The normal human vertebral disc is surrounded by nerves in the tissues that surround it (the annulus fibrosis). A tear in this layer, through overloading the spine, may trigger the formation of new nerve endings in the injured area and these nerves may then be irritated by the chemicals within any spinal fluid leakage resulting from a herniated disc, producing pain. All discs degenerate with age but not all discs shown by MRI to be abnormal will produce pain. Indeed research shows almost half of people with no back pain have abnormal discs.
To aid diagnosis, discography is therefore usually used in patients who are being evaluated to determine a specific cause of pain so a new treatment plan, possibly including surgery, can be developed. Because discography is an invasive procedure - it involves putting needles into the disc - it is not performed early in the diagnostic and treatment process. Contrast (a liquid that shows up on x-ray), is injected into the centre of each disc. If the disc is normal, the contrast remains in the centre of the disc. If the disc is abnormal, the contrast spreads through the tears in the disc. CT scanning is often performed after disc injection.