What causes behind knee lump?

A Baker’s cyst is a common cause of a lump behind the knee. This condition develops when joint fluid leaks out from the joint cavity into the tissues behind your knee. Other causes include infections, bleeding, trauma, and, rarely, tumors.

Infectious causes of behind knee lump

Behind knee lump may be caused by infectious processes including:

  • Abscesses
  • Boils

Traumatic causes of behind knee lump

Behind knee lump can arise related to traumatic events. Traumatic causes include:

  • Fracture of bone
  • Hematoma (collection of blood in body tissues)
  • Sprains and strains

Tumors that may cause behind knee lump

Both benign and malignant tumors of the skin, soft tissues, or bone can cause a lump behind the 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)
  • Nevi (moles of the skin)
  • Nonmelanoma skin cancers
  • Osteosarcoma (type of bone cancer)
  • Sarcoma (soft-tissue cancerous tumor)

Serious or life-threatening causes of behind knee lump

In some cases, behind knee lump 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

Questions for diagnosing the cause of behind knee lump

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

  • How long have you felt your behind knee lump?
  • Is the lump getting bigger?
  • Is the lump painful?
  • Is the lump 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 behind knee lump?

Lumps caused by cancers may have life-threatening consequences, which depend on the type and stage (extent) of the cancer. Left untreated, lumps due to abscesses or serious infections may lead to widespread infection in the body. Knee lumps 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
  • Nerve problems that cause pain, numbness or tingling
  • Permanent or chronic pain
  • Spread of cancer
  • Spread of infection


  1. Baker’s cyst. PubMed Health. http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002202/.
  2. Knee problems. FamilyDoctor.org. http://familydoctor.org/online/famdocen/home/tools/symptom/542.html.
  3. Hayashi D, Roemer FW, Dhina Z, et al. Longitudinal assessment of cyst-like lesions of the knee and their relation to radiographic osteoarthritis and MRI-detected effusion and synovitis in patients with knee pain. Arthritis Res Ther 2010; 12:R172.


What is behind knee lump?

A swollen area or lump that can be felt behind your knee can be caused by several different conditions. The lump may create an unsightly appearance, limit knee flexibility, or generate discomfort. A Baker’s cyst is a collection of synovial fluid (the fluid that is present within joint spaces) that bulges out through the back of the knee joint and can be felt as a lump behind the knee. Knee inju... Read more about behind knee lumpintroduction


What other symptoms might occur with behind knee lump?

Behind knee lump may accompany other symptoms, which vary depending on the underlying disease, disorder or condition.

Localized symptoms that may occur along with behind knee lump

Behind knee lump may accompany other localized symptoms including:

Medical Reviewer: William C. Lloyd III, MD, FACS Last Annual Review Date: Aug 23, 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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