A synovial joint is a joint in which the opposing bone surfaces are covered by cartilage and separated by a free space filled with a small quantity of synovial fluid produced by synovial membrane.
Synovial fluid contains nutrients for the cartilage (which is not vascularised and is provided with nutrition by diffusion) and serves as a joint lubricant. The synovial membrane is normally a one cell layer thin inner cover of the joint capsule. It is well vascularised, but poorly innervated. The joint is surrounded by a capsule.
In synovitis such as in rheumatoid arthritis, synovial fluid may be abundant and clearly detectable on physical examination.
The function of synovial fluid is