What causes knee swelling?

Knee swelling can be a result of infection, inflammation, trauma or injury, bleeding, and, rarely, tumors.

Infectious causes of behind knee swelling

Knee swelling may be caused by infectious processes including:

  • Abscesses
  • Cellulitis (infection of the skin and subcutaneous tissue)

Inflammatory causes of knee swelling

Knee swelling can be caused by inflammatory diseases that may also affect multiple joints within the body including:

  • Ankylosing spondylitis (inflammation of joints between the vertebrae of the spine)
  • Bursitis (inflammation of a bursa sac that cushions a joints)
  • Gout (type of arthritis caused by a buildup of uric acid in the joints) and pseudogout
  • Osteoarthritis
  • Psoriatic arthritis (arthritis associated with psoriasis of the skin)
  • Rheumatoid arthritis (chronic autoimmune disease characterized by joint inflammation)
  • Septic arthritis (infectious arthritis)
  • Systemic lupus erythematosus (disorder in which the body attacks its own healthy cells and tissues)

Traumatic causes of knee swelling

Knee swelling can arise due to trauma or injury. Traumatic causes include:

  • Bite and sting injuries
  • Fracture of bone
  • Fragments of bone or cartilage within joint space
  • Hematoma (collection of blood in body tissue)
  • Sprains and strains
  • Torn meniscus

Tumors that may cause knee swelling

Both benign and malignant tumors of the skin, soft tissues, or bone can cause a swollen knee. Examples include:

  • Fibroma (benign tumor composed of fibrous or connective tissue)
  • Lipoma (benign fatty growth)
  • Melanoma (cancer arising in the melanocytes, or pigment-producing cells, in the skin or other parts of the body)
  • Osteosarcoma (type of bone cancer)
  • Sarcoma (soft-tissue cancerous tumor)

Serious or life-threatening causes of knee swelling

In some cases, knee swelling may be a symptom of a serious condition that should be immediately evaluated. These include:

  • Deep vein thrombosis (blood clot in the leg that can break loose from the leg and cause a pulmonary embolism in the lung, heart attack, or stroke)
  • Malignant tumors
  • Popliteal artery aneurysm
  • Septic arthritis (infectious arthritis)

Questions for diagnosing the cause of knee swelling

To diagnose your condition, your doctor or licensed health care practitioner will ask you several questions related to your knee swelling including:

  • Have you experienced a recent animal or insect bite?
  • Have you recently traveled outside the United States?
  • How long have you felt your knee swelling?
  • Is the swelling painful?
  • Is the swelling the result of an injury?
  • Do you have any symptoms in other joints?
  • Does anything relieve or worsen your symptoms?

What are the potential complications of knee swelling?

Swelling related to cancers may have life-threatening consequences, which depend on the type and stage (extent) of the cancer. Left untreated, swelling due to abscesses or serious infections may lead to widespread infection in the body. Knee swelling and associated symptoms can be due to serious diseases, so failure to seek treatment can result in serious complications and permanent damage.

Once the underlying cause is diagnosed, it is important for you to follow the treatment plan that you and your health care professional design specifically for you to reduce the risk of potential complications including:

  • Decreased athletic performance
  • Disability
  • Joint deformity and destruction
  • Joint instability
  • Nerve problems that cause pain, numbness or tingling
  • Permanent or chronic pain
  • Spread of cancer
  • Spread of infection


  1. Knee problems. FamilyDoctor.org. http://familydoctor.org/online/famdocen/home/tools/symptom/542.html.
  2. Knee problems. National Institute of Arthritis and Musculoskeletal and Skin Diseases. http://www.niams.nih.gov/Health_Info/Knee_Problems/default.asp.
  3. Kahan S, Miller R, Smith EG (Eds.). In A Page Signs & Symptoms, 2d ed. Philadelphia: Lippincott, Williams & Williams, 2009.

What is knee swelling?

Swelling of the knee can be caused by injuries to the joint as well as many different medical conditions. Depending upon the cause, knee swelling may be painful or may not produce any other symptoms. In some cases, you may notice tenderness, warmth, difficulties with moving your knee joint, muscle weakness, or bleeding or brui... Read more about knee swellingintroduction


What other symptoms might occur with knee swelling?

Knee swelling may accompany other symptoms, which vary depending on the underlying disease, disorder or condition. Symptoms that affect the knee may, in some cases, also involve other body systems.

Localized symptoms that may occur along with knee swelling

Knee swelling may accompany other localized symptoms including:

  • Bleeding or bruising
  • Read more about knee swellingsymptoms

Medical Reviewer: William C. Lloyd III, MD, FACS Last Annual Review Date: Sep 6, 2013 Copyright: © Copyright 2014 Health Grades, Inc. All rights reserved. May not be reproduced or reprinted without permission from Health Grades, Inc. Use of this information is governed by the HealthGrades User Agreement.

This Article is Filed Under: Bones, Joints and Muscles

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