Lump Behind Knee – Causes, Other Symptoms, Treatment

Finding a lump behind the knee can be concerning for most people but it usually is not a serious condition. Often it is due to collection of fluid within the knee joint. Sometimes even pus or blood may collect behind the knee and be felt as a lump. There are instances where solid lumps behind the knee can be tumors and even be cancerous. However, this is rare.

The knee is a joint that is composed of several structures. Firstly, the bones of the thigh and lower leg comprise the knee joint along with patella. The joint is lined by synovium and surrounded by a capsule. Ligaments provide strength and support to the knee joint. Muscles and tendons that move the different parts of the limb also strengthen and supports the joint.

A lump behind the knee (popliteal region) is just a symptom of some knee problem. Sometimes it is unrelated to the knee joint despite its location. It is important to have any lump assessed by a medical professional, irrespective of its location, size or consistency. A lump behind the knee may not always seem serious since it is not near a vital organ but should still be investigated by a doctor.

Other Symptoms of Knee Problems

The knee has to bear extensive force throughout life, both when we are moving (walking, running or standing) and even stationary (standing). It is one of the hardest working joints in the body and is therefore prone to a host of diseases and injuries. This can present with various symptoms, most notably pain and stiffness.

Redness, swelling, heat, joint deformities and abnormal sounds from the knee may be other symptoms that are experienced with the various knee problems. A lump behind the knee is not one of the common symptoms of a knee problem but it can occur. It is important to monitor the lump for any growth in size, whether it is fixed or movable and if there is any discharge from it.

Read more on pressure in the knee.

Causes of Lump Behind Knee

The exact cause of a lump behind the knee must be diagnosed by a medical doctor. This may require diagnostic investigations like an x-ray, CT (computed tomography) scan, ultrasound or magnetic resonance imaging (MRI). Immediate medical attention is necessary if there is bleeding or an inability to stand or walk due to joint instability or muscle weakness.

Baker’s Cyst

A Baker’s cyst is one of the common causes of a lump behind the knee. Cysts are filled with fluid and form when there is injury or inflammation to the area. A Baker’s cyst is filled with synovial fluid which is secreted in excess due to conditions like arthritis or various forms of knee injury.

It may be accompained by pain, swelling of the entire knee and stiffness. However, in some cases there is no pain but there is a sensation of tightness or pressure in the knee. Usually the cyst appears as a bulge behind the knee. Symptoms worsen with standing or walking for long periods.

Read more on knee arthritis.

Abscess or Boils

Abscesses and boils are one of the possible causes of a lump behind the knee. Both are collections of pus with a boil being more superficial (just under the skin) while abscesses lie deeper. Abscesses and boils are a result of an infection, usually bacterial.

It forms when the body attempts to wall of the invading microbe. These microbes may enter through the skin, like with a cut, or sometimes can spread from neighboring infected sites in the leg or even spread from distant sites through the bloodstream.


Injury to the knee joint or surrounding bone, muscles, tendons or ligaments can also cause a lump or swelling behind the knee. This may occur with muscle and tendon strain, ligament stretching (sprain) or tears and even fractures.

Blunt force trauma to the knee can cause swelling as a result of inflammation. It can also cause accumulation of blood under the skin if blood vesels are broken. This is known as a hematoma and may appear red to purple or even blue in color.

Blood Vessels and Clots

Swelling may occur with various circulatory problems of the leg, including varicose veins and deep vein thrombosis (DVT).  However, the swelling in these cases are usually not isolated to the back of the knee (popliteal region).

One blood vessel condition that can cause a lump in the popliteal fossa is a popliteal aneurysm. This is where a region in the popliteal artery becomes weak and balloons. If the aneurysm ruptures then blood will leak into the popliteal fossa and may be visible as a hematoma.

Widening of the superficial veins that run behind the knee is a common problem. It is known as varicose veins. When the smaller veins are affected it is referred to as telangiectasia or spider veins. One of the common sites whether this occurs is the area behind the knee.


Various tumors can occur behind the knee. These tumors may be benign (non-cancerous) or malignant (cancerous). It may arise in the skin (ranging from moles/nevi to melanomas), connective tissue (fibroma), fat tissue (lipoma), soft tissue (sarcoma) or even the bone (osteosarcoma). These tumors are not specific to the region behind the knee.

Treatment of Lump Behind the Knee

There is no specific treatment for a lump behind the knee as the choice of treatment depends on the underlying cause. Medication like anti-inflammatory drugs may help to reduce inflammation but this is only effective if the lump is due to inflammation. Invasive procedures like incision and drainage or fine needle aspiration may be helpful for abscesses or cysts.

In the event of injury or strain it is important to implement a few conservative measures known as the R.I.C.E. method until medical attention can be sought. This includes resting the knee and leg, applying an ice pack to the area, compression of the affected region and elevating the limb. It is only useful for soft tissue injuries.

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