Common Causes of Knee Swelling

A swollen knee or “water on the knee” as some people like to call it,  is a common problem that I see regularly in my practice.  Sometimes it can be a bit of a challenge to determine the actual cause of the swelling but by taking a good patient history and a physical examination we can usually nail down the diagnosis.  If you haven’t already, I may ask you to have some diagnostic imaging tests done such as x-rays, ultrasounds or MRIs to help in differentiating possible causes.

Swelling Due To Fluid Inside The Knee Joint

Acute Injuries

If you’ve experienced a traumatic event that has occurred within the past 24-48 hours then this applies to you.

Acute injuries are distinct from chronic conditions because the knee swelling developed suddenly as a result of the injury such as a fall, blow to the knee, sudden twists and popping sensations, etc.  If the injury is acute, the next step is to determine the type of fluid within the knee. There are specific injuries that can cause bleeding in the knee, and others that may cause a sudden increase in the production of synovial fluid.

  • Blood in the Knee
    • Two conditions commonly cause the accumulation of blood within the knee.  Depending on how you sustained the injury, there could be an ACL tear or a fracture of the bone and cartilage of the knee. These injuries allow blood to enter the joint and will create a large, swollen knee. When bleeding is the cause of knee swelling, the onset is rapid, and the swelling can be intense. Fluid usually accumulates within minutes of the injury.
  • Non-Bloody Fluid in the Knee
    • Acute injuries that cause the accumulation of non-bloody fluid within the knee include meniscus tears and ligament sprains.   If you feel like your knee got “twisted up”, this is probably you.  Typically you will notice fluid accumulation hours to days after the injury rather than within minutes as seen with bleeding into the knee.   There can still be a significant amount of fluid but it is not typically as severe as seen with blood accumulation.

Chronic Conditions

Chronic injuries cause a gradual onset of knee swelling. This fluid may fluctuate in amount and the symptoms may come and go. This is the most common type of swelling and often seen as a result of knee arthritis or wear-and-tear.

Knee Arthritis

  • Osteoarthritis of the knee causes the body to produce extra fluid in the joint and the amount of fluid tends to fluctuate over time.  Patients with knee arthritis often notice the affected knee is larger than the other. The amount of fluid often corresponds with the amount of activity the patient has been doing lately.  For example, more swelling after gardening on the weekend or a soccer game.

Rapid Onset of Swelling But No Injury

The last general category of swelling is the rapid onset of fluid within the joint, but no recent injury to the knee. The most common causes of this type of fluid accumulation are due to infection or gout.


  • Infections can cause fluid to accumulate within the knee joint. Infections can be caused by contamination in the knee, such as from a recent surgery or a wound to the knee.  Alternatively, you may have had a systemic infection that spread to the joint. Infections inside a joint are quite problematic because your body has a hard time fighting infections within this space.  You may need to be referred for surgery in order to clean out the infection.


  • Gout is due to the accumulation of uric acid crystals within the fluid of various joints including the the big toe, ankle or knee. Uric acid is a substance produced as part of digestion and it passes through the kidneys and out of the body in urine.  However, uric acid can build up in the blood when:
    • The body increases the amount of uric acid it makes
    • The kidneys do not get rid of enough uric acid
    • A person eats too many foods high in purines

Fluid Outside The Knee Joint

If the fluid resides mainly in the soft tissues surrounding the knee, the most common cause is prepatellar bursitis. This condition causes fluid accumulation in the bursa just on top of the kneecap. Usually the fluid can be felt on top of the kneecap, rather than underneath the kneecap.

Prepatellar bursitis is often caused by pressure from constant kneeling.  So people like plumbers, roofers, carpet layers and gardeners are susceptible to this.  As well, athletes who participate in sports in which direct blows or falls on the knee are common, such as football, wrestling, or basketball, are at greater risk for the condition.

Fluid can also accumulate in the soft tissue around the knee after an injury such as a contusion to the knee. A hard impact to the soft tissue surrounding the knee can cause fluid and/or blood to accumulate, giving the appearance of a swollen knee.

Need help with a knee problem?  Give us a call to see what therapies are most appropriate for you!

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