The Effect of Different Controlling Force Directions of Spinal Orthosis on the Patients with Adolescent Idiopathic Scoliosis: A Preliminary Study.

Adolescent idiopathic scoliosis (AIS) is a three-dimensional deformity, yet the direction of controlling force applied in orthotic treatment remains largely empirical. Whether this force direction is aligned with patient-specific structural orientation has not been adequately evaluated in clinical cohorts. This retrospective study investigated the alignment between controlling force direction (CFD) and the plane of maximum curvature (PMC) and explored its relationship with first in-orthosis correction. A cohort of 265 patients with AIS treated with CAD/CAM-manufactured thoracolumbosacral orthoses was analyzed. CFD was estimated from digital orthosis models using transverse-plane analysis, and PMC orientation was derived from biplanar radiographs. Force-structure alignment was quantified as the angular difference between CFD and PMC. The study outcomes were changes in coronal Cobb angle and PMC-Cobb angle from baseline to the first in-orthosis radiograph. The mean CFD was 41.1° ± 11.3°, the mean PMC orientation was 66.0° ± 14.9°, the mean signed CFD-PMC difference was -24.9° ± 17.6°, and the mean absolute difference was 26.1° ± 15.8°, indicating a systematic posterior offset of CFD relative to PMC. In the analyses, absolute misalignment was not associated with first in-orthosis coronal Cobb change (r = -0.071, p = 0.248) but was associated with less favorable first in-orthosis PMC-Cobb change (r = 0.124, p = 0.045). The signed difference showed a similar association with the first in-orthosis PMC-Cobb change (r = -0.137, p = 0.026), whereas the corresponding coronal association was also non-significant. In an exploratory ≤20° grouping, aligned and misaligned cases showed similar coronal correction (-12.7° vs. -13.3°, p = 0.448), while the aligned group demonstrated numerically better PMC-Cobb correction (-14.0° vs. -12.5°, p = 0.112). These preliminary findings suggest that the force-structure alignment may be more obvious at the deformity-plane than the coronal plane in the first in-orthosis correction, although the early signal appears modest and exploratory.