The synovial membrane lines the knee joint and secretes synovia into the joint cavity. This membrane, like other tissues of the body, can become irritated and inflamed, resulting in chronic inflammation or a chronic infection, depending on the cause. Torn cartilage, joint mice (see section on loose bone or cartilage below), chondromalacia, overuse, and other causes account for these changes. When the membrane is irritated, it secretes more synovia, oftentimes causing the joint to swell, a condition commonly called "water on the knee," or synovitis.
Generalized swelling and pain, from mild to severe, accompany this reaction. Pain can be particularly severe on motion, and no weight may be borne by the knee.
Keeping off the knee should help, unless the cause is something other than overuse. If the cause is joint mice (see section on loose bone or cartilage below), the condition may require surgery.
This is a section from Dr. Jack E, Jensen’s book The One Stop Knee Shop. Read the next section Fractures.