Herniated nucleus pulposus (HNP) in the lumbar spine is a type of spine degeneration that can cause lower back pain and sciatica. The intervertebral discs that normally cushion the spine are thick, spongy pads. The outer layer of these discs is a tough fibrous material, known as the annulus fibrosis, and the inner layer is a gel-like fluid called the nucleus pulposus. When a disc weakens and a tear develops in the outer walls, the nucleus pulposus can rupture into the spinal column, potentially causing pain if the material comes into contact with the spinal cord or a nerve root.