Partial Intraoperative Global Alignment and Proportion Scores Do Not Reliably Predict Postoperative Mechanical Failure in Adult Spinal Deformity Surgery

Study Design: Retrospective radiographic review. Objectives: The Global Alignment and Proportion (GAP) score allows sagittal plane analysis for deformity patients and may be predictive of mechanical complications. This study aims to assess the effectiveness of predicting mechanical failure based on partial intraoperative GAP (iGAP) scores. Methods: A retrospective radiographic review was performed on 48 deformity patients between July 2015 to January 2017 with a 2-year follow-up. Using the same methodology as the original GAP study, the partial iGAP score was calculated with the sum of the scores for age, relative lumbar lordosis (RLL), and lordosis distribution index (LDI). Therefore, the iGAP score (0-7) was grouped into proportional (0-2), mildly disproportionate (3-5), and severely disproportionate (6-7). Logistic regression was performed to assess the ability of the partial iGAP score to predict postoperative mechanical failure. Results: The mean iGAP for patients with a mechanical failure was 3.54, whereas the iGAP for those without a mechanical failure was 3.46 (P =.90). The overall mechanical failure rate was 27.1%. The mechanical failures included 8 proximal junctional kyphosis, 7 rod fractures, and 1 rod slippage from the distal end of the construct. Logistic regression analysis revealed that the partial iGAP score was not able to predict postoperative mechanical failure (χ² = 1.4; P =.49). Conclusion: The iGAP scores for RLL or LDI did not show any significant correlation to postoperative mechanical failure. Ultimately, the proposed partial iGAP score did not predict postoperative mechanical failure and thus, cannot be used as an intraoperative alignment assessment to avoid postoperative mechanical complications.