The etiology and pathogenesis of idiopathic scoliosis

Idiopathic scoliosis is a complex three-dimensional deformity and in the thoracic region the essential lesion lies in the sagittal plane in the form of an area of inappropriate lordosis. The thoracic kyphosis is normally protected from buckling by being behind the axis of spinal column rotation but when the thoracic lordosis develops it brings the apical region anterior to this axis and thus under compression with resultant buckling failure of the spinal column. The condition of idiopathic thoracic scoliosis is the opposite to idiopathic hyperkyphosis (Scheuermann's disease), the latter being rotationally stable and not moving out of the sagittal plane. The two frequently co-exist in the same spine with thoracic hyperkyphosis above an area of lumbar lordo-scoliosis. There is a spectrum of normal lateral profile and flat backs at the one end are in danger of buckling (lordo-scoliosis) while round backs at the other end of the spectrum are in danger of being defined as Scheuermann's disease. There is no requirement for a specific pathological process. Engineers describe only two ways in which a flexible column can fall into mechanically-angular collapse (kyphosis) and column buckling (lordo-scoliosis). A number of factors favour column buckling (Euler's law) and thus the bigger a deformity the more likely it will be to continue progressing and the taller and more slender the column the more likely it will be to fail and this we see in our patients with idiopathic scoliosis. Not only is lordosis the essential lesion but it is also the primary abnormality which can be demonstrated in children before lateral curvature and rotation develop.