It's the synovium that allows for smooth movement of the joints.
From the latimes.com
The synovium grows abnormally and invades cartilage and bone, causing them to soften and crumble.
From the sciencedaily.com
Schett, G. Cells of the synovium in rheumatoid arthritis.
From the nature.com
More examples
Synovial membrane: a thin membrane in synovial (freely moving) joints that lines the joint capsule and secretes synovial fluid
(synovial) relating to or secreting synovia
Synovial membrane (or synovium) is the soft tissue that lines the non-cartilaginous surfaces within joints with cavities (synovial joints).
(synovial) pertaining to synovia, the lubricating fluid of joints.
(synovial) adjective, Greek syn = with, and ovum = egg; hence pertaining to the fluid in freely movable joints resembling egg-white.
A thin layer of connective tissue with a free smooth surface that lines the capsule of a joint. Synovial fluid lubricates and facilitates movement of the joint.
A soft tissue present in varying quantities in normal healthy joints. It produces synnovial fluid, a biological lubricant.
A fibrous envelope that produces a fluid to help to reduce friction and wear in a joint.
This is an inflammation of the lining in spinal joints.