Aspects on the influence of the thoracic wall on the pathogenesis of idiopathic scoliosis

Experimental and clinical studies were performed in order to study the possible influence of the thoracic wall on the pathomechanism of adolescent idiopathic scoliosis, IS. Experimental longitudinal elongation of one rib resulted in a deviation of the vertebra in the frontal plane to the contralateral side of the elongated rib, an anterior displacement in the sagittal plane and a rotation towards the convexity of the curve, thus a deviation identical to the 3-D configuration of the spine seen in IS.

Study I. The very early 3-D movements of a short segment of the spine were studied, during progressive increase of the length of one rib in two pigs. An instantaneous and simultaneous deviation of the spine in all three planes was registered, identical to the 3-D configuration of the spine seen in IS. Other movements of the vertebrae were also registered that could explain the typical structural changes seen already in patients with early IS.

Study II. CT-scans of patients with normal spines were examined and it was found that in a majority of normal spines the vertebra is rotated towards the right and that the left pedicle is slightly longer than the right. It seems thus as if the normal spine is predisposed to rotate towards the right, a characteristic finding in IS.

Study III. The position of the aorta in relation to the vertebra was studied on CT-scans of patients with right convex idiopathic scoliosis and of patients with no signs of spinal disorder. A marked posterior displacement of the aorta in relation to the vertebra was documented in the scoliotic patients followed by a significant increase of the distance from the aorta to the sternum along the thoracic wall on the right, compared to the left side. This increase of length, which is assumed to reflect the length of the intercostal arteries, could explain earlier documentations of anatomical asymmetries between the right and left side of the thorax seen in IS.

Study IV. The rib-vertebral angles, RVA:s, of the spines of patients with adolescent idiopathic scoliosis and of others with neuromuscular scoliosis were analyzed. The angles were measured and found to follow the same pattern in these two types of scoliosis with presumably different etiology and with muscular forces exerted on the spine that apparently must be different. The RVA:s of these two clinical groups were compared with the RVA patterns obtained from three groups of rabbits where spinal curvatures were induced experimentally, in group I, by unilateral electrostimulation of three muscle groups of the thorax and back, in group II, by manual bending of the spine of two dead rabbits and in group III, by mechanical rib elongation of one rib. In all these experimentally induced curvatures, the patterns of the RVA:s were similar to the patterns of the patients in the clinical material. Together with the results of a previous study where statistical differences of the RVA:s of the concave and the convex sides were documented only in curves beyond 8¡, it is concluded that the importance of the muscles for the development of the RVA:s seems questionable and that the angles develop regard less of the magnitude of underlying muscular dysfunction.

Study V. From these studies it is concluded that the muscles of the thorax probably do not initiate the typical curvature of the spine seen in IS. The normal spine seems to be predisposed to rotate towards the right and an unknown factor further accentuates this rotation and also initiates a frontal deviation of the spine. Rib elongation could be of importance for this deterioration since experimental and clinical findings suggest that it could result, not only in the typical 3-D configuration of the scoliotic spine but also in early structural changes of the vertebrae seen in this condition. The asymmetry of the vascular anatomy of the normal thorax, which further increases during the progression of the scoliotic deformity, could explain the initiation and further accentuation of the scoliotic deformity.