According to the Mayo Clinic, about one in three Americans over the age of 45 experiences some type of knee pain or discomfort, and knee pain and swelling is one of the most common reasons a person visits his doctor or the emergency room. Knee pain and swelling are often related, although swelling can occur in the absence of knee pain and vice versa.
Trauma
Traumatic accidents or injuries--including motor vehicle accidents and sports injuries--can cause knee pain and swelling. According to the University of Minnesota's Sports Medicine Institute, trauma-induced knee pain and swelling is often caused by blood leaking into the knee joint--known as acute hemarthrosis--and the most common injury that's associated with acute hemarthrosis is an anterior cruciate ligament tear. The ligament stabilizes the knee and prevents forward movement of the lower leg bone relative to the thigh bone. The University of Minnesota's Sports Medicine Institute states that other common trauma-induced knee injuries associated with knee pain and swelling include patellar dislocations, meniscal tears, articular cartilage injuries and other ligament injuries, including sprains or ruptures of the lateral collateral, medial collateral and posterior cruciate ligaments.
Overuse
Overuse injuries are a common cause of knee pain and swelling. The U.S. National Library of Medicine and the National Institutes of Health state that knee pain or knee pain and swelling usually result from overuse, along with poor technique or form during sports or physical activity, failing to warm up or cool down effectively and failing to stretch before activity. Many repetitive overuse injuries causing knee pain and swelling self-resolve with rest and time, and being overweight predisposes a person to knee pain and swelling. Bursitis is one of the most common repetitive overuse injuries causing knee pain and swelling. According to the American Academy of Family Physicians, plumbers, roofers, carpet layers and other people who spend a significant time on their knees often develop pain and swelling in the front of the knee, as the constant friction irritates the bursa--a small fluid-filled sac--situated in front of the kneecap. An inflamed knee bursa will fill with fluid and will cause swelling at the top of the knee.
Medical Conditions
Certain medical conditions can cause knee pain and swelling. According to the University of Minnesota's Sports Medicine Institute, medical conditions that may cause knee pain and swelling include benign tumors in the synovium or a joint's fibrous coating. Bleeding problems, such as hemophilia, along with rheumatoid arthritis and infection can cause knee pain and swelling, as can gout or pseudogout. Gout and pseudogout are caused by the deposit of crystalline substances within the knee joint, which causes tissue irritation and swelling. Joint aspiration or arthrocentesis--the extraction of fluid from a joint using a syringe--may be necessary to definitively diagnose gout or pseudogout as the cause of knee pain and swelling. A person with osteoarthritis or degenerative joint disease may also experience pain and swelling in the knee joint, and an increase in physical activity among those with osteoarthritis can occasionally lead to significant knee joint swelling.
