The intervertebral disc is a structure made up of two parts that sits between the bones of the spine, or vertebrae, and acts as a sort of 'shock absorber'. The two parts of the disc are the anulus fibrosus, a ligament, and the nucleus pulposus, a jelly like substance. The disc is, almost literally, formed like a jelly doughnut with the ligament on the outside and the jelly in the center.
Because the nucleus has a very high water content, it can't be compressed but, rather, any pressure placed on it is redistributed, evenly, around the interior of the ligamentous anulus fibrosus. This is how it redistributes and diffuses compressive energy to act as a 'shock absorber'.
In some breeds of dogs, though, the nucleus pulposus isn't normal. These dogs are called chondrodystrophic or hypochondroplastic. Several breeds are predisposed to this condition and, therefore, to intervertebral disc disease. This includes the dachshund, poodle, basset hound, and more. When the nucleus pulposus degenerates in these animals it loses water content or 'dries out'. After degeneration, the disc can no longer act as a shock absorber and so the ligament tears and the jelly, or nucleus pulposus, pushes out of the center of the disc and into the vertebral canal to cause compression of the spinal cord or the nerve roots. This is also called a 'slipped disc'.
Intervertebral disc disease can be treated with medicine or with surgery. Which option is best depends on several factors, but the prognosis for recovery is almost always much better with surgery. In addition, there are procedures, called fenestrations, that we perform at surgery that lower the chances for another disc problem in the future.