Ageing and degenerative changes of the intervertebral disc and their impact on spinal flexibility. 
Degeneration of the intervertebral disc is associated with various morphological changes of the disc itself and of the adjacent structures, such as reduction of the water content, collapse of the intervertebral space, disruption and tears, and osteophytes.  These morphological changes of the disc are linked to alterations of the spine flexibility.  This paper aims to review the literature about the ageing and degenerative changes of the intervertebral disc and their link with alterations in spinal biomechanics, with emphasis on flexibility.  Narrative literature review.  Clinical instability of the motion segment, usually related to increased flexibility and hypothesized to be connected to early, mild disc degeneration and believed to be responsible for low back pain, was tested in numerous in vitro studies.  Despite some disagreement in the findings, a trend toward spinal stiffening with the increasing degeneration was observed in most studies.  Tests about tears and fissures showed inconsistent results, as well as for disc collapse and dehydration.  Vertebral osteophytes were found to be effective in stabilizing the spine in bending motions.  The literature suggests that the degenerative changes of the intervertebral disc and surrounding structures lead to subtle alteration of the mechanical properties of the functional spinal unit.  A trend toward spinal stiffening with the increasing degeneration has been observed in most studies.