The membrane that covers the spinal cord and nerve roots in the spine is called the dura membrane and the space that surrounds this is the epidural space. Nerves travel through this to the back and legs. Irritation from a damaged disc or contact with the bony structure of the spine will inflame nerve roots in the epidural space, causing pain.
An epidural injection places anti-inflammatory medicine into this epidural space to decrease inflammation of the nerve roots, thereby reducing the pain in the back or legs. The epidural injection may help the injury to heal by reducing inflammation and may provide permanent relief or provide a period of pain relief for several months while the injury/cause of pain heals.