The biomechanical behaviour of the spine significantly varies in relation to the age of the spine. Particularly, the elastic behaviour of the intervertebral discs has been proved to change during the spine growth, which changes the disc reaction to externally imparted forces.
The biomechanical analysis of the G modulus of torsion rigidity of the intervertebral disc shows that the G values progressively increase through growth, which favours the progression of early scoliotic curves. At the same time, however, early structural scoliosis is more amenable to conservative treatment owing to the residual growth potential of the spine.
Whereas indications to surgical treatment of scoliotic curves has been based upon the magnitude of the curves as measured according to the Cobb method, two additional factors affect the chance of correcting a scoliotic curve,
The first is the residual growth potential of the vertebrae. In fact, a longer residual growth allows for external forces to be applied so as to change the growth model of the scoliotic spine, which ensures a stable correction of the deformity when these external forces are removed. The second is the magnitude of the elastic deformation of the intervertebral discs. It has been suggested that a deformation beyond the disc elastic behaviour, by producing hysteresis of the disc, renders the disc less susceptible to transferring the load to the neighbouring vertebral bodies, thus impairing remodelling.
It ensues that both the age and the magnitude of rotation affects the success of conservative treatment and not only the magnitude in Cobb degrees. The curve localization adds to these two parameters, thoracic curves being stiffer than thoraco- lumbar and lumbar curves.