Sagittal misalignment patterns and pathomechanical hypothesis of adolescent idiopathic scoliosis: a radiographic analysis

Purpose: This study aimed to identify sagittal misalignment patterns in adolescent idiopathic scoliosis (AIS) by measuring sagittal segmental slope angles of the lower cervical, upper and lower thoracic, and upper and lower lumbar segments relative to the global horizontal line, as well as sagittal deviations of C4, T7, and L3 from the sagittal central sacral line on lateral radiographs of AIS patients. This study also examined the sagittal pathomechanics of AIS, including compensatory mechanisms underlying these misalignment patterns. Methods: A retrospective radiographic analysis was conducted using standing full-spine lateral radiographs from 100 AIS patients randomly selected from a scoliosis clinic database. Five sagittal segmental radiographic spinal alignment parameters (RSAPs) and three sagittal deviational angles (SDAs) were measured. Radiographs were classified into distinct sagittal misalignment types based on combinations of SDA values and sagittal segmental RSAP measurements. Statistical analyses were performed to confirm the distinctiveness of each type. Results: Seven sagittal misalignment types were identified, ranging from near-normal alignment to patterns involving mixed combinations of of the following features: pronounced posterior tilting of the thoracic spine with excessive posterior decompensation of the superior segments; thoracic hypokyphosis with or without cervical decompensation; vertical straightening of the upper lumbar segments, lower thoracic segments, or both; and lumbar hyperlordosis with pronounced anterior tilt of the lower lumbar segment. These profiles reflect regional responses—anterior displacement or compensatory hyperextension—to increased extension moments. Conclusion: AIS sagittal misalignment is driven by increased extension moments and compensatory hyperextension, with regional responses governed by facet joint orientation. Recognizing these mechanisms supports universal 3D biomechanical principles for orthotic design, enhancing sagittal correction and overall treatment outcomes.