Characterization of the mechanical behaviour parameters of the costo-vertebral joint.

This in vitro study introduces a new method to determine quantitative parameters characterizing the mechanical behaviour of the costo-vertebral joint. These parameters are useful in building numerical models of the thoracic spine, taking into account the thoracic cage. Nine thoracic cages were isolated from fresh human cadavers. From each cage, three functional units were tested: T1-T2, T5-T6, T9-T10. Loads were applied according to the joint local coordinate system. Every functional unit was tested first intact and again after section of successive costo-transverse ligaments. We used an opto-electronic system to follow the three-dimensional motion of the joint, and obtained non-linear load/displacement curves according to the primary rotation axis. A statistical analysis of these curves allowed the calculation of parameters describing the joint mechanical behaviour: total range of motion, motion in the low-stiffness zone, and flexibilities in the positive and negative quasi-linear zones. These values can be used as a database for mechanical modeling of the spine.