The knee joint is lined with a very thin tissue of specialized cells that produce synovial fluid. This fluid provides nutrition and lubrication to the joint. If the lining becomes swollen and inflamed this is called synovitis. The inflamed synovium can lead to a large amount of fluid being produced and result in a swollen knee (irritation, swelling and thickening with extra fluid production). Synovitis is usually painful when the joint is moved but can cause pain at rest as well. There are many causes of knee joint synovitis.
The synovial membrane of the knee joint is the largest and most extensive in the body. It starts at the upper border of the kneecap and travels beneath the Quadriceps muscle (front thigh muscle) on the lower part of the front of the thigh bone (femur). There is often a connection to the bursa between the tendon and the front of the femur. The pouch of synovial membrane between the Quadriceps and front of the femur is supported, during the movements of the knee, by a small muscle, the Articularis genu. It travels on boths sides of the knee cap and is separated from the patella tendon by the infrapatellar pad. The synovial membrane passes downward from the femur, lining the capsule to the point where is attached to the menisci (cartilages, shock absrobers). At the back part of the lateral (outside) meniscus it coats the tendon of the Popliteus and passes across the front of the cruciate ligaments (which are therefore situated outside the synovial cavity).
Synovial fluid is a transparent, viscous fluid produced by the synovial membrane and found in joint cavities, bursae and tendon sheaths. A normal knee has less than 5ml of fluid in it but a very swollen knee can have 200ml or more. Analysis of synovial fluid from a joint can confirm or rule out various joint problems such as osteoarthritis, gout, pseudogout, lupus, infection and rheumatoid arthritis. Synovitis is more commonly found in rheumatoid arthritis than in other forms of arthritis.
Synovitis causes joint tenderness, pain, swelling and redness (erythema). It can be difficult to differentiate between the inflammation of synovitis and infection in the joint. Removing some of the fluid and sending it to the laboratory may be the only way to decide (joint aspiration looking for bacteria or crystals).
The symptoms of synovitis are often relieved with anti-inflammatory drugs (NSAIDs). A cortisone injection can be given into the knee which acts as a direct anti-inflammatory agent. Certain types of arthritis respond to medical therapies which are usually provided by a rheumatologist.
The cause of rheumatoid arthritis is unknown but autoimmunity definitely plays a role in its chronicity and progression. It is considered a systemic autoimmune disease and creates masses of inflammatory cells in the joint. The joint looks swollen and feels puffy or boggy to the touch and since there is increased blood flow the joint warm. The cells release enzymes into the joint space which causes further pain and irritation. The synovial lining and the enzymes gradually ‘digest’ the cartilage and bone of the joint leading to chronic pain and destroying the joint.