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Abstract
The overall hypothesis of this work was that rib asymmetrical growth can cause the initial instability of the spine in scoliosis. A new stereoradiographic method was developed to measure rib shape of patients with adolescent idiopathic scoliosis. A standard posterior-anterior x-ray view and a second view with the tube moved up, forming an angle of 20$\sp\circ$ was used to make a stereo pair of the rib cage. Rib midline images were fitted by cubic spline functions and an iterative procedure was used for optimizing the correspondence between points on both rib images. Accuracy studies showed a mean error less than 1.5 mm (S.D. = 1.1 mm; maximum error $<$ 6 mm). The shape of ribs was described by "intrinsic" measurements and their orientations by "extrinsic" measurements.
Thirty patients with right thoracic scoliosis were compared to a control group of 10 subjects. Both intrinsic and extrinsic rib asymmetries were generally correlated with the lateral deviation of the vertebra along the spine. The maximum rib asymmetries were found at or close to the level of spinal apex. The majority of the patients had longer rib on the convex side but no significant difference was found compared to the symmetrical case and the control group. No significant correlation was found between Cobb angle and rib arc length asymmetries while the extrinsic rib asymmetry measurements and the posterior curvature correlated with the severity of the spinal curvature. In general, rib asymmetry measurements correlated within themselves.
A two-dimensional finite element model simulating growth and bone remodeling, was developed to study the effect of rib growth asymmetries on the vertebral displacement and rib shape deformation. This model was used to simulate the mean rib arc length asymmetry (6.3%) measured in the 16 patients with longer ribs on the right (convex) side of the scoliosis. The effect of the adjacent rib anatomical levels, bone remodeling and lateral forces were simulated. The growth asymmetry simulation produced rotation and lateral deviation of the vertebra similar to that seen in the patients. This model simulated a mechanism of rib asymmetrical growth causing scoliosis. However, the clinical results did not confirm the hypothesis that rib asymmetrical growth is the primary cause of scoliosis, suggesting that rib cage asymmetries are secondary.
