Symptoms and diagnosis of a swollen knee.


A swollen knee is a common problem that can affect both young and old people. Many people call it "lap water" because of its often fluffy appearance. Sometimes it can be difficult to determine the cause of a swollen knee.

It can be an acute condition caused by a traumatic injury or a chronic condition that progresses slowly over time. The location of the tumor can also vary, sometimes within the knee joint and sometimes in the surrounding soft tissue.

Get Medication Information / Brianna Gilmartin

Knee joint

The knee joint is surrounded by a capsule. This capsule forms a "joint space" in which a small amount of lubricating fluid (called synovial fluid ) helps the knee move. Certain conditions can cause this fluid to build up. When this happens, the knee can swell, a condition commonly called a knee effusion .

When to seek emergency help

Go to the nearest emergency room or emergency center if you have a knee injury with:

  • A snap at the time of injury.
  • Rapid swelling of the knee.
  • Strong pain
  • Knee deformity
  • Inability to put weight on an injured knee


The first step in treating a stroke is determining the cause. Your doctor will first evaluate the appearance of your knee.

When the inflammation is in the knee joint, the patella is usually well defined and easily felt under the skin (although it may appear to be slightly extended). When the tumor is in soft tissue, the kneecap may not be visible or may not be easily felt.

Based on the results of the physical exam, the doctor can examine some of the more common causes of knee effusion.

Fluid off the knee

The most common cause of excess fluid in the soft tissue surrounding the knee is prepatellar bursitis . This is an inflammation of the fluid-filled sac (bursa) that softens the kneecap (kneecap). The buildup can be seen and felt at the top of the kneecap. This is not what you see below the knee.

Trauma, such as bruising (soft tissue contusion ), can also cause localized edema. In some cases, the accumulation of blood and fluid can simulate an acute knee injury.

Fluid inside the knee joint.

If the knee joint is an effusion area, we generally investigate three possible causes: acute trauma, chronic condition, and acute non-traumatic condition .

Acute injuries

Acute injuries are those that have occurred in the last 24 to 48 hours and have caused rapid inflammation of the knee. In this case, we determine whether the fluid in the knee is bloody or not:

  • Blood in knee fluid is usually the result of an anterior cruciate ligament (ACL) tear in the knee or a fracture of the bone and cartilage of the knee. When bleeding is the source of the tumor, the onset will be quick and fast. intensely, usually within a few minutes.
  • Non-bloody fluid can be caused by a sprain or tear of the meniscus in the elastic disc that cushions the knee . The swelling is usually slower and often occurs only a few hours or days after the injury. The volume of fluid can be significant, but generally not as much as the blood pool.

Chronic spills

Chronic effusions are characterized by the gradual appearance of edema. The swelling can often change as symptoms come and go. In addition to the wear and tear associated with aging, there are two common causes of chronic knee effusion:

  • Osteoarthritis can cause excess fluid production in response to underlying inflammation. In knee osteoarthritis, the affected knee is usually larger than the other. Swelling tends to get worse with exertion, especially when the knee is stressed . The pain usually goes away when the knee relaxes.
  • Rheumatoid arthritis , an autoimmune form of arthritis, can cause the same effect. Rheumatoid arthritis most often affects multiple joints, with more persistent edema due to ongoing underlying inflammation .

Acute onset without injury

A rapid onset of edema without injury is a broad category in which fluid accumulation is not associated with trauma or a chronic condition, for example:

  • The infection can cause fluid to build up in the joints, often as a result of surgery, a knee injury, or a systemic infection that spreads to the joint. Treatment can be challenging, as it is difficult for the body to rid itself of the infection from this space. Surgery may be required to completely clear the septic infection .
  • Gout and pseudogout are associated with the accumulation of crystals in the knee fluid. In gout , uric acid used to transport waste products can accumulate and crystallize in various joints in the body, causing severe swelling and pain. Calcium crystals are the cause of pseudogout .

Frequently asked questions

  • Several conditions can cause swelling that spreads to the back of the knee, but if you have a slight visible lump, you most likely have a Baker's cyst, which is often the result of injury, arthritis, or other conditions. This type of cyst is usually not serious, but your doctor should evaluate it and prescribe the correct treatment.

  • In the absence of infection, bursitis should clear up with basic home care, including rest, ice, lifting, and the use of non-steroidal anti-inflammatory drugs (NSAIDs).

  • In addition to swelling, you may feel warm in the joint, and if you have a knee infection, you may have a fever. To make a diagnosis, your doctor will do a blood test. You may also have an X-ray and fluid sample from your knee to determine the type of bacteria.

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