Measurements of the three-dimensional shape of the rib cage

A stereoradiographic method was developed to measure the three-dimensional shape of the rib cage in vivo in order to provide descriptive data and to study symmetry in the normal population. The method is also intended for use in description of rib asymmetry in scoliosis. Rib midlines were reconstructed from digitized points on lines drawn through the middle of each rib image in stereo-radiographs of the rib cage. The method relied on pairing points in one image with a corresponding point found in the second image. The error term obtained from a Direct Linear Transformation (DLT) reconstruction program was used for optimizing this correspondence. The method was used on test objects of known shape where the standard deviation of measurement errors was found to be 0.72 mm. Studies in which different sets of points on the rib midlines were selected for digitizing showed that the precision of the method was 1.04 mm (S.D.). Rib shape was described by intrinsic measurements (arc and chord length, maximum curvature and enclosed area) and rib orientation by extrinsic measurements (frontal and lateral angulations and posterior rib cage rotation). No statistically significant rib shape asymmetry was found among ten subjects without spinal or thoracic abnormalities, although a trend of inequality of rib angulation at all anatomical levels was observed.