Orthopedics Hip & Knee Swollen Knee Symptoms and Causes By Jonathan Cluett, MD Jonathan Cluett, MD LinkedIn Twitter Jonathan Cluett, MD, is a board-certified orthopedic surgeon with subspecialty training in sports medicine and arthroscopic surgery. Learn about our editorial process Updated on October 11, 2022 Medically reviewed by Cara Beth Lee, MD Medically reviewed by Cara Beth Lee, MD LinkedIn Cara Beth Lee, MD, is a board-certified Orthopedic Surgeon with over 20 years of practice experience and subspecialty expertise in hip preservation surgery, focusing on patients with pre-arthritic hip disorders. Learn about our Medical Expert Board Print A swollen knee is a common problem that can affect the young as well as old. Many people refer to it as "water on the knee" because of its often spongy appearance. Determining the cause of a swollen knee can sometimes be challenging. A swollen knee may be an acute condition caused by a traumatic injury or a chronic one that has developed slowly over time. The location of the swelling can also vary, sometimes occurring within the knee joint and, at others, in the soft tissues surrounding the knee. Verywell / Brianna Gilmartin The Knee Joint The knee joint is surrounded by a capsule. This capsule forms the "joint space" where a small amount of lubricating fluid (called synovial fluid) keeps the knee moving easily. Certain conditions can cause this fluid to accumulate. When this happens, the knee can swell, a condition typically referred to as a knee effusion. When to Seek Urgent Care Go to the nearest emergency room or urgent care center if you experience a traumatic knee injury accompanied by:A popping sound at the time of the injuryRapid swelling of the kneeIntense painKnee joint deformityInability to place weight on the injured knee Diagnosis The first step in treating an effusion is to pinpoint the cause. Your healthcare provider will first look at the physical appearance of the knee itself. When the swelling is within the knee joint, the kneecap is usually well-defined and easily felt under the skin (although it may seem pushed out a bit). When the swelling is in the soft tissue, the kneecap may not be visible or easily felt. Based on the outcome of the physical exam, the healthcare provider can then explore some of the more typical causes of knee effusion. Fluid Outside the Knee Joint The most common cause of excessive fluid in the soft tissue surrounding the knee is prepatellar bursitis. This is inflammation of a fluid-filled sac (called the bursa) which cushions the kneecap (called the patella). The buildup can be seen and felt at the top of the kneecap. It is not something you would see under the knee. An injury such as a contusion (soft tissue bruise) may also cause localized swelling. In some cases, the buildup of blood and fluid may mimic an acute injury of the knee joint. Fluid Inside the Knee Joint If the knee joint is the area of effusion, we typically explore three possible causes: an acute injury, a chronic condition, and an acute condition not related to an injury. Acute Injuries Acute injuries are those that have occurred within the past 24 to 48 hours, resulting in rapid swelling of the knee. In this instance, we would determine whether the fluid in the knee is bloody or non-bloody: Blood in the knee fluid is usually caused by either a torn anterior cruciate ligament (ACL) in the knee or a fracture of the bone and cartilage of the knee. When bleeding is the source of the swelling, the onset will be rapid and intense, usually within minutes. Non-bloody fluid can be caused by a ligament sprain or a meniscus tear in the rubbery disk that cushions the knee. The swelling is typically slower and often only noticed hours or days after the injury. The volume of fluid can be significant but is not typically as profound as a blood accumulation. Chronic Effusions Chronic effusions are characterized by the gradual onset of swelling. The swelling can often fluctuate as the symptoms come and go. In addition to aging-related wear-and-tear, there are two common causes for a chronic knee effusion: Osteoarthritis can cause excess fluid production in response to underlying inflammation. With knee osteoarthritis, the affected knee is often larger than the other. Swelling tends to worsen with activity, particularly when the knee bears weight. The pain will often disappear once the knee is relaxed. Rheumatoid arthritis, an autoimmune form of arthritis, can cause the same effect. Rheumatoid arthritis will most often affect multiple joints accompanied by a greater persistence of swelling due to the ongoing, underlying inflammation. Acute Onset Without Injury Rapid onset of swelling with no injury is a broad category wherein the accumulation of fluid is not due to an injury or a chronic condition, such as: Infection can result in joint fluid accumulation, often as a result of surgery, a knee wound, or systemic infection that spreads to the joint. Treatment can be a problem as the body has a tough time clearing infection from this space. Surgery may be required to fully clean out a septic infection. Gout and pseudogout involve a buildup of crystals in the knee fluid. With gout, the uric acid used to transport waste can accumulate and crystallize in various joints of the body, causing intense swelling and pain. With pseudogout, the culprit is calcium crystals. When to See a Healthcare Provider About Knee Pain Frequently Asked Questions Why is there a lump and swelling behind my knee? Several conditions can cause swelling that extends to the back of the knee, but if you have a soft, visible bulge, it’s very likely that you have a Baker’s cyst, which is often caused by injury, arthritis, or other conditions. This type of cyst is usually not serious, but your doctor should evaluate it and determine the proper treatment. Learn More: What You Should Know About Baker's Cysts Can knee bursitis go away on its own? As long as there's no infection, bursitis should go away with basic at-home care including rest, ice, elevation, and the use of nonsteroidal anti-inflammatory drugs (NSAIDs). Learn More: Choosing the Right Anti-Inflammatory Medication How do you know if knee swelling is an injury or infection? In addition to swelling, your joint may feel warm, and you may have a fever when you have an infection in the knee. To make a diagnosis, your doctor will run blood tests. You may also have an x-ray, and a sample of fluid may be collected from the knee to determine the type of bacteria. Learn More: What Is Septic Arthritis? 19 Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Gerena LA, DeCastro A. Knee effusion. StatPearls. Gupte C, St Mart JP. The acute swollen knee: diagnosis and management. J R Soc Med. 2013;106(7):259–268. doi:10.1177/0141076813482831 Hui AY, McCarty WJ, Masuda K, Firestein GS, Sah RL. A systems biology approach to synovial joint lubrication in health, injury, and disease. Wiley Interdiscip Rev Syst Biol Med. 2012;4(1):15–37. doi:10.1002/wsbm.157 Mathison DJ, Teach SJ. Approach to knee effusions. Pediatr Emerg Care. 2009;25(11):773-86. doi:10.1097/PEC.0b013e3181bec987 Chatra PS. Bursae around the knee joints. 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Horowitz DL, Katzap E, Horowitz S, Barilla-LaBarca M-L. Approach to septic arthritis. Am Fam Physician. 2011;84(6):653-660. Additional Reading The American Association of Orthopaedic Surgeons. Common Knee Injuries - OrthoInfo - AAOS. [online] Orthoinfo.aaos.org. By Jonathan Cluett, MD Jonathan Cluett, MD, is board-certified in orthopedic surgery. He served as assistant team physician to Chivas USA (Major League Soccer) and the United States men's and women's national soccer teams. See Our Editorial Process Meet Our Medical Expert Board Share Feedback Was this page helpful? Thanks for your feedback! What is your feedback? Other Helpful Report an Error Submit