The anatomy of spinal deformity: a biomechanical analysis

The essential lesion in idiopathic scoliosis is a lordosis at the curve apex. For the rotational instability provided by a lordosis to progress, asymmetry must be present in another plane. A biomechanical analysis of spinal shape demonstrates a delicate balance between the median, transverse, and coronal planes. The normal cervical and lumbar lordosis which are inherently rotationally unstable are protected by: a) prismatic shaped vertebral bodies with their bases anterior, b) considerable available intersegmental flexion, and c) powerful posterior soft tissue support. In contrast, the thoracic vertebral bodies are shaped as prisms with their apices anterior. This rotationally unstable configuration is protected by a kyphosis with the axis of spinal rotation situated anteriorly. The thoracic vertebral prisms are asymmetric, their apices lying to the right of the median plane. In the presence of a lordosis the apices of the prisms will be directed toward the right producing a right-sided scoliosis. Any significant degree of left-sided coronal asymmetry can override the influence of the transverse plane and therefore left-sided curves are not uncommon. Thoracic idiopathic scoliosis is located at T8/T9 and the lordosis here is often an upward continuation of the normal lumbar lordosis such that the asymmetric thoracic prisms are no longer protected.