Fig. 2

FE models of four different grades of degenerated discs. a Grade 0: healthy model with detailed view of the intervertebral disc included the collagen fibers which were embedded in the annulus ground substance in radial direction around the nucleus in eight layers, b Grade 1: mildly degenerated disc model, c Grade 2: moderately degenerated disc model, d Grade 3: seriously degenerated disc model