Overview

A swollen knee occurs when excess fluid collects in or around your knee joint. Health care providers might refer to this condition as an effusion (uh-FU-zhun) in your knee joint.

A swollen knee may be the result of trauma, overuse injuries, or an underlying disease or condition. To find the cause of the swelling, your provider might need to test a sample of the fluid for infection, disease or blood from an injury.

Removing some of the fluid may help reduce the pain and stiffness associated with the swelling. Once the underlying cause is known, treatment can begin.

Symptoms

Signs and symptoms typically include:

  • Swelling. The skin around your kneecap can puff up noticeably, especially when you compare the affected knee to your other one.
  • Stiffness. When your knee joint contains excess fluid, you might not be able to bend or straighten your leg completely.
  • Pain. Depending on the cause of the fluid buildup, your knee might be very painful — to the point that it's impossible to bear weight on it.

When to see a doctor

See your health care provider if self-care measures, such as ice and rest, don't improve symptoms. Seek immediate medical attention if one knee becomes red and feels warm to the touch compared with your other knee. This can be a sign of infection within the joint.

Causes

Many types of problems, ranging from traumatic injuries to diseases and other conditions, can cause a swollen knee.

Injuries

Damage to any part of your knee can cause excess joint fluid to accumulate. Injuries that can cause fluid buildup in and around the knee joint include:

  • Torn ligament, particularly the anterior cruciate ligament (ACL)
  • Cartilage (meniscus) tear
  • Irritation from overuse
  • Broken bones

Diseases and conditions

Underlying diseases and conditions that can produce fluid buildup in and around the knee joint include:

  • Osteoarthritis
  • Rheumatoid arthritis
  • Infection
  • Gout
  • Pseudogout
  • Bursitis
  • Cysts
  • Tumors

Risk factors

Factors that may increase your risk of a swollen knee include:

  • Age. Your likelihood of developing a swollen knee related to arthritis increases as you age.
  • Sports. People who participate in sports that involve twisting the knee, such as basketball, are more likely to experience the types of knee injuries that cause swelling.
  • Obesity. Excess weight puts added stress on the knee joint, contributing to the tissue and joint overload and knee degeneration that can lead to a swollen knee.

Complications

Complications of a swollen knee can include:

  • Muscle loss. Fluid in the knee can harm the working of your muscles and cause thigh muscles to weaken and atrophy.
  • Fluid-filled sac (Baker cyst). The buildup of fluid in your knee can lead to the formation of a Baker cyst in the back of your knee. A swollen Baker cyst can be painful, but usually improves with icing and compression. If the swelling is severe, you might need to have fluid removed with a needle.

Prevention

A swollen knee is typically the result of an injury or chronic health condition. To manage your overall health and prevent injuries:

  • Strengthen the muscles around your knee. Strong muscles around a joint can help ease pressure on the joint itself.
  • Choose low-impact exercise. Certain activities, such as water aerobics and swimming, don't place continuous weight-bearing stress on your knee joints.
  • Maintain a healthy weight. Excess weight contributes to the wear-and-tear damage that can lead to a swollen knee.