Orthoses are widely used to treat scoliotic deformities of the trunk, but the way the corrective forces are transmitted from the thorax to the spine remains not well understood, and several undesired effects such as the reduction of sagittal curvatures or weak derotations are often reported. A biomechanical finite element model of the trunk was used to investigate the hypothesis that there exist coupling mechanisms between the scoliotic spine and rib cage which may explain incomplete and unexpected results obtained by orthotic treatments. Forces of 40 N were applied on the rib hump and on the lateral side of the thorax, and their individual effects were evaluated in 3-D on the spine and thorax using several geometrical indices (transverse plane translations, axial, sagittal and frontal rotations, Cobb angles). These biomechanical simulations demonstrated the existence of coupled motions between the spine and rib cage subjected to orthotic loads. It showed that reduction of physiological sagittal curvatures (up to 30%) are possibly related to anterior orthotic loads applied on the rib hump. These loads also contribute to increase lateral shift of the spine (up to 7 mm) as well as scoliotic frontal curvatures (up to 4 degrees). Based on the results found in this study, a simple and more optimal approach to treat scoliotic deformities was proposed and consisted to apply loads laterally on the convex side and on the anterior thorax opposite to the rib hump, with a system that mechanically constrains the posterior rib hump to move backward. It was simulated on 4 scoliotic patients presenting thoracic curves between 22 degrees and 54 degrees to evaluate its practicability and it was found that derotation of the trunk (between 7 degrees and 13 degrees) and reduction of frontal curvatures (up to 4 degrees) could be done without reducing physiological sagittal curvatures. More simulations on different scoliotic configurations are necessary to find the most optimal combination of forces to produce a real 3-D correction of scoliotic deformities.