Swollen Knee -

A swollen knee occurs when excess fluid collects in or around your knee joint. Healthcare professionals 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 healthcare team 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.

Signs and symptoms typically include:

Swelling. The skin around your kneecap can puff up noticeably, especially when you compare the affected knee with 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 healthcare professional if self-care measures, such as ice and rest, don’t improve symptoms. Seek immediate medical attention if one knee has a change in skin color and feels warm to the touch compared with your other knee. This can be a sign of infection within the joint.

Many things can cause a swollen knee. They include traumatic injuries to diseases and other conditions.

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, also called ACL.
Tear in the meniscus cartilage.
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.

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 of a swollen knee can include:

Muscle loss. Fluid in the knee can harm how well your muscles work and cause thigh muscles to weaken.
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.

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

Your healthcare team is likely to start with a detailed history and physical examination. After that you likely will need tests to find out what’s causing your swollen knee.

Imaging tests

Imaging tests can help show where the problem is located. Options include:

X-ray. An X-ray can rule out broken or dislocated bones and determine if you have arthritis.
Ultrasound. This test uses sound waves to check for disorders affecting the tendons or ligaments.
MRI. Using radio waves and a strong magnetic field, MRI can detect tendon, ligament and other soft tissue injuries that aren’t visible on X-rays.

Joint aspiration (arthrocentesis)

A needle is used to remove fluid from inside your knee. This fluid is then checked for the presence of:

Blood, which may stem from injuries or bleeding disorders.
Bacteria that may be causing an infection.
Crystals common to gout or pseudogout.

Treatment varies depending on the cause of the swollen knee, its severity and your medical history.

Therapy

Physical therapy exercises can improve your knee’s function and strength. In some situations, a knee brace may be helpful.

Surgical and other procedures

Treating the underlying cause of a swollen knee might require:

Arthrocentesis. Removing fluid from the knee can help relieve pressure on the joint. After removing some of the joint fluid, your healthcare professional might inject a corticosteroid into the joint to treat inflammation.
Arthroscopy. A lighted tube called an arthroscope is inserted through a small incision into your knee joint. Tools attached to the arthroscope can remove loose tissue or repair damage in your knee.

References

Ferri FF. Knee sprain. In: Ferri’s Clinical Advisor 2025. Elsevier; 2025. https://www.clinicalkey.com. Accessed Jan. 20, 2025.
Firestein GS, et al. Hip and knee pain. In: Firestein & Kelley’s Textbook of Rheumatology. 12th ed. Elsevier; 2025. https://www.clinicalkey.com. Accessed Jan. 20, 2025.
Beutler A, et al. Physical examination of the knee. https://www.uptodate.com/contents/search. Accessed Jan. 20, 2025.
Evaluation of the knee. Merck Manual Professional Version. https://www.merckmanuals.com/professional/musculoskeletal-and-connective-tissue-disorders/approach-to-the-patient-with-joint-symptoms/evaluation-of-the-knee?query=evaluation%20of%20the%20knee. Accessed Jan. 20, 2025.
Roberts WN. Joint aspiration or injection in adults: Indications and technique. https://www.uptodate.com/contents/search. Accessed Jan. 20, 2025.
Common knee injuries. OrthoInfo. https://orthoinfo.aaos.org/en/diseases–conditions/common-knee-injuries. Accessed Jan. 20, 2025.
Kellerman RD, et al. Common sports injuries. In: Conn’s Current Therapy 2025. Elsevier; 2025. https://www.clinicalkey.com. Accessed Jan. 20, 2025.
Goldman L, et al., eds. Bursitis, tendinopathy, and other periarticular disorders and sports medicine. In: Goldman-Cecil Medicine. 27th ed. Elsevier; 2024. https://www.clinicalkey.com. Accessed Jan. 20, 2025.
Miller M, et al. Knee diagnosis and decision-making. In: DeLee, Drez, and Miller’s Orthopaedic Sports Medicine: Principles and Practice. 5th ed. Elsevier; 2020. https://www.clinicalkey.com. Accessed Jan. 20, 2025.
Knee pain. Merck Manual Professional Version. https://www.merckmanuals.com/professional/injuries-poisoning/sports-injury/knee-pain?query=knee%20pain%20treatment#Diagnosis_v1116664. Accessed Jan. 21, 2025.

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Swollen Knee

When to see a doctor

Injuries

Diseases and conditions

Imaging tests

Joint aspiration (arthrocentesis)

Therapy

Surgical and other procedures

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