neutral UIV tilt angles, and 15(12.8%) had negative, kyphotic UIV tilt angles. 2) UIV screw angle: 38(32.5%) had positive, cranially-directed screws, 4(3.4%) had neutral screws, and 75(64.1%) had negative, caudally-directed screws. The mean UIV tilt angle was 12.9±13.6°, while the mean UIV screw angle was -3.2±8.0°. Multivariate regression showed that both lordotic UIV angles (OR=1.04, 95%CI=1.00-1.08,p=0.046) and positive, cranially-directed screws (OR=7.86, 95%CI=2.54-24.35,p < 0.001) significantly increased the odds of PJK, with a more pronounced effect of UIV screw angle compared to UIV tilt angle (Wald test, 11.80 vs. 2.99).
Conclusion : While both lordotically angled UIV endplate and cranially-directed UIV screws significantly increased the odds of PJK, UIV screw angle appeared to be more strongly associated with development of PJK, with a lesser contribution from vertebral tilt angle.
