Evaluation of global alignment and proportion score in an independent database

Background Context: Sagittal spinopelvic alignment has been associated with patient-reported outcome measures and mechanical complication rates. Recently, it was claimed that linear numerical values of pelvic tilt and lumbar lordosis measurements may be misleading for patients that have different magnitudes of pelvic incidence. The use of “relative” measurements embedded in a weighted scoring of Global Alignment and Proportion (GAP) was proposed. Purpose: The purpose was to evaluate the GAP scorein an independent database. Study Design/Setting: Retrospective Cohort Study Patient Sample: Adult spinal deformity patients who underwent ≥7 levels posterior fusion to the pelvis between 2004 and 2014 were included. Outcome Measures: Mechanical Complication Rates. Methods: Demographic, clinical, surgical and radiographic patient characteristics were recorded. Cochran-Armitage tests were used to compare mechanical complication rates in GAP categories. Uni and multivariable logistic regression analyses were used to obtain crude and adjusted Odds Ratios, of predictor (GAP categories) and the outcome (mechanical complication), and Risk Ratios were calculated. The diagnostic performance of the GAP score was tested using the area under the receiver operating characteristic curve, sensitivity, specificity, positive predictive value, negative predictive value and accuracy in predicting mechanical complications. Results: A total of 322 patients (285F, 37M) with a mean age of 58.2±9.6 were analyzed. Mean follow-up was 69.7 months (range 24 to 177). Mechanical complications occurred in 52.2% of the patients. Mechanical complication rates in proportioned (GAP-P), moderately (GAP-MD) and severely disproportioned (GAP-SD) patients were 21.8%, 55.1%, and 70.4%, respectively. AUC for the GAP score, at 2 years, was 0.682 (95% CI, 0.624 to 0.741, p<.001). AUC at minimum 5 years follow-up was similar at 0.708, while AUC at minimum 7- and 12-year follow-up were 78.5 and 90.7, respectively. Having a postoperative spinopelvic alignment of GAP-MD and GAP-SD resulted in 2.5 and 3.2 folds of relative risk in incurring a mechanical complication when compared to having a proportioned spinopelvic state, respectively. Conclusions: This study reports an association between the GAP Score and mechanical complications in an independent database. Increased association was noted as the years of follow-up increased. Aiming to achieve proportionate GAP Score postoperatively seems to be a viable option as lower GAP scores were associated with lower rates of mechanical complications, and vice versa.