Lump behind Knee, in Child, Cancer, Small, Painful, Hard, Causes & Treatment

Lump behind the knee or a painful swollen area felt behind your knee or any part of your body can be caused by several conditions. In adults or child, the lump can be small, hard and painful. The lump creates an unsightly appearance, this might limit the knee flexibility or be a great cause of discomfort. Common causes of this kind of lumps include cancer, most commonly lymphoma, and baker’s cyst which is the accumulation of synovial fluid behind the knee or simple knee injuries.

For baker’s cyst, most affected people will show no symptoms. For other causes such as arthritis, and damage to the cartilage of the knee, severe pain and swelling are the most common symptoms. Here is an insight on the causes, the treatment and how to relieve the pain and discomfort caused by lump behind the knee.

Lump behind the Knee Picture

Lump behind the Knee Picture

Lump behind the knee

Pain caused by a lump behind the knee can be from a variety of causes. The lump in most cases will be a problem occurring in the bones, muscles, tendons, ligaments and blood vessels. Most of these complications can be managed without using surgical intervention. A lump limiting flexibility and causing discomfort behind the knee is in most cases caused by Baker’s cyst also known as popliteal cysts.

A knee lump may be painful or fail to produce any symptoms at all, especially if it is caused by a Bakers cyst. Same people especially if the lump is occurring in children, then they are likely to experience tenderness, warmth, bleeding and bruising and at times difficulties in movements of the knee joint.

If the underlying cause of the lump is an injury, you will need to seek immediate medical attention, you will also need to do so if you notice the following symptoms:

  • Instability of the joints
  • Painful swelling behind knee
  • Redness and warmth of the skin
  • Inability or difficulties in moving the knee
  • Pus or blood discharge
  • Pain that can be described as dull, sharp, burning, stabbing or aching
  • Swelling in other parts of the body
  • Fever and chills

Some of the symptoms caused by a lump behind the knee can be bearable and manageable. However, there are those symptoms that might indicate a serious underlying condition that will need to be evaluated in an emergency setting. These symptoms will include the following:

  • High fever in both adult and in children
  • Loss of sensation in the lower parts of the body
  • Deformity and breakage
  • Complete inability to move the body
  • Severe and uncontrolled pain
  • Hemorrhage or uncontrolled bleeding

When left untreated, the lump could become infected causing painful boils and abscesses. The lump can also be a tumor or a malignant growth, in such cases, the cancerous cells will start to spread to other parts of the body. The lump occurring behind the knee might also be caused by a serious or life-threatening condition, this can be very dangerous.

For a condition such as cancer-causing the lump, a Potential complication of behind knee lump could be life-threatening. This will depend on the stage and type of cancer. Boil and abscesses could also lead to widespread infection in the body. It could also lead to permanent damage. Some of the common complications will include:

  • Disability
  • Decreased athletic performance
  • Nerve problems causing tingling, numbness and severe pain
  • Spread of cancerous cells to other parts of the body
  • Spread of infection.

What causes behind knee Lump?

A painful swollen leg or knee is a common clinic scenario. A baker’s cyst is the most common cause of this lump. A baker’s cyst develops when joint fluid leaks out from the joint cavity into the tissues behind the knee. The lump can also be caused by an infectious process such as a boil or abscesses, trauma and rarely tumor.

Here are some of the possible causes of knee lump:

Lump behind knee cancer

Lump behind the knee can be caused by both benign and malignant tissue growth (cancer) of the skin, soft tissue or knee bones. Common cancer that might cause these bumps are:

  • Lymphoma  malignant cancer of the lymph nodes cells
  • Osteosarcoma a type of bone cancer
  • Sarcoma soft tissue cancerous tumor

Baker’s cyst

Also known as a popliteal cyst, this is a benign swelling of the knee joint. Baker’s cyst result from a problem with your knee joint, this includes conditions such as arthritis or a cartilage tear. This condition causes the knee to produce too much fluid which can lead to baker’s cyst.

Soft Lump behind the Knee, Baker's Cyst

Soft Lump behind the Knee, Baker’s Cyst

This is a painless lump that resolves on its own with no medical intervention.


This is a painful inflammation and stiffness of joints. According to arthritis foundation, arthritis is very common but not very well understood the condition. They clarify that arthritis is not a single disease but just a formal way of referring to joint pain or diseases.

The symptoms of arthritis might be mild, moderate or severe, they might also come and go from time to time. Common arthritis joint symptoms will include:

  • Swelling
  • Pain
  • Stiffness
  • Decreased range of motion

Damage to the knee cartilage

A knee cartilage is a connecting tissue found in the knee. Through it is a tough and flexible material, a cartilage can easily be damage. With a cartilage damage, you are more likely to experience joint pain, stiffness, and inflammation. Seek immediate medical attention in case of this kind of damage.

Abscess on knee

A lump behind the knee can also be an abscess, a swollen area within body tissue that contain an accumulation of pus.

Injury or fracture causing deformity and bleeding

Injury and fractures are the main cause of swelling, inflammation, and lump behind the knee. Depending on the extent of the injury you might have cartilage damage, a sprain, a strain or deep vein thrombosis. All these are known to cause lumps and swelling.


It is an abnormal collection of blood outside of a blood vessel. A hematoma occurs due to a damage of blood vessels wall. It may be tiny or large and could cause a significant amount of swelling.

Lump behind knee blood clot

A blood clot forming lump behind knee will form due to a condition known as deep vein thrombosis. Deep vein thrombosis DVT occurs when a blood clot forms in one or more of the deep veins in your body. DVT is common in legs, it can cause leg pain, or may occur at times with no symptoms.

The condition can occur if you have a certain medical condition that affects how your blood clot. It is also common from the condition to occur when you don’t move for a long time, this is common after surgery or following an accident.

Lump behind knee, not bakers cyst

Not all swelling occurring behind the knee are caused by a baker’s cyst. Though baker’s cyst is the most common cause of lump behind the knee, there is an array of other possible cause that might lead to the swelling or formation of the bump.

Common such cause is a traumatic event occurring behind the knee. Traumatic causes are also common and include such causes as fracture or bone from accidents and injuries, hematoma which is the collection of blood in body tissues and sprain and strains.

A sprain is the stretch r tear of a ligament, the severity of the injury will depend on the extent of the injury and the number of ligament involved, and on the other hand, a strain is an injury to either a muscle or a tendon. Depending on the severity of the injury, a strain could either be a simple overstretch of muscle or tendon or could result in a partial or complete tear.

Other causes of knee lump that are not baker’s cyst will include damage to the knee cartilage, cancer though very rare, and infection such as boil and abscess.

Lump behind knee cap

The knee cap is the convex bone in front of the knee joint. Also known as the patella or kneepan, it is a thick circular-triangular bone which articulates with the femur (thigh bone). It covers and protects the anterior articular surface of the knee joint. The knee cap is the largest sesamoid bone in the body.

A lump occurring behind knee cap or causing swelling can be caused by the following condition:

  • Compartment syndrome that includes excessive bleeding or swelling following surgery or injury. Injuries can result in increased pressure that affects the blood flow, this can result in tissue death necessitating amputation, nerve or muscle damage. This is a medical emergency, it can either be chronic or acute
  • Bone fractures and Knee injuries
  • Melanoma, common and the most dangerous for of skin cancer. Melanoma is the leading cause of death from skin disease. Melanoma involves skin cells called melanocytes, which produce a skin pigment known as melanin. This pigment (melanin) is responsible for the dark color of skin and hair.
  • Lymphadenitis which is the inflammation of one or several lymph glands also known as lymph nodes.
  • Varicose veins are the other leading cause of swelling and lump behind the knee cap. Varicose veins are veins that have become enlarged and twisted. Though common in legs, varicose veins can occur on any part of the body.
  • Sebaceous cyst caused by the clogging and accumulation of sebum on hair follicles can be the cause of the lump occurring behind the knee.

Lump behind knee in child

A swelling or lump occurring on the popliteal space or the area at the back of the knee is common as a result of baker’s cyst. This lump is common with both children and adults. In a child, this lump will present as a painless lump behind the knee. In most cases, the lump will only affect one leg.

In children and younger people, a baker’s cyst may develop just behind an otherwise healthy knee joint. When this happens, the lump or swelling is referred to a primary or idiopathic baker’s cyst. This is said to be caused by a connection between the knee joint and the popliteal bursa behind the knee. It, therefore, means that synovial fluid from inside the joint can pass into the popliteal bursa forming the baker’s cyst.

In children, the lumps are common at ages 4 to 7. The cyst especially if caused by baker’s cyst, are more common in adults than in children. Primary Baker’s cyst in children will often get better and disappear on its own with time.

Lump behind knee on tendon

The tendon is a flexible but inelastic cord of strong fibrous collagen tissue attaching a muscle to the bone. A lump behind the knee on tendon can be caused by conditions such as tendonitis. Also known as tendinitis, this is the inflammation of the tendon. The term tendinitis according to Wikipedia, should be reserved for tendon injuries that involve large-scale acute injuries accompanied by inflammation.

Gastrocnemius tendonitis, a calf muscle that crosses behind the knee and attaches above the knee joint can become strained when the knee is extended while the toe points upward, this can pull on the tendon.  This is common in cyclist.

Fatty lump behind knee

A lump behind the knee can also be caused by both benign and malignant tumors of the skin, soft tissue or bone. Common such fatty lump will include fibroma, a benign tumor composed of fibrous or connecting tissue, lipoma which are benign fatty growth, melanoma which is cancer resulting from the melanocyte in the skin or part of the body.

Other possible causes of this fatty growth behind knees will include:

  • sarcoma, a soft tissue cancerous tumor
  • osteosarcoma a type of bone cancer and
  • non-melanoma skin cancer

Hard lump behind knee

A hard knee lump behind the knee can represent a joint deformity due to a chronic condition such as arthritis or arising from knee and leg injury. A hard lump can be formed by a number of the condition. If you notice a hard lump that start to get painful or limit flexibility, you need to have it checked out as soon as possible.

Depending on what the underlying cause of the hard lump is, the lump may be single or multiple. The lump may also grow rapidly or may not change in size, it might be painful or painless, whatever the case, you need to have it diagnosed by a professional health care provider since some of the underlying cause of the hard lump might be life threatening.

Painful lump behind knee joint

Pain in one or all of the knees can be as a result of overusing the knee or caused by an injury. Cases of painful lump behind the knee are common and most do not require agent medical attention. When you run or jump, the knee joint is particularly vulnerable to damage and pain as it takes the full weight of the body and any exerted force.

As you grow older, you are more likely to experience this kind of knee pain. The other group of people with higher risk are those who are overweight and those who do a lot of sports. Common causes of painful lump behind knee might include simple sprain or strain, damage to the menisci or cartilage, osteoarthritis, and tendonitis.

Large lump behind knee no pain

A large lump behind the knee with no pain is the most common symptom of a Baker’s cyst. Also known as a popliteal cyst (to mean it occurs behind the knee) is a benign a noncancerous swelling of the semimembranosus or other synovial bursa found behind the knee joint.

A painless lump can also be a melanoma, a tumor of melanin-forming cells. Melanoma is a type of cancer that develops from the pigments that contain cells known as melanocytes, it typically occur on skin but may also rarely occur in the mouth, intestine or eyes [Wikipedia]

Small lump behind knee lymphoma

Lymphoma is the cancer of the lymph nodes. Lymph nodes contain cells knows as lymphocytes which are the infection-fighting cells of the immune system. Lymphocytes are present in the spleen, thymus, bone marrow and other parts of the body.

Lump behind Knee could be Lymphoma

Lump behind Knee could be Lymphoma

Small lump behind knee caused by lymphoma can be of two types. Non-Hodgkin or Hodgkin lymphoma. The actual cause of lymphoma is unknown, the risk of developing lymphoma increases if:

  • you have a week immune system
  • you take immunosuppressant
  • you have been exposed to a common virus

Treatment for Lump behind the knee

The treatment for a lump behind knee will depend on what the underlying cause is. After a medical diagnosis, your health care provider will be able to identify what the underlying cause is. This way the best treatment option for the underlying condition can be prescribed.

If for instance the underlying cause of the lump is cancer, the treatment option available would include surgery radiation and chemical therapy.  If on the other hand the cause is lymphoma, the health care provider might prescribe a transplant, biologic therapy, radiation or chemical therapy where strong chemical drugs are used to destroy the cancerous cells.

According to the University of Maryland Medical center, most cysts and lumps do not require medical treatment. Simple tricks like resting your leg throughout the day might help relieve inflammation and prevent further strain in knee. Apply an ice warp around knee to further reduce the inflammation caused by the lump.

If you have to stand for long, then you will need to wrap a bandage around knee, this helps support your muscles and ensure proper blood flow. You can also increase blood flow by elevating your leg during rest.

Our References and Sources