Flexibility Assessment of Adolescent Idiopathic Scoliosis Using Fulcrum Bending Radiographs in the Modern Era

Fulcrum bending (FB) radiographs provide a standardized, passive assessment of spinal flexibility in adolescent idiopathic scoliosis (AIS) and have emerged as a reliable auxiliary planning tool for guiding fusion level selection and implant strategy. This narrative review synthesizes current evidence on the development, technique, and clinical application of FB in surgical planning. Compared with traditional side-bending radiographs, FB more accurately predicts postoperative coronal correction, with fulcrum Cobb angles closely approximating postoperative alignment. The FB correction index further refines this relationship by normalizing achieved correction to preoperative flexibility, enabling comparison across constructs. In selective thoracic fusion for Lenke 1 curves, FB-based “squared fusion mass” criteria (Cobb ≤20°, fusion mass shift ≤20 mm) enable shorter constructs and safely preserve motion segments without increasing coronal decompensation. FB also informs construct strategy, confirming that maximal correction does not require full segmental screw density when levels are appropriately chosen. Beyond coronal assessment, lateral FB radiographs provide insight into sagittal flexibility and prediction of postoperative thoracic kyphosis, supporting more comprehensive 3-dimensional planning. In Lenke 5C curves undergoing anterior spinal fusion, FB in combination with active contralateral side-bending radiographs facilitates selective shortening of constructs in flexible deformities, with early postoperative imbalance often resolving over time. In the modern era of AIS surgery, FB remains a validated, objective tool for preoperative planning, integration with sagittal alignment criteria, spinopelvic targets and static landmarks such as the last touched vertebra and the substantially touched vertebra is essential for comprehensive patient-specific and 3-dimensional correction.