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Swelling Behind the Knees - Causes, Symptoms and Treatments

Swelling behind the knees may be caused by a variety of conditions.

A Bakers Cyst occurs when the popliteal bursa at the back of the knee becomes inflamed. 

Sometimes you may experience pain and swelling at the back of the knee, while there is only a lump with no pain at other times. 

Swelling can affect one or both knees. You can treat the swelling if it’s not severe by a mix of exercises, ice, compression bandages, physical therapy, and rest. However, there are certain cases where you will require to perform knee surgery.

The condition can be chronic if you are experiencing pain. For instance, arthritis is known for gradually damaging the knee. 

If you are asking yourself this question, why is the back of my knee swollen? Here are a few conditions that may cause swelling of the knees and what to do if you experience it.

Why is the Back of My Knee Swollen?

Various factors can lead to swelling behind the knees. The most common cause of back knee swelling is an abnormal growth or a build-up fluid in the popliteal space (the soft tissue at the back of the knee).

This article highlights common reasons for a swollen knee, symptoms, causes and how to examine if it’s severe, and the best ways to treat them.

Bakers Cyst

Bakers cyst, also known as the popliteal Cyst, is one of the common causes of swelling behind the knees.

What is a Baker’s Cyst?

The popliteal bursa (a small fluid-filled sac) behind your knee will be inflamed for a baker’s cyst to develop. 

swelling behind the knee

The popliteal bursa is located between the semimembranosus hamstring tendon and the gastrocnemius calf muscle. The bursa reduces friction between the two muscles, allowing for painless knee movement.

Causes of A Bakers Cyst

A fluid-filled sac surrounds the knee joint by lubricating and supplying it with much-needed nutrients to keep it healthy. 

In case of knee damage or wound, it will produce more fluid to protect the knee joint, lubricate it, and speed up the healing. 

However, when the fluid becomes excess, it will travel into the popliteal bursa through the joints and pool causing inflammation and later developing a Baker’s Cyst.

Osteoarthritis is one of the most common causes of swelling behind the knees from a Baker’s Cyst, and most knee arthritis patients suffer from one at some point.  

Popliteal Cyst can also be caused by Gout knee, inflammatory arthritis, ligament strain, or knee injuries such as meniscus rupture. 

Here are the main symptoms of a Baker’s Cyst: 

  • Swelling and discomfort behind the knee
  • Throbbing pain in the knee and calf
  • It may feel as if your knee is giving way

A Baker's Cyst in the popliteal fossa causes a palpable pocket of swelling that feels like a squashy orange.

People with a Popliteal Cyst may detect a modest amount of swelling at the back of their knee before experiencing any knee pain.

There will be more noticeable swelling, and pain below the knee as the knee cyst fills with fluid and grows, especially while bending the knee, which squashes the popliteal cyst.

The fluid will leak into the leg when a popliteal cyst behind the knee bursts, causing pain to travel to the ankle.

A popliteal cyst that causes swelling and pain below the knee is easily treatable. Still, if not treated appropriately, it has a significant risk of recurrence.

Popliteal Aneurysm

An abnormal swelling in the popliteal artery (the main artery behind the knee) is referred to as a popliteal aneurysm. 

If the back of your knee is swollen, a popliteal aneurysm may be the leading cause of the swelling. These swellings usually have no discomfort but are asymptomatic.

Popliteal aneurysms may cause swelling behind the knees. It is easy to detect the swelling behind the knee if you bend your knee. One can tell if the lump at the back of your knee is a popliteal aneurysm through the palpable pulsating lump. 

Bilateral popliteal aneurysms, or swelling behind both knees, occur in around half of all cases. One can feel the pressure on the surrounding nerves if the popliteal aneurysm is large. You may experience numbness, pins, and needles, or weakness in the calf.

Smoking, atherosclerosis, inflammatory processes, and hereditary factors are some of the causes of popliteal aneurysms; however, the reasons are unclear. 

swelling behind the knee

Build up of tiny blood clots and weakening of the arterial wall may lead to popliteal artery swells. Decreased blood flow and discomfort on your knee can be triggered by blood clots breaking off, hence blocking the smaller arteries in the leg.

If this happens, it can result in tissue death and amputation if you do not address the situation promptly.

To confirm why the back of your knee is swollen, an ultrasound can help with diagnosis, and one may require knee surgery.

Synovial Sarcoma

A cancerous growth that develops a swelling behind the knee is known as synovial sarcoma. It is one of the rare causes of swelling behind the knees, but it is dangerous and requires immediate attention and treatment. 

Young adults are prone to experience swelling behind the knee. Here is a brief of how sarcoma looks like:

  • It is usually larger than 5cm
  • It expands in size
  • It’s not painful at all times

According to health experts, it’s prudent to seek medical attention if the soft tissue lesion is larger than 5cm in diameter and continues to expand in size. CT scan or ultrasound may be ideal for carrying out the diagnosis.

Depending on the results, you may be assigned to a specialist for treatment if the scan reveals a sarcoma. Treatment may range from chemotherapy, surgery, or radiation according to the grade and stage of the sarcoma.

swelling behind the knee


Your life may be at risk when the swelling relates to malignancy. Do not ignore any type of swelling since it can quickly spread throughout the body if left untreated, depending on the stage and type of cancer. 

Swelling behind knees and related symptoms can cause severe disorders if you fail to seek medical attention leading to long-term damage and major complications.

Once the problem has been established, it would be best if you stick to the recommended treatment plan by your health care expert. Failure to stick to it, you’re exposing your body to severe consequences such as:

  • Spread of cancer
  • Spread of infection
  • Disability
  • Back of knee swollen
  • Chronic pain

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