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

Sammanfattning: Experimental and clinical studies were performed in order to study the possible influence ofthe thoracic wall on the pathomechanism of adolescent idiopathic scoliosis, IS..Experimental longitudinal elongation of one rib resulted in a deviation of the vertebra in thefrontal plane to the contralateral side of the elongated rib, an anterior displacement in thesagittal plane and a rotation towards the convexity of the curve, thus a deviation identicalto 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, duringprogressive increase of the length of one rib in two pigs. An instantaneous and simultaneousdeviation of the spine in all three planes was registered, identical to the 3-D configuration ofthe spine seen in IS. Other movements of the vertebrae were also registered that couldexplain 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 ofnormal spines the vertebra is rotated towards the right and that the left pedicle is slightlylonger than the right. It seems thus as if the normal spine is predisposed to rotate towardsthe 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 withright convex idiopathic scoliosis and of patients with no signs of spinal disorder. A markedposterior displacement of the aorta in relation to the vertebra was documented in thescoliotic patients followed by a significant increase of the distance from the aorta to thesternum along the thoracic wall on the right, compared to the left side. This increase oflength, which is assumed to reflect the length of the intercostal arteries, could explainearlier documentations of anatomical asymmetries between the right and left side of thethorax seen in IS. Study IV.The rib-vertebral angles, RVA:s, of the spines of patients with adolescent idiopathicscoliosis and of others with neuromuscular scoliosis were analyzed. The angles were measuredand found to follow the same pattern in these two types of scoliois with presumably differentetiology and with muscular forces exerted on the spine that apparently must be different. TheRVA:s of these two clinical groups were compared with the RVA patterns obtained fromthree groups of rabbits where spinal curvatures were induced experimentally, in group I, byunilateral electrostimulation of three muscle groups of the thorax and back, in group II, bymanual bending of the spine of two dead rabbits and in group III, by mechanical rib elongationof one rib. In all these experimentally induced curvatures, the patterns of the RVA:s weresimilar to the patterns of the patients in the clinical material. Together with the results of aprevious study where statistical differences of the RVA:s of the concave and the convex sideswere documented only in curves beyond 8¡, it is concluded that the importance of the musclesfor the development of the RVA:s seems questionable and that the angles develop regardlessof the magnitude of underlying muscular dysfunction. Study V.From these studies it is concluded that the muscles of the thorax probably do not initiate thetypical curvature of the spine seen in IS.The normal spine seems to be predisposed to rotate towards the right and an unknownfactor further accentuates this rotation and also initates a frontal deviation of the spine.Rib elongation could be of importance for this deteroration since experimental and clinicalfindings suggest that it could result, not only in the typical 3-D configuration of thescoliotic 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 increasesduring the progression of the scoliotic deformity, could explain the initiation and furtheraccentuation of the scoliotic deformity.