Swollen Knee Symptoms and Causes

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 injury
  • Rapid swelling of the knee
  • Intense pain
  • Knee joint deformity
  • Inability to place weight on the injured knee


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.

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.

  • 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).

  • 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. 

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.
  1. Gerena LA, DeCastro A. Knee effusion. StatPearls.

  2. Gupte C, St Mart JP. The acute swollen knee: diagnosis and managementJ R Soc Med. 2013;106(7):259–268. doi:10.1177/0141076813482831

  3. Hui AY, McCarty WJ, Masuda K, Firestein GS, Sah RL. A systems biology approach to synovial joint lubrication in health, injury, and diseaseWiley Interdiscip Rev Syst Biol Med. 2012;4(1):15–37. doi:10.1002/wsbm.157

  4. Mathison DJ, Teach SJ. Approach to knee effusions. Pediatr Emerg Care. 2009;25(11):773-86. doi:10.1097/PEC.0b013e3181bec987

  5. Chatra PS. Bursae around the knee jointsIndian J Radiol Imaging. 2012;22(1):27–30. doi:10.4103/0971-3026.95400

  6. Kakarlapudi TK, Bickerstaff DR. Knee instability: isolated and complexWest J Med. 2001;174(4):266–272. doi:10.1136/ewjm.174.4.266

  7. Everhart JS, Sojka JH, Kaeding CC, Bertone AL, Flanigan DC. The ACL injury response: A collagen-based analysis. Knee. 2017;24(3):601-607. doi:10.1016/j.knee.2017.01.013

  8. Fox AJ, Bedi A, Rodeo SA. The basic science of human knee menisci: structure, composition, and functionSports Health. 2012;4(4):340–351. doi:10.1177/1941738111429419

  9. Levy DR. The swollen kneeCan Fam Physician. 1983;29:2197–2203. PMID: 21283481

  10. Goldring MB, Otero M. Inflammation in osteoarthritisCurr Opin Rheumatol. 2011;23(5):471–478. doi:10.1097/BOR.0b013e328349c2b1

  11. Peeler J, Christian M, Cooper J, Leiter J, Macdonald P. Managing Knee Osteoarthritis: The Effects of Body Weight Supported Physical Activity on Joint Pain, Function, and Thigh Muscle Strength. Clin J Sport Med. 2015;25(6):518-23. doi:10.1097/JSM.0000000000000173

  12. Guo Q, Wang Y, Xu D, Nossent J, Pavlos NJ, Xu J. Rheumatoid arthritis: pathological mechanisms and modern pharmacologic therapiesBone Res. 2018;6:15. doi:10.1038/s41413-018-0016-9

  13. Shirtliff ME, Mader JT. Acute septic arthritisClin Microbiol Rev. 2002;15(4):527–544. doi:10.1128/cmr.15.4.527-544.2002

  14. Sidari A, Hill E. Diagnosis and Treatment of Gout and Pseudogout for Everyday Practice. Prim Care. 2018;45(2):213-236. doi:10.1016/j.pop.2018.02.004

  15. Rock KL, Kataoka H, Lai JJ. Uric acid as a danger signal in gout and its comorbiditiesNat Rev Rheumatol. 2013;9(1):13–23. doi:10.1038/nrrheum.2012.143

  16. Molloy ES, Mccarthy GM. Calcium crystal deposition diseases: update on pathogenesis and manifestations. Rheum Dis Clin North Am. 2006;32(2):383-400, vii. doi:10.1016/j.rdc.2006.02.001

  17. Cleveland Clinic. Baker’s cyst.

  18. American Academy of Orthopaedic Surgeons. Prepatellar (kneecap) bursitis.

  19. Horowitz DL, Katzap E, Horowitz S, Barilla-LaBarca M-L. Approach to septic arthritis. Am Fam Physician. 2011;84(6):653-660.

Additional Reading

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.