A model for studies of the deformable rib cage.

An earlier model for the study of rib cage mechanics was modified so that rib deformity in scoliosis could be better represented. The rigid ribs of that model were replaced by five-segment deformable ribs. Literature data on cadaver rib mechanical behavior were used to assign stiffnesses to the new individual model ribs so that experimental and model rib deflections agreed. Shear and tension/compression stiffnesses had little effect on individual rib deformation, but bending stiffnesses had a major effect. Level-to-level differences in mechanical behavior could be explained almost exclusively by level to level differences in the rib shape. The model ribs were then assembled into a whole rib cage. Computer simulations of whole rib cage behaviors, both in vivo and in vitro, showed a reasonable agreement with the measured behaviors. The model was used to study rib cage mechanics in two scolioses, one with a 43 degrees and the other with a 70 degrees Cobb angle. Scoliotic rib cage deformities were quantified by parameters measuring the rib cage lateral offset, rib cage axial rotation, rib cage volume and rib distortion. Rib distortion was quantified both in best-fit and simulated computer tomography (CT) scan planes. Model rib distortion was much smaller in best-fit planes than in CT planes. The total rib cage volume changed little in the presence of the scolioses, but it became asymmetrically distributed.