Despite numerous years of dedicated research into the origin of idiopathic scoliosis, the pathogenesis of this classic orthopaedic disorder has so far remained elusive. A striking feature of idiopathic scoliosis is the fact that it does not occur in vertebrates other than humans, despite many similarities in the basic architecture of the vertebral column across the species. From recent work by Castelein et al, it was demonstrated that the fully erect posture, which is unique to humans, significantly alters spinal loading conditions. It was shown that the backward inclined or 'declive' segments of the human spine are subject to dorsal shear loads. These dorsal shear loads are an important distinction between the human spine and the spine of all other vertebrates. In this thesis, it was postulated that excessive dorsal shear loads during the period of rapid growth, could lead to rotational instability of the declive segments of the spine. If progressive, these dorsal shear loads not only reduce rotational stability, but also can enhance a slight preexistent vertebral rotation. 1. In a biomechanical in vitro study on porcine and human spinal segments, we showed that at the mid and lower thoracic levels, significantly more vertebral rotation occurred under dorsal shear loads than under ventral shear loads. These data show that, in humans and in quadrupeds, the thoracic spine is less rotationally stable under dorsal shear loads than under ventral shear loads. We believe that these dorsally directed shear loads can, under critical circumstances during growth, introduce vertebral rotation, whereas ventrally directed loads counteract rotation. 2. The normal, nonscoliotic human spine shows a preexistent pattern of rotation in the transverse plane. The thoracic vertebrae are predominantly rotated to the right side, similar to what is seen in adolescent idiopathic scoliosis. We believe that if the spine starts to decompensate into scoliosis, it seems logical that this built-in rotational pattern will be followed. 3. In neuromuscular scoliosis, it was demonstrated that despite the difference in origin, curve patterns in neuromuscular disease show many similarities with those in idiopathic scoliosis, such as a strong prevalence of right convex thoracic curves. We concluded that preexistent rotation, rather than muscle imbalance, also plays an important role in directing subsequent rotation in neuromuscular disease, as it does in idiopathic scoliosis. 4. We also showed the existence of a predominant rotation of the thoracic vertebrae to the right side in the quadruped spine, similar to what was found in humans. It was concluded that preexistent rotation can be considered as a physiological process in the normal development of the spine, independent of the pathogenesis of idiopathic scoliosis. 5. Finally, we demonstrated that the normal, nonscoliotic spine of humans with a complete mirror image reversal of the internal body organs (situs inversus totalis) shows a preexistent pattern of vertebral rotation opposite to what is seen in humans with normal organ anatomy. It was concluded that the asymmetrical anatomy of the thoracic organs is likely to play an important role in the development of this preexistent rotational tendency
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