When two bones come together to form a joint, the scraping of the bones against each other would quickly become painful without some type of padding or lubricant. The synovial membrane provides that padding and lubricant. It is a tissue that acts as a cushion, but more importantly, the membrane secretes a lubricant that allows the two bones to move freely against each other. Just as a car engine needs oil to keep its moving parts from wearing out or freezing up, the movable joints of the body need to be kept well oiled.
The synovial membrane is found only in synovial joints, which are the most common body joints in humans. Types of synovial joints include the knees, elbows, shoulders, wrists and hips. Although some synovial joints have a greater range of motion than others do, they are all able to move to some degree. The ability of the joints to move is directly proportional to the risk that the joints could be injured more easily. In other words, the greater movement is possible, the greater the chance of injury, such as seen in the knee.
Human knees have the greatest range of motion of any joints in the body. As their structure is representative of all synovial joints, an explanation of the structure of knees may be helpful. The knee is where the thigh and shin bones meet; the intersection is covered by the kneecap or patella, which in turn connects to the joint capsule. Within the joint capsule, synovial fluid, produced by the synovial membrane, lubricates the ends of the bones and the cartilage between them.
This lubrication is essential for ease of movement and the prevention of joint pain. Certain diseases, such as rheumatoid arthritis, can make the body's own defenses attack healthy areas. When the synovial membrane is subjected to such an attack, it cannot produce sufficient synovial fluid, resulting in decreased mobility and increased pain. As the synovial membrane also functions as a seal to keep the fluid in the joint, a damaged membrane can result in leakage into areas where it does not belong.
If the leakage is too great, or the synovial membrane is damaged too severely, the cartilage in the joint may not receive an adequate supply of blood and other nutrients. When this happens, the cartilage may literally starve to death. It is also possible that the body may respond by releasing enzymes that can consume the cartilage, resulting in further pain and immobility in the joint.