A lump on the side of knee – either inside or outside- is most likely a meniscal cyst on the side of the knee. A meniscal cyst is a collection of thick fluid arising from a tear of the meniscal cartilage, which is the cushioning of the knee. However, not all lumps in the knee are meniscal cysts. How do we diagnose a meniscal cyst on the side of the knee from other causes and what do we do about them?

How does a meniscal cyst develop? 

Usually, a tear forms in the meniscal cartilage from degeneration or an acute injury. Fluid from the knee joint then leaks out of the tear forming a cyst. So, a meniscal cyst is not a true cyst but an outpouching of displaced knee fluid. Also, the cartilage tear can form a one-way valve preventing the fluid from going back into the knee. The fluid then collects as a cyst below knee cap.

Often, meniscal cysts form in people with a past history of a knee injury, cartilage damage, or contact or twisting sports. Also, the presence of arthritis predisposes to the formation of a cyst on side of knee.

meniscal cyst

Clinical findings of a lump on the side of knee

Not all lumps on the side of the knee are obvious or painful. In cases where the lump is due to a meniscal cyst, the more common symptoms include pain with standing, swelling in the knee joint, and a lump or bump on the side of the knee that can change in size.

When examining the knee, we often find a lump close to the joint line. The lump is usually soft and may or may not be painful. Also, twisting or rotating the knee can cause pain.

Generally, we use an MRI to diagnose the cyst and the corresponding tear. However, an ultrasound can also confirm a meniscal cyst.

meniscal cyst

What is the difference between a meniscal cyst and a Baker’s cyst? 

A Baker’s cyst is a swelling behind the knee caused by an outpouching of the capsule at the back of the joint. Similar to a meniscal cyst, a Baker’s cyst can form from a meniscal tear. However, they also form from any knee condition that leads to an increase of knee joint fluid such as arthritis or ligament damage. Generally, we can tell the difference between a meniscal cyst and a Baker’s cyst based on the location and the size of the lump.

Could a lump in the knee be something else? 

Yes. Other causes of a lump in the knee include Housemaid’s knee, knee joint swelling and tumours such as lipoma. If the lump is growing in size or painful, you should speak to your doctor about a referral for further investigations such as an ultrasound or MRI.

Meniscal cyst treatment

As meniscal cysts arise from a degenerative torn meniscus, treatment reflects meniscal tear treatment. Generally, we recommend simple treatments such as anti-inflammatory tablets and exercise to build strength. In cases where the cyst causes pain, a cortisone injection guided by ultrasound into the cyst can help. We suggest surgery if pain continues despite simple treatments or you develop mechanical symptoms such as locking or giving way. The good news is that removing the cyst is not necessary. Simply removing the torn meniscus by arthroscopy is enough to stop it from coming back.

More on injections for lump on side of the knee

ultrasound-guided meniscal injection

In general, injections are useful for meniscal cysts that are large and painful. Under ultrasound guidance, we pierce the cyst using a needle. First, a numbing solution is injected into the skin and outside the cyst. Second, a needle is placed into the middle of the cyst. The cyst is then decompressed using more numbing fluid. Finally, we inject cortisone into the cyst to reduce inflammation and prevent a recurrence.

Sometimes, PRP injections are useful for meniscal tears especially if we want to treat the tear as well.

Final word on Sportdoctorlondon about a lump on side of knee

A lump on the side of the knee is often due to a meniscal cyst. We treat these cysts with simple treatments followed by an injection. Overall, we should limit surgery to those cases that fail simple treatments.

Other related knee conditions:

Dr. Masci is a specialist sport doctor in London. 

He specialises in muscle, tendon and joint injuries.

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