Disk degeneration and disk herniation: the contribution of mechanical stress.

Experimental studies on the role for mechanical stresses in the genesis of disk degeneration and herniation are reviewed. Simple mechanical stimulations of functional vertebral segments cannot cause a disk herniation: a complex mechanical stimulation combining forward and lateral bending of the spine followed by violent compression is needed to produce posterior herniation of the disk. Intervertebral disk degeneration seems to influence the development of posterior disk herniation or foraminal disk protrusion. Furthermore, direct mechanical stimulation of the disk tissue or cells generates complex metabolic and cellular responses that lead to qualitative and quantitative modulation of disk matrix proteins. Thus, it is becoming increasingly likely that physical and metabolic factors act in concert to produce disk herniation.