Sagittal alignment goals in adult spinal deformity surgery: a narrative review focusing on proximal junctional complications and clinical outcomes

Adult spinal deformity (ASD) is a complex condition associated with significant disability and reduced health-related quality of life (HRQOL). Surgical correction has increasingly emphasized restoration of sagittal alignment; however, the optimal radiographic targets and their relationships to clinical outcomes and mechanical complications remain subjects of debate. This narrative review summarizes five major alignment strategies in ASD surgery and examines their relevance to HRQOL and the prevention of proximal junctional kyphosis/ failure (PJK/PJF). The Scoliosis Research Society-Schwab classification introduced the first standardized thresholds for sagittal imbalance that demonstrated strong associations with HRQOL, although its ability to predict PJK/PJF is limited. Age-adjusted alignment goals highlighted the importance of avoiding overcorrection, demonstrating that functionally appropriate targets in older patients can reduce junctional complications while maintaining HRQOL benefits. The Global Alignment and Proportion (GAP) score proposed a proportionality-based framework and demonstrated early promise in predicting mechanical complications; however, subsequent validation studies have reported inconsistent results across different populations. The Roussouly classification emphasized restoration of a patient's inherent sagittal profile, with lower complication rates observed when type-matched correction was achieved. More recently, vertebral-pelvic angle-based metrics, including the T1 pelvic angle and the T4-L1-hip axis, have shown strong correlations with HRQOL and PJK risk while offering reproducible and practical intraoperative applicability. Although each system provides valuable insights, no single approach is universally superior. Future research should focus on integrating radiographic, biological, and functional factors into predictive models and validating these approaches through prospective multicenter studies to better guide individualized alignment strategies.