Causes of knee swelling include injuries like torn ligaments, torn cartilage, irritation and broken bones, states Mayo Clinic. When a portion of the knee becomes damaged, extra joint fluid tends to build up, leading to knee swelling. Underlying conditions that may cause fluid accumulation include gout, infection, rheumatoid arthritis and osteoarthritis. Tumors, cysts and pseudogout also lead to knee swelling.
In patients with osteoarthritis, the cartilage supporting the edges of the bones wears down, leading to swelling in the knees and other joints that mostly carry the body's weight, explains WebMD. Those with rheumatoid arthritis experience pain, stiffness and swelling in joints, including the knees, hands and feet.
Gouty arthritis often occurs unexpectedly and causes severely swollen, painful, warm and red joints, states WebMD. Joint injuries also cause painful and swollen joints. In some cases, the ligaments, tendons or muscles around the affected joint become damaged. Sprains, strains, dislocations, tendinitis and bursitis also lead to joint swelling. Dull pain, warmth, stiffness and difficulty moving the joints naturally are symptoms of joint swelling.
Doctors identify the cause of a swollen knee by acquiring a sample of the joint fluid and testing it for injury, disease or infection, notes Mayo Clinic. They usually remove the excess fluid from the affected knee to relieve pain and stiffness caused by the swelling.