Fig. 6

Comparison of the changes of the axial effective stress in the posterior annulus subjected to different load cases for all models. Compared with the changing rates of axial effective stress in the center of the nucleus under Cases 2 to 4, the changing rates of axial effective stress in the posterior annulus under Cases 2 to 4 were low. a Grade 0, b Grade 1, c Grade 2, d Grade 3