How does a herniated disk cause sciatica?

A herniated disk causes sciatica through the mechanical displacement of the disk's internal structure. In the lumbar region of the spine, the soft, pulpy center of an intervertebral disk—known as the nucleus pulposus—can push through tears in the disk's fibrous outer ring.12

When this displacement occurs, the protruding disk material impinges upon, or compresses, the adjacent nerve roots. Because the sciatic nerve is formed from branches of these lumbar and sacral nerve roots, this compression results in radiating pain, tingling, or numbness that travels along the course of the nerve from the lower back down the legs. This specific manifestation of nerve root irritation caused by disk pathology is clinically referred to as sciatica.1234