Biomechanical factors affecting progression of structural scoliotic curves of the spine.

The development of the spine is affected by both biological and mechanical factors. As the geometry of the motion segment changes throughout growth, so does the mechanical behaviour of the spine owing to changing vectors acting on a variable geometry system. The biomechanical analysis of the growing spine enables the assessment of the role played by biological and mechanical factors in the pathogenesis of progressive scoliosis to be assessed and its comparison with factors acting on an adult scoliotic spine. The knowledge of these principles is instrumental to setting the right parameters for treatment and to design braces that may be capable of obtaining correction of the deformity. The elastic behaviour of child and adult spine differs because of both physiologic and pathologic conditions. In child scoliosis an abnormal geometry causes a persistent stress concentration in crucial areas of the motion segment. This induces a progressive elasto-plastic strain which modifies the geometry of the motion segment, thus worsening the stress concentration and the excessive strain through a vicious cycle. In adult scoliosis, on the other hand, deformation primarily affects the viscous-elastic structures, namely the intervertebral disc and the capsulo-ligamentous structures. This produces instabilty of the motion segments and slow deformation of the vertebrae through remodelling. It therefore ensues that the aim of the treatment differs in both conditions. In the child spine the aim is to prevent the motion segment deformity by means of braces which adequately modifies the stress distribution acting on the spine, thus enabling the spine to grow according to a quasi-physiological model. In adult scoliosis a stable equilibrium is required in order to prevent further deformation of the motion segment.