What is a 4mm synovial cyst?

Answer 1

Synovial cysts are benign, fluid-filled sacs that develop in the facet joints of the lumbar spine as a result of degeneration. If large enough, these sacs can cause spinal stenosis - a narrowing of the spinal canal that places pressure on spinal nerves and causes pain.

The synovium is a thin film of tissue that produces fluid to help lubricate the joints. When facet joints in the lumbar spine begin to degenerate, this fluid may build up in an attempt to protect the joint. In some patients, small amounts of the fluid escape from the joint capsule but remain within the synovium, creating a sac-like protrusion. These cysts are not under tremendous pressure and, even if quite large, rarely cause neurological problems or cauda equina (loss of bowel or bladder function). Many older patients have synovial cysts in their lumbar spines but have no symptoms. Occasionally, however these cysts can cause pain in the lower back that travels down the legs. The pain is relieved when sitting as this position widens the spinal canal and relieves pressure on the nerves.