Part of disc
The disk between vertebrae has an annular fibrotic portion and a "jelly-like" center (nucleus pulposus
) that amortiguates the weight.
When the annular portion is broken (due to a combination of extreme forces and degeneration due to aging) the nucleus escapes and it can compress nerves or the spine. At the level of l4-5 (between the lumbar vertebras 4th and 5th) it can only compress nerves.