From spine to trunk hyperboloid

 The external surface of the trunk can be assimilated to one or two hyperboloids in case of double curvature.

 The thoracic region and the lumbar region each constitute a hyperboloid.

 For the Lyon Method, the two scapular  and pelvic girdles form the non-deformable base of the two hyperboloids.

It is impossible to obtain a detorsion without a fixed base.

 The first thoracic vertebrae, the clavicle and the scapulae constitute the upper non-deformable base.

 The last lumbar vertebra and the pelvis form the lower non-deformable base.

 For double curve scoliosis, there is an intermediate fixed zone at the intersection of the anterior and posterior muscle chains at the thoraco-lumbar junction. 

 This model simplifies the biomechanics of spinal correction, which is performed along a single Z axis for both hyperboloids.

The two orthogonal horizontal axes X and Y will be independent for each hyperboloid.

The geometrical detorsion is performed by translation along the Z axis.

The mechanical detorsion is carried out at the level of the X and Y axes thanks to the coupled movements in the frontal plane and in the sagittal plane.

In the sagittal plane, isostatic balance is restored according to the angle of pelvic incidence. In the frontal plane, depending on the orientation of the posterior joints, we perform a bending at the thoracic level and a shift at the lumbar level.

 The displacement in the frontal and sagittal planes creates a rotation around the Z axis. This is the mechanical detorsion.
For the physiotherapist, the correction is done first in the sagittal plane (sagittalometer), then in the frontal plane by trying to move the apical vertebra on the midline.