A medial branch block is a potent anesthetic injection in the medial branch nerves, which supply the facet joints. The facet joints (or zygapophysial joints) are small interlocking bumps on the bony structure of the spine. In the presence of some acute chronic conditions, the facet joints become inflamed. In the lower back (lumbar) region pain usually manifests as an ache that radiates across the lower back, buttocks, and upper thighs. Prolonged standing or bending backwards often increases the pain. Inflamed facet joints in the neck can cause pain in the neck and shoulders, with pain increasing with head movement in any direction.