Your shoulder joints are held together by a " [no-glossary]capsule[/no-glossary] " of tissues and [no-glossary]ligaments[/no-glossary]. The innermost tissue of the [no-glossary]capsule[/no-glossary] is a membrane called the synovium.

The synovial membrane secretes a clear fluid called [no-glossary]synovial fluid[/no-glossary] that lubricates joint surfaces and provides the cartilage with nutrients. Sometimes this [no-glossary]capsule[/no-glossary] becomes inflamed. The result is the painful condition called synovitis.

Here's what happens when synovitis occurs:

• Disease (such as rheumatoid arthritis) or injury causes white blood cells to move from your blood stream into your synovium
• The synovium cells grow and divide abnormally. Fluid collects as the synovium becomes thickened and inflamed
• The synovial cells release enzymes
• The enzymes may eventually destroy joint cartilage and bone, as well as surrounding [no-glossary]muscles[/no-glossary], [no-glossary]ligaments[/no-glossary], and [no-glossary]tendons[/no-glossary]