Mechanical properties and failure mechanics of the spine under posterior shear load: observations from a porcine model.

Few studies have focused on failure patterns of the entire motion segment and on the function of the individual structures resulting from dynamic posterior loading. The current study investigated the mechanical properties of the motion segment under posterior shear loading and documented the resulting injuries. Twenty-six porcine motion segments were separated into three groups (whole segments, no posterior ligaments, and no posterior ligaments and facet joints). The specimens were tested under posterior shear loading to failure in a custom-designed jig adapted to a testing machine. Load-deformation curves were collected to obtain the following parameters: the energy absorbed to failure, the deformation at failure, the ultimate shear load, and the stiffness. The disk was found to carry approximately 1,540 N (74% of the intact specimen) of the ultimate load. Stiffness was dominated by the intervertebral disk complex, accounting for approximately 85% of the average stiffness. The predominant injuries resulting from posterior shear loading were avulsions of the end plate.