Medical illustration showing a lumbar disc herniation at the level of L4-L5 compressing the nerves of the ponytail. The lumbar disc herniation (contents of the nucleus of the vertebral disc that protrudes behind the vertebra in the vertebral canal and compresses the root of the nerve, causing inflammation) represents 90% of the causes of sciatica. In this illustration the hernia on the left is represented on a superior view of a lumbar vertebra. On the right, it is represented on a median sagittal section of the lumbar spine and the beginning of the sacred column .