An Overview of Meniscal Cysts

A meniscal cyst is a collection of joint fluid caused by a tear within the meniscus cartilage, the joint pad over the knee. It may have no symptoms, or it may cause pain, swelling, or mechanical issues with the knee. Meniscal cysts are most commonly reported in 20- to 30-year-old males and are associated with a specific type of meniscal tear called a horizontal cleavage tear, which may be caused by over-rotation of the knee or direct impact to the knee.

An MRI or ultrasound may be necessary to diagnose a meniscal cyst. Treatment involves lancing and draining the cyst, although it will commonly return unless the underlying meniscus tear is also treated.

Meniscal cyst symptoms may not always appear, but when they do, the most common symptoms are:

• Pain in the knee when standing
• Tenderness directly over the joint line of the knee
• A bump or lump at the cyst site: Cysts are usually found near the lateral meniscus (outside of the knee)
• A cyst that becomes more visible as the knee extends, though the bump itself may be painless
• Size fluctuations in the cyst, though it may also remain seemingly unchanged
• Swelling in the knee joint or locking of the knee joint

Cysts may be caused by degeneration of the meniscus itself (related to age or osteoarthritis) or by direct injuries, such as impact to the front or side of the knee, an over-rotation of the knee, or disproportionate force on the knees, such as when running on uneven surfaces. Any of these instances may result in a tear of the meniscus cartilage, which then allows synovial fluid to leak out of the joint. 

The body is constantly making new synovial fluid and absorbing excess fluid. However, when fluid escapes the joint, it can collect and form the pouch that is a meniscal cyst. The cartilage tear can act as a one-way valve, where fluid can escape the joint into the cyst, but doesn't come back out the other way. Because of this, the cyst will continue to collect fluid.

Risk factors for meniscal cysts include:

• Previous knee injury or meniscal injury
• Participation in contact sports or any sports in which there's a frequent twisting of the knee joint, such as football, soccer, rugby, or tennis
• Increasing age or osteoarthritis, both of which cause a breakdown in menisci
• Associated ligament injuries, such as a torn anterior cruciate ligament (ACL)

A physician will ask specific questions related to knee pain, popping sounds, and any recent injuries or impact to the knee to first determine if a meniscal tear has occurred. A meniscal cyst can generally be palpated, and a doctor will then test the knee's range of motion to ensure no torn pieces of cartilage are trapped in the joint.

The common test for assessing meniscal tears and other knee injuries is the McMurray Test. Further diagnosis may be confirmed by magnetic resonance imaging (MRI) to see both the cyst and the corresponding tear.

Pain resulting from an associated meniscal cyst can be treated with occasional ice application and dosing of anti-inflammatory medications, while the cyst itself may be drained with a needle in a physician's office.

However, unless the underlying cause of the tear is addressed (or the tear heals itself), the cyst will usually return.

Surgery

Surgery may not always be necessary for a meniscal tear, though if your doctor recommends it, the best method is generally a minimally-invasive arthroscopic surgical approach.

Once the tear has been addressed, the meniscal cyst will usually decompress and less commonly returns. While it is possible for the cyst to come back, it is unlikely.

Meniscal cysts are similar to popliteal or Baker's cysts, but the latter are located in the back of the knee joint. In addition, a popliteal cyst is seen with many types of knee joint problems that lead to fluid accumulation or knee swelling. Therefore, they can occur when there is a meniscus tear, but also with arthritis, ligament injuries, and other problems that lead to knee swelling.

A Word From Verywell

It is important to understand that while called a cyst, a meniscal cyst is really just a pouch of dislocated joint fluid. It is not an abnormal growth and it is not a tumor. It is simply a collection of normal synovial fluid that has escaped to an abnormal location. If you aren't experiencing any pain, you may resume normal activities right away. If you do feel some pain, swelling, or locking of the knee, work with your care provider to determine the next best steps.