Hard, Fatty, Painful Lump Behind Knee: Causes, Symptoms, Diagnosis, Treatment

A hard lump behind the knee is most likely a Baker’s cyst. If painful, it could be due to other causes such as injuries, arthritis or skin abscess. If the lump is causing no pain, it could be a benign tumor. If it keeps increasing in size, it could be a malignant tumor.

It is quite tempting to ignore a lump behind your knee. This is especially so if it is not causing any form of discomfort or pain. Ignoring a lump can bring undesired consequences. For one, lumps are closely associated with malignancy. Also, some may lead to the spread of infections in the body if left untreated. Severe and spreading infections are indicated by signs such as a hard painful lump under the armpit, in the throat or the groin area. This article examines Hard, Fatty, Painful Lump behind Knee: Causes, Symptoms, Diagnosis and Treatment.

Hard lump behind knee; Baker’s cyst

The most common cause of a hard lump behind knee is a Baker’s cyst (popliteal cyst). Basically, it is a sac-like structure under the skin behind the knee that is filled with fluid. Baker’s cysts cause skin tightening and get painful when the leg is fully stretched.


Just like grease is needed in moving parts of machinery, your knee joints require lubrication of their own, known as synovial fluid. Sometimes, too much of the fluid get produced, more than is needed. The fluid accumulates and forms a Baker’s cyst. This can be encouraged by:

  • Damaged cartilages
  • Arthritis
  • Inflammation of the joints
  • Knee injuries


A Baker’s cyst may form a small hard lump or grow larger . In either case, it will be noticeable by careful visual or physical examination. Other than that, the following symptoms may occur:

  • skin stiffness
  • pain when stretching your leg
  • swollen calf muscle

Some Baker’s cysts may go for a long time without being noticed. This is because most of them do not produce very noticeable symptoms normally.

Diagnosis and treatment

Your doctor will conduct a physical examination. He may also compare the affected knee with the healthy one to check the range of stretching.

An MRI scan will be considered to see if the cyst is due to injured knee cartilage. Ultrasound scan can also be used to confirm diagnosis. This is mostly if the cyst is accompanied by symptoms such as difficulties in walking or pain in the affected knee. It not due to injury, the cyst may be due to arthritis. This can be checked through X-rays scan.

Most Baker’s cysts do heal without treatment. If necessary, your doctor will choose from the following options:

  • Draining – draining is a popular treatment against cysts. For a Baker’s cyst, a needle will be inserted into the cyst. This usually requires ultrasound to guide the doctor.
  • Medications – Steroid based medications can be injected into the cyst to reduce inflammation. This treatment is meant to ease any pain caused by the cyst but not to keep it from coming back.
  • Physicaltherapy – Cysts generally respond well to cold and warm compress. It is also important to keep the affected knee busy with simple strengthening activities.

Painful lump behind knee; arthritis


There are three types of arthritis that can cause a painful lump behind knee:

  • Osteoarthritis – most common of all types. It progressively wears and tears the cartilages.
  • Rheumatoid arthritis – causes inflammation of the joints.
  • Post-traumatic arthritis – occurs after an injury


If the lump behind your knee is due to arthritis, you will also experience the following symptoms:

  • Pain in the knees. The pain will most likely develop gradually.
  • Swelling around the knee joints. The skin will also feel tender. This mostly occurs after sitting or being inactive for long.
  • Cracking sound in the knees, especially after sitting for long. It is caused by rubbing of knee joint bones against each other.
  • Weakness in the knee. For example, you will experience difficulty bending or straightening your knees. This symptom comes and goes.
  • Lost ability to travel long distances


Arthritis is not curable. However, there are a number of medications that can be taken to improve it.

Non-steroidal medications are mostly used to reduce inflammation and ease pain. Analgesics can be used in place of non-steroidal medications. Corticosteroids too are sometimes prescribed. For rheumatoid arthritis, immune modulators can be considered.

Your doctor may use injections to administer pain and inflammation reliving medications into the affected knee joint. A needle can also be inserted to draw out excess fluid.

Severe cases are treated through surgery. The whole joint can be replaced with a prosthetic one. A corrective surgery to replace the damaged parts of the knee joint can also be performed.

Painful lump on back of leg behind knee; injury

There are many types of injuries that can cause a lump on the back of the leg behind the knee. Such an injury may involve:

  • Cartilages – a cartilage is a tough tissue that cushions bones at joints. It plays the role of a shock absorber and a lubricator. Cartilages can get damaged during extreme sports, activities of by diseases.
  • Tendons – tendons are strong cords made of connective fibrous tissue. They are flexible but not elastic. They serve the role of connecting muscles to bone.
  • Ligaments – a ligament serves the role of connecting a bone to another bone. they are made of fibrous connective tissues. Ligaments can sprain or strain, leading to injuries and swelling.
  • Muscles – muscles are a number of tissues brought together to perform a similar task. Behind the knee, calf muscles are most prominent.

Any form of injury or trauma on any of the above structures could be the cause of a painful lump on back of leg behind knee. The severity and pain of the lump will depend on how serious the injury is. The same is the case for treatment options.

Soft and painful lump behind knee; abscess

A painful, tender lump behind your knee can as well be an abscess. Abscesses are mostly caused by bacterial infections. They can be encouraged by athletic activities and the conditions in a bodybuilding center. Symptoms of abscess include:

Abscesses are not hard to treat. If left alone, they can lead to serious complications and spreading of infection into the rest of the body. For treatment, antibiotics and painkillers are mostly used.

Fatty lump behind knee

A fatty lump behind knee is a lipoma. Lipomas are fat nodules that form under the skin. They form tender and soft lumps. The lumps feel a bit movable if pushed. Lipomas are not harmful.

In most cases, treatment to get rid of a fatty lump due to lipoma will not be necessary. If need be, it can be removed surgically. Lipomas are closely related to fibromas. Fibromas are lumps that form on connective tissues.

Lump behind knee in child

A lump behind knee in a child can be a Baker’s cyst. It could also be a lipoma or an abscess. It will depend on the symptoms.

It can sometimes get a bit difficult to diagnose a medical condition in a child. This is mostly because they can’t sufficiently provide information on the symptoms. For such reasons, consider getting medical help for a lump behind the knee in a child.

Hard lump behind knee; cancer

Rarely will a hard lump behind your knee be a sign of cancer. Cancer lumps are mostly identified by their tendency to indefinitely gain mass. Most of them are hard and painless especially in the first stage. When left untreated, cancer cells spread into the lymphatic system and ultimately to other body organs. By then, you will notice masses such as small lump on testicle, hard lump on breasts or in the lower belly.

If a lump behind your knee persists, shows suspicious signs, causes discomfort or is infected, consult your doctor for the available medical treatment.

More cases malignant tumors are being reported today. While experts fail to agree on whether self-examination at home reduces mortality due to cancer, it is known that early diagnosis increases chances of recovery. For this reason, consider learning more on topics such as what does a testicular lump look like, how to check for breast lumps, meaning of a lump on skin and such. Also remember to regularly go for checkups as your doctor recommends.