Structural characterization of human rib cage behavior under dynamic loading

Tests were conducted on four rib cages to characterize their 3D deformation and the kinematics of the costovertebral joints. The influence of the structural properties, i.e. geometry and initial rib slopes, and of the costo-vertebral joints on the deformation capacity of the rib cage during a dynamic loading was studied. Each rib cage was loaded four times by increasing, up to 40% of the initial thickness, the mid-sternum deflection. The spine was rigidly fixed without constraining the costo-vertebral joints and the rib motions were computed from 3D video analysis. In addition, numerical simulations were performed with subject-specific models obtained from the rib cage geometry and a method for model personalization. The objective was to numerically evaluate the sensitivity to solely changes in geometry. The rib rotations were determined from the motion of 3D-markers close to the costo-vertebral joints and the 3D rib deformations were assessed from the motion of markers along the ribs. The rib rotations varied with the costal level (mean value 5.8 degrees [max. 7.9 degrees, min. 3.5 degrees], 2.9 degrees [4.8 degrees, 1.0 degree], 2.5 degrees [4.8 degrees, 1.1 degrees] and 2.2 degrees [3.5 degrees, 0.8 degree] for rib 2, 4, 6 and 8 respectively) and among the subjects (mean variation from 3.3 degrees to 7.1 degrees). The rib deformations were mainly in the sagittal plane for the upper ribs and in the rib plane for the lower. Although, no statistically significant correlations were found with different morphometrics parameters, a link (R2>0.4) was found between the initial rib slope and the amount of rotation and deformations, according to the assumption described by Kent et al (2005). The costovertebral joint was described by a functional rotation axis (i.e. helical axis) that does not correspond to the physiological axis of rotation. The orientation and the position of this helical axis changed with the level of deflection and varied with the costal level.