The Influence of Relative Curve Correction and Upper Instrumented Vertebra (UIV) Tilt Angle on Post-operative Shoulder Balance Following Posterior Spinal Fusion (PSF) in Lenke Type 1 and 2 Adolescent Idiopathic Scoliosis (AIS) Patients

Study Design: Retrospective study. Objective: To investigate the relative curve correction of Proximal Thoracic (PT) and Main Thoracic (MT) scoliosis curves and their relationship with post-operative UIV tilt angle as well as post-operative shoulder imbalance. Methods: 151 AIS patients with Lenke type 1 and 2 curves who underwent PSF were reviewed. Relative PT/MT Correction Rate (RCR), Relative PT/MT Residual Cobb angle (RRCA) and Post-operative UIV Tilt Angle were investigated for their association with post-operative T1 tilt (medial shoulder balance), Cervical Axis (neck balance) and Clavicle Angle (Lateral Shoulder Balance). Results: There were 107 patients with Lenke 1 curves and 44 patients with Lenke 2 curves. Overall, the prevalence of medial shoulder, neck and lateral shoulder imbalance was 27.8%, 23.2% and 9.9%, respectively. There was a significant association between RRCA and medial shoulder, lateral shoulder and neck balance but RCR had a significant association with lateral shoulder and neck balance. There was a significant and strong correlation (r = 0.708) between post-operative UIV tilt angle and post-operative T1 Tilt. Unlike RCR, RRCA had shown a statistically significant correlation to post-operative UIV tilt angle with an r-value of 0.406. Conclusions: RRCA had shown a statistically significant correlation with the post-operative UIV tilt angle, and post-operative T1 tilt ie, medial shoulder balance. However, the post-operative UIV tilt angle showed the strongest strength of correlation with the post-operative T1 tilt (r = 0.708).