O1050: Rule Breakers: Upper Instrumented Level Selection and Shoulder Balance in Adolescent Idiopathic Scoliosis

Background: Current upper instrumented vertebra (UIV) selection guidelines are based on pre-operative relative shoulder height. The purpose of this study was to compare shoulder balance at 2-year follow up in patients designated as “rule followers” versus “rule breakers” based on commonly used UIV selection guidelines. We hypothesized that violation of UIV selection guidelines would not impact shoulder balance after posterior spinal fusion (PSF) for adolescent idiopathic scoliosis (AIS).

Methods: AIS patients who underwent PSF between June 2015 and May 2017 at a tertiary academic center with minimum of 2-year follow up and complete clinical and radiographic data were included. Patients with Lenke type 5 curves were excluded. Radiographic measurements of shoulder balance included first rib angle (FRA), T1 tilt, coracoid process height difference (CPH), and clavicle angle (CA) and were documented at pre-op, post-op, 6 months, 1 year, and 2-year time points. Strict shoulder balance was defined as CPH < 1 cm. Relative preoperative shoulder height was utilized to create rule breaker and rule follower groups based on commonly utilized UIV selection guidelines. Postoperative outcomes were compared between groups using mixed models adjusting for pre-operative values and including surgeon as a random effect.

Results: : A total of 92 patients met inclusion criteria. Age, body mass index and gender distribution were not significantly different between groups (p>0.05). Pre-operative T1 tilt, FRA and CPH were similar between RB and RF groups (p>0.05). The proximal thoracic curve was significantly higher in the RB group (28.4º vs. 23.7º, p=0.038). While the clavicle angle was higher in the RF group (3.1º vs. 2.4º, p=0.048). Overall, immediate post-surgical outcomes didn’t show any significance difference between RF or RB in T1 tilt (4.5° vs. 5.1°, p=0.309), FRA (3.4° vs. 4.3°, p=0.144), and CPH (10.3 vs 11.4 mm, p=0.306). This observation has persisted at 2-year follow up: T1 tilt (4.3° vs. 5.2°, p=0.198), FRA (3.2° vs. 4.2°, p=0.059), CA (2.3° vs. 2.2°, p=0.764), and CPH (8.6 vs. 8.1 mm, p=0.733). Shoulder balance (defined as CPH was < 1cm) was achieved in 60.0% of RF and 67.4% of RB at 2 years post-operatively (p=0.517).

Conclusion: Adherence to commonly accepted UIV selection guidelines did not predict better shoulder balance in this cohort. At two-year follow up, shoulder balance was similar between the rule breakers and rule followers. The results of this study suggest the potential for improvement in UIV selection guidelines.