Relationship between intervertebral disc and facet joint degeneration: A probabilistic finite element model study.
Both intervertebral disc (IVD) and facet joint (FJ) degeneration are frequently associated with chronic low back pain. While genetic factors are considered the most relevant in the onset of degeneration, the mechanics play an important role in its progression. Degenerative changes in one of these two structures are believed to induce degeneration in the other. However, despite decades of research, there is no consensus on the mechanical interplay between the two structures. On the basis of a parametric finite element model of a human L4-L5 spinal motion segment, one thousand individual segments were probabilistically generated covering all grades of degeneration in both structures. The segments were subjected to combined compression and flexion/extension loads. Correlation matrices were created to identify the effect of individual degeneration parameters of each structure on the mechanical stresses in the corresponding counterpart. In the non-degenerated group, a strong positive and a moderate negative correlation was found between the strain of the capsular ligament and the disc height and the nucleus compressibility, respectively. With increasing degeneration, the correlation between IVD morphologies and the FJ loads gradually decreased, whereas the correlation between FJ morphologies and disc load gradually increased. The results suggest that early mechanical changes associated with IVD degeneration have the greatest effect on the FJ loading. With progression of degeneration, this effect is diminished, whereas the appearance of FJ degeneration increasingly influences the disc loading, which might indicate an increasing support of the disc degeneration.