The lumbar disc and low back pain.

The lumbar disc serves to sustain compression loads and is subject to tension and shear in forward bending and rotation. Its outer third is innervated and can be a source of pain. The annulus fibrosus may be injured in rotation and flexion of the lumbar spine and may become symptomatic as a ligamentous injury. Compression injuries of the disc are initially asymptomatic but may set in train a degradative process that, in time, leads to internal disc disruption, which becomes symptomatic as a result of chemical or mechanical irritation of nociceptors in the annulus fibrosus. Disc prolapse is but one possible end stage of internal disc disruption and represents the culmination of a series of destructive processes affecting the disc. This condition can be symptomatic while the external appearance of the disc remains normal and before nerve roots are affected in any way.