Swollen Knee Symptoms and Causes

A swollen knee is a common problem which can affect the young as well as the 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. It may an acute condition caused by a traumatic injury or a chronic one which 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.

When to seek urgent care for a knee injury
Verywell/Brianna Gilmartin


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.

The first step in treating an effusion is to pinpoint the cause. We do this by first looking 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 doctor can then explore some of the more typical causes of knee effusion.

Fluid Outside the Knee Joint

When there is excessive fluid in the soft tissue surrounding the knee, the most common cause is prepatellar bursitis, the 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 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 is usually caused by either a torn knee ligament (called an ACL tear) 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 tear in the rubbery disk which cushions the knee (called the meniscus). The swelling is typically slower and often only noticed hours or days after the injury. While the volume of fluid can be significant, it is not typically as profound as a blood accumulation.

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 the body to produce excess fluid in response to the underlying inflammation. People with knee arthritis often notice that the affected knee is larger than the other. The swelling tends to worsen with activity, particularly when the knee is weight-bearing. The [ain 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.

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 the accumulation of fluid in the knee joint, often as a result of surgery, a knee wound, or a systemic, all-body infection which spreads to the joint. Treatment can be problematic as the body has a difficult time clearing infection from this space. As such, surgery may be required to fully clean out a septic infection.
  • Gout and pseudogout both involve the buildup of biochemical 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 which similarly stimulates an inflammatory response in the joints.

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
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Article Sources

  • Gupte, C. and St. Mart, J. "The acute swollen knee: diagnosis and management." Journal of the Royal Society of Medicine. 2013; 106(7): 259–268.