Assessment of segmental flexibility at each disk level of the scoliotic curve in patients with adolescent idiopathic scoliosis (AIS) has rarely been performed, despite its importance for surgical planning of fusion levels. The fulcrum-bending radiography method could be used to assess flexibility at specific spinal segments of the scoliotic curve for preoperative planning. Therefore, our aim was to use the fulcrum-bending method to evaluate the pattern of segmental flexibility in patients with AIS.

Our retrospective analysis was based on preoperative fulcrum-bending radiographs in 21 patients presenting with main thoracic curve scoliosis. The apex vertebra was defined as the “0” level, with the sequential cranial disk levels defined as “+1 to +5” and the sequential caudal levels as “−1 to −5.” The segmental Cobb angle was measured on both, standing and fulcrum-bending radiographs. The segmental flexibility index (FI) was calculated as: [(the segmental Cobb angle in standing position−the segmental Cobb angle in the fulcrum-bending position)/the segmental Cobb angle in standing position]×100. The FI was measured from the upper-most to the lowermost end vertebrae of the main curve.

There was a significant difference in the FI among segments, with the greatest flexibility detected at the level of the −5 disk (median FI, 50%) and least flexibility at the periapical segments (+1 and −1; FI, 50%–66.7%). The curve around the apex and upper thoracic segments in thoracic AIS is relatively rigid. Understanding differences in FI between each disk level is important for optimal corrective surgery.