Fibrosis is the main cause of ligamentum flavum hypertrophy, and fibrosis is caused by the accumulation of mechanical stress with the aging process, especially along the dorsal aspect of the ligamentum flavum. TGF-beta released by the endothelial cells may stimulate fibrosis, especially during the early phase of hypertrophy.

Accumulation of fibrosis (scarring) causes hypertrophy of the ligamentum flavum. Inflammation-related gene expression is found in the ligamentum flavum. It might be possible to prevent the hypertrophy of ligamentum flavum with antiinflammatory drugs.

Decreased elasticity of LF in the elderly is due to the loss of elastic fibers and a concomitant increase of collagenous fibers in the dorsal aspect. LF hypertrophy could be due to the thickening of the normal elastic layer as well as of the abnormal collagenous layer.

Spacers less stiff than the graft will: (1) provide stability similar to titanium cages in the presence of posterior instrumentation, (2) reduce the stresses in endplates adjacent to the spacers, and (3) increase the load transfer through the graft, as evident from the increase in stresses in graft.

The hybrid testing protocol is advocated because it better reproduces clinical observations in terms of motion following surgery, using pure moments. Using this approach, we found that the Charité artificial disc placement slightly increases motion at the implanted level, with a resultant increase in facet loading when compared to the adjacent segments, while the motions and loads decrease at the adjacent levels. However, in the load control mode that we believe is not that clinically relevant, there was a large increase in motion and a corresponding increase in facet loads, as compared to the intact.