wiseGEEK: What Is the Synovial Membrane?

The synovial membrane is a type of soft tissue found in certain joints in the body. Also called the synovium, this membrane helps joints move more freely and plays a role in protecting against from wear and tear. Inflammation, cancer and and other disorders can sometimes affect this membrane and cause significant joint problems.

A joint is an area where two separate bones meet and are connected together by soft tissues. In certain joints, a thin tissue called synovial membrane is present in the space between the bones. It often covers many different joint structures, including tendons and ligaments, which hold bones and muscles together, as well as the ends of the bone that face inward toward each other. Joints that contain this membrane are generally called synovial joints.

Inside a synovial joint, layers of membrane often form a sac, similar to a balloon. The sac is usually filled with synovial fluid. This is a thick fluid secreted by the synovial membrane itself and designed to help lubricate the joints, allowing them to move freely without the bones rubbing against each other. Joints that contain this membrane and fluid, therefore, are generally those that are meant to have wide ranges of movement, including the ankles, knees, toes, fingers, wrists and hips. While the membrane can hold this thick fluid inside of it, it is otherwise permeable to many smaller chemicals in the body, such as oxygen and nutrients, which help keep the membrane and other joint structures alive and healthy.

Like most other body tissues, the synovial membrane is at-risk for some possible problems and diseases. One of the most common disorders is irritation and inflammation of the membrane, also known as synovitis. Sometimes synovitis occurs for no known reason, and similarly, resolves on its own; other times, however, it can occur due to an underlying disease, such as rheumatoid arthritis. In this form of arthritis, the immune system often mistakes the synovium as a foreign invader. The immune system mounts an attack against the membrane, causing it to thicken and swell inside the joint, which can lead to pain, limited movement of the joint and eventual erosion of bone tissue.

Other synovial disorders often tend to cause similar symptoms. For example, synovial sarcoma — a rare form of cancer where tumors grow on the synovial membrane inside a joint — often causes pain and swelling. Similarly, scleroderma, a condition that affects many of the soft tissues of the body, sometimes including the synovium, can cause stiffness and discomfort in the joints it affects. These disorders further highlight the essential function of the synovial membrane: creating a lubricative environment to protect and support movement in key articulating joints.

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