Knee pain and swelling are most often caused by an injury to the knee. Other causes are arthritis, bursitis, major or minor ligament injuries, torn cartilage and a Baker's cyst. Less common causes for pain and swelling in the knee include autoimmune diseases such as psoriasis; in rare instances, it can be the result of a blood clot or cancer. Knowing whether the knee pain is on one side or both sides, and whether it is transient or constant provides a doctor with clues pointing to the correct diagnosis. While a person should consult a doctor whenever there is pain and swelling in the knee, rest and home care usually allows knee pain to resolve on its own.
The knee can be injured by overuse, poor footwear, bad athletic form, or trips and falls. Knee injuries usually only affect one knee and often involve the anterior cruciate ligament or medial collateral ligament. These injuries can be simple strains and sprains or be severe enough to result in internal bleeding in the knee, increasing knee pain and swelling. Runners or skiers may develop tendonitis, an inflammation of the tendons around the knee. A twisting or over-flexion of the knee can cause a meniscal tear, while a popping sensation may occur at the time of the injury and the knee joint will repeatedly lock with pain and swelling.
In athletic adolescents, repeated small injuries can cause pain and swelling around the knee. This condition is called Osgood-Schlatter disease and is seen most often in boys ages 10 to 15 who are involved in sports. Bursitis, or housemaid's knee, results from an excessive period of pressure on the knees and occurs in jobs that require kneeling, such as carpet layers, cleaners or tile layers. Occasionally, knee injuries will require surgery, though an injured knee often resolves with rest and treatment such as acetaminophen or ibuprofen.
Several forms of arthritis can cause knee pain and swelling. Rheumatoid arthritis is an autoimmune disease that can affect any joint of the body, and multiple joints are often affected at the same time. Osteoarthritis occurs from a lifetime of wear-and-tear and is common with advanced age. Gout is a form of arthritis that results from uric acid crystals building up in the joints. Pseudogout is similar, except calcium pyrophosphate crystals form in the joint fluid; the most common joint affected by pseudogout is the knees.
A swelling behind the knee that feels like a water balloon may be a Baker's cyst. This cyst forms from an excess of synovial fluid, the fluid that lubricates the knee. It may be painful with swelling, or it may be painless with no other symptoms. Baker's cysts can accompany arthritis and injuries such as meniscal tears.
Autoimmune diseases that cause widespread systemic inflammation can also result in knee pain and swelling. Psoriasis is a disease in which the skin develops red, itchy and scaly patches. Psoriasis patients can also develop psoriatic arthritis affecting both knees. Lupus and Crohn's disease may also cause painful, swollen knees.
Less commonly, knee pain and swelling may indicate a serious condition. Knee pain may be a symptom of a blood clot in the leg, or deep venous thrombosis. Infection in the knee joint is usually accompanied by pain, swelling and fever. Bone tumors may also present as pain and swelling in the knee.