Is sciatica only caused by a herniated disc?

No. First, sciatica is a bit of a misnomer. Strictly, sciatica means pain in the distribution of the sciatic nerve. In medicine, sciatica commonly is used to describe lower extremity pain caused by the irritation of the nerve roots that form the sciatic nerve. This can be caused by a herniated disc or anything that puts pressure on the nerve root; such as spinal stenosis, severe facet arthritis, bone spurs, tumor, or cyst. Any of the above can cause inflammation of the involved nerve roots that typically will cause low back and lower extremity pain or “sciatica”.