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Injuries to the intervertebral disc occur when compressive or torsional strain is applied to the disc, which in turn damages either the annulus or the endplate. Injuries may occur due to repeated micro trauma rather than a single traumatic event. The outer third of the anulus fibrosis in the posterolateral portion of the lumbar disc and the endplate adjacent to the nucleus pulposus contain nerve fibers sensitive to pain. The development of disc pain is thought to progress as follows:

• Dense ingrowth of small unmyelinated nerve fibers, granulation tissue and vascular tissue occurs in degenerated discs in response to injury.
• Changes in the matrix of the nucleus pulposus result in loss of hydrostatic pressure, which in turn leads to buckling of annular layers.
• Increased mobility and sheer stress to the annular wall leads to fissuring.
• An inflammatory reaction develops producing long-term chemical changes.
• Tissue breakdown is accelerated.
• Destruction of glycoproteins within the disc makes them more susceptible to injury.
• Radial tears develop which cause back pain if they reach the outer third of the anulus.

Understanding the patho-anatomy of lower back pain can help physicians engaged in the process of differential diagnosis.