Facet joints are 2 small joints located at each segment of the spine. These facet joints supplied by 2 medial nerves that carry pain signals from spine to brain.

Medial nerves named as cervical, thoracic & Lumbo sacral medical branch nerves depending on segment of spine.

This medial branch nerves don’t have role in muscle function or carrying sensation from arms or legs to brain so there is no harm with this injection.