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Lump around the knee, what is the cause?

There are a few causes for a lump around the knee joint. Assessment by a specialist doctor followed by the appropriate imaging is essential to establish the diagnosis. So, what are the causes of a lump around the knee?

Baker’s cyst

A Baker’s cyst is by far the most common cause we see for a swelling and refers to build-up of fluid at back of the knee. Sometimes, it is associated with excess production of fluid within the knee joint (referred to as “knee joint effusion”) and part of this fluid may be pushed to the back of the knee resulting into a Baker’s cyst. Therefore, a Baker’s cyst is usually secondary to an underlying knee joint condition that results in build-up of fluid, such as knee joint inflammation (for example in conditions like Rheumatoid arthritis), knee joint wear and tear (osteoarthritis) and meniscal tears. If the cyst is large enough, it can restrict the joint pain, particularly knee extension.

A Baker’s cyst can sometimes leak (referred to as a ruptured Baker’s cyst). This usually presents with sudden pain and swelling in the calf and can be mistaken for a more serious condition, called deep vein thrombosis or “DVT”.

Baker's cyst ultrasound assessment

Ultrasound examination is very useful for the assessment here. It will confirm the diagnosis of a Baker’s cyst and rule out other serious conditions (like DVT). It will also assess for presence of excess fluid within the knee joint (joint effusion). If the ultrasound shows that the lump is unlikely to be a Baker’s cyst, then an MRI examination might be needed to further assess the swelling at the back of the knee. other conditions that can cause swelling at the back of the knee includes a soft tissue tumour or enlargement of the artery that runs at the back of the knee (called popliteal artery aneurysm). Therefore, it is very important to see a doctor if you develop a swelling around the knee. Ultrasound is usually the first modality used for the assessment, to help get the right diagnosis and decide about any further management plans.

If you are considering having aspiration of a Baker’s cyst, please see our article.

Meniscal (para-meniscal) cyst

A meniscal cyst is a small pocket of fluid usually seen at the margin of the knee (either the inner or outer margin) but can be also seen at the front or the back of the knee, especially if they are large. This type of cysts is almost always associated with a meniscal tear.

The meniscus is a C shaped structure made of a special type of cartilage (called hyaline cartilage). It acts as a shock absorber and plays an essential role in knee stability. There are two menisci in each knee (a lateral meniscus at the outer aspect of the knee joint, and a medial meniscus at the inner aspect of the knee joint). The meniscus is susceptible to injury particularly during a sudden twisting or rotation movement. This can happen in sport activities, and this type of injury is usually referred to as (acute meniscal tear). Tears can also happen without a specific injury, especially in the older population; this type is referred to as (chronic degenerative meniscal tear). The MRI picture below shows a small tear within the posterior part of the meniscus (arrow).

A meniscal cyst happens when the meniscal tear allows the fluid to go from the knee joint into a small out pouching at the side of the joint. A Meniscal cyst usually presents as a painful lump at the side of the knee joint. The fluid within these cysts is usually thick and viscous and therefore the lump can feel hard. The cyst can change in the size as the fluid within the cyst can move from the knee joint into the cyst, and also in the opposite direction. Sometimes the meniscal tear act as a one-way valve, allowing fluid to go into the cyst but not out of it. In this case, the size of the cyst will increase gradually. There is often associated knee joint arthritis.

MRI is the modality of choice for assessment of meniscal tears and parameniscal cysts. It will assess the meniscal tear and the joint cartilage in details providing very useful information for the management. Ultrasound is also useful for the assessment and often will diagnose a meniscal cyst. Please see our article about the role of MRI vs. Ultrasound in assessment of knee problems. The MRI image below shows a large para meniscal cyst (White) at the side of the knee joint.

The management of a meniscal cyst consists of treatment of the underlying cause (ie the meniscal tear) and knee joint arthritis. Sometimes, the management is conservative including rest, physiotherapy and anti-inflammatory tablets. If the cyst is large or painful, then an ultrasound guided aspiration and steroid injection can be considered, especially if the cyst is associated with a degenerative type meniscal tear and there is underlying knee joint arthritis. Surgical treatment might be indicated sometimes to repair the torn meniscus, to prevent the cyst from coming back.

What are other causes for a lump at the side of the knee?

There are other causes for swelling/lump at the side of the knee. A sinister cause (a tumour arising from the bone or soft tissues around the knee) is rare but possible. Therefore, it is very important to see your doctor if you notice a swelling around your knee. Imaging is very useful for the assessment. This usually starts with an ultrasound examination which is very useful to assess soft tissue lumps particularly if it is close to the skin. Ultrasound can give the diagnosis in a good number of cases. If there is uncertainty, then an MRI would be indicated to further assess the lump. A knee x-ray can be also useful for the assessment if there is suspicion of a bony growth.

Knee conditions and treatment


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Specialist Consultant Musculoskeletal Radiologist Doctor with extensive experience in image-guided intervention

To book a consultation:

Call us on 020 3442 1259 or Book online

The Musculoskeletal Ultrasound & Injections clinic

Unit 3, Brentside Executive Park

Brentford, TW8 9DR

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