Predicting shoulder balance using novel intraoperative radiographic measures in adolescent idiopathic scoliosis

Retrospective case-series OBJECTIVE: The purpose of this study is to validate novel radiographic measures, specifically Rib-2 height (R2H) and T1-Rib-2 Change (T1R2C), as predictors of postoperative shoulder balance following adolescent idiopathic scoliosis (AIS) surgery. Shoulder balance following posterior spinal fusion (PSF) in AIS continues to be important for optimal aesthetic outcomes and patient satisfaction. Using currently accepted radiographic measures intraoperatively (e.g., T1-tilt) for the achievement of shoulder balance remains a challenge. Power analysis determined 28 patients were needed to achieve 80% power with an effect size Pearson's r = 0.5.

AIS patients who underwent PSFs were retrospectively identified when no further deformity correction was completed after the last intraoperative long-cassette radiograph. Traditional radiographic measures were completed. For intra-op patients, a reference vertical was used for all measurements. All measures were Pearson correlated with radiographic shoulder height (RSH) 6 weeks post-op (6wk) and 2y.

There were 29 patients (26 female, 90%) whose mean age at surgery was 14.1 years. The mean RSH changed from -14.9mm pre-op to 5.4mm 2y post-op (p =1.9e-6). T1R2C and R2H demonstrated strong positive correlations pre-op (T1R2C r=0.7, p =0.0001) / (R2H r=0.6, p =0.001) to post-op (T1R2C r=0.8, p =2.23e-7) / (R2H r=0.6, p =2e-4). All radiographic variables showed significant correlations with RSH pre-op to post-op (p<0.001). Intraoperative R2H had intermediate associations with RSH at 6wks (r=0.48, p=0.018) and 2y post-op (r=0.4, p =0.04). Intra-op T1R2C and remaining coronal measurements showed no correlation with RSH. Linear regression models show a significant predictive relationship between RSH at 2y and intra-op R2H. The model was not improved when corroborated with intra-op T1R2C.

R2H was a significant predictor of RSH pre-op to post-op and intra-op to post-op, performing better than T1R2C and other radiographic measures. R2H could be used as an objective radiographic measurement tool to plan surgery and during surgery to optimize shoulder balance following PSF including T1-Rib-2 Change.