Three-dimensional osseo-ligamentous model of the thorax representing initiation of scoliosis by asymmetric growth.

A biomechanical model of the human thorax was constructed to investigate how asymmetric growth of the thorax might initiate spinal lateral curvature and axial rotation as seen in scoliosis deformities. Geometric data specifying nodal points of the model were taken from stereo-radiographs of an adolescent subject. An initially symmetrical geometry was created by 'mirroring' measurements of a hemi-thorax and spine. Published data provided cross-sectional measurements of the ribs, material properties of tissues and global flexibilities of the intervertebral motion segments. The ribs, sternum, intervertebral motion segments and intercostal ligaments were represented by elastic elements. Model deformations were calculated by the direct stiffness finite element method, with growth represented by an initial strain term in the constitutive law. Non-linear behavior was accommodated by running the model recursively, with updated node locations at each step. Both stress relaxation and stress modulation of growth in the component tissues were simulated. Thoracic growth of 20% with asymmetric growth of the ribs was simulated to give rib length asymmetries of 11%. similar to that observed in a previous study of patients with idiopathic scoliosis. This resulted in the model having a small thoracic scoliosis curvature convex toward the side of the longer ribs. Variations of the model which permitted free motion at the costo-vertebral joints or produced changes in the curvature of the posterior parts of the ribs resulted in axial rotation of the vertebrae similar to that observed clinically. The model supports the idea that growth asymmetry could initiate a small scoliosis during adolescence.