The Double Rib Contour Sign (DRCS) in lateral spinal radiographs: aetiologic implications for scoliosis

All lateral spinal radiographs in idiopathic scoliosis show a DRC sign of the thoracic cage, a radiographic expression of the rib hump. The outline of the convex overlies the contour of the concave ribs. The aim of this study is to assess this DRC sign in children with and without Late Onset Idiopathic Scoliosis (LOIS) with 10 degrees -20 degrees Cobb angle, and to examine whether in scoliosis the deformity of the thorax or that of the spine develops first.

The radiographs of 133 children referred to hospital in a school screening study were examined. There were 47 boys and 86 girls, 13.28 and 13.39 years old respectively. The Cobb angle was measured and the radiological lateral spinal profile (LSP) was appraised from an angle made by a line drawn down the posterior surface of each vertebral body (T1-L5) and by the vertical. The children, boys and girls, were divided in 5 groups, namely: 1) with straight spines, 2) with spinal curvature having a Cobb angle <10 degrees, 3) with thoracic, 4) thoracolumbar and 4) lumbar curves 10 degrees -20 degrees. For quantification of the DRC sign, the "rib index" was defined as d1/d2 ratio, where dl expresses the distance from the most extended point of the most projecting rib contour (RC) to the posterior margin of the corresponding to point vertebra and d2 expresses the distance from the posterior margin of the same vertebra to the most protruding point of the least projecting RC. In a symmetric and non-deformed thorax, these two RC lines are superimposed and the "rib index" is 1.

The statistical descriptive of d1 and d2 in boys and girls are presented together because they are not statistically different. There are no sex differences of the "rib index" which is 1.45, 1.51, 1.56, 1.59, 1.47 for the 5 respectively aforementioned groups. According to statistical analysis, there is no correlation of the Cobb angle with the "rib index" of thoracic, thoracolumbar and lumbar scoliosis groups. The DRC sign is present in all referrals and scoliotics. The data show a correlation of the "rib index" with each of T2, T3, T4, T5, T6 and T7 LSP in girls with lumbar curvatures.

The DRCS primarily appears because of the rib deformation and secondarily because of the vertebral rotation, as it could be present in straight spines with no vertebral rotation. In all our school-screening referrals, (having ATI > or = 7 degrees), the thorax deformity, in terms of the DRC sign, has already been developed. 70% of these children were scoliotic. The others had a curvature of less than 9 degrees of Cobb angle (10%) or they were children with straight spines (20%) who were followed because of their existing rib hump. The non-scoliotics were 1,5-2 years younger than the ones who had already developed scoliosis, and they had both approximately a "rib index" of 1,5. The DRC sign is present in all referrals. In contrary, there is no scoliotic spine without it, as the DRC sign is always present in scoliotic lateral spinal radiographs with no exception. This observation supports our hypothesis that in idiopathic scoliosis, the deformity of the thorax develops first and then the deformity of the spine follows.