The synovium (or synovial membrane) is the thin, richly-supplied tissue that lines the inside of every movable joint and produces a small amount of slippery synovial fluid to lubricate cartilage. In inflammatory arthritis — most classically rheumatoid arthritis — the immune system attacks this lining, so it thickens, floods the joint with fluid and releases enzymes that gradually erode cartilage and bone. Ultrasound and MRI can pick up an inflamed synovium (synovitis) even before X-ray changes appear.
What is synovium?
Synovium: The synovium is the thin membrane lining the inside of a joint capsule. It produces synovial fluid, which lubricates and nourishes cartilage. In rheumatoid arthritis, the immune system attacks the synovium, causing swelling, warmth and long-term joint damage.
Last updated 2026-07-04. This is general information, not a substitute for personalised medical advice.