Between each of the vertebrae of the spine rests a cushion known as an intervertebral disc. Like any other cushion, this disc is composed of a soft, inner portion that is encased by an outer shell. A disc bulge occurs when this shell ruptures, and the inner portion of the disc bulges out. A disc herniation is a more serious condition in which this disc material bulges out and moves more significantly backward toward the spinal cord. Either condition leads to pain by causing compression of the nerves of the spine.
Disc bulges and herniations are most frequently caused by disc degeneration due to aging (in which the discs dry up and become less flexible) or trauma (such as with an auto accident or sports injury). Trauma is not always a sudden experience; over time, poor posture can also injure the discs of the spine.