Swelling On Side Of Knee

Written By: Chloe Wilson, BSc(Hons) Physiotherapy
Reviewed by: KPE Medical Review Board

Swelling on the side of the knee is a common problem which may or may not be painful.

There are lots of different things that can cause lumps and swelling on the side of the knee but in most cases it’s nothing serious.

Side knee swelling usually means there is some kind of inflammation, tear, or damage in the ligaments or cartilage that hold your knee together but may also indicate an underlying medical issue.

There may be swelling on the inside of the knee or the outside of the knee, a hard lump on the side of the knee or generalised swelling around the joint.

It is very uncommon for a knee lump to be cancerous but any new growth should be evaluated by your doctor.

Here we will look at the common causes of swelling on the side of the knee and how to identify which one you have. We will then look at the best ways to treat side knee swelling. If you are more bothered by pain than swelling, check out the Inner Knee Pain or Outer Knee Pain articles.

The most common cause of swelling on the side of the knee is a cartilage tear aka torn meniscus.

The knee has a very thick layer of cartilage known as the meniscus that helps to protect the joint.

The knee meniscus is a flexible, rubbery pad that sits between your thighbone and shinbone.

It cushions the knee and helps to improve stability and movement of the joint, acting like a shield to keep the bones from grinding against each other when you move your leg.

Cartilage tears are a common cause of swelling on the side of the knee and may be caused by:

• An Injury: sudden overloading or pinching of the meniscus e.g. sporting tackles where the knee twists awkwardly, particularly when wearing cleats
• Wear & Tear: repetitive force or friction through the knee can slowly wear away the meniscus resulting in cartilage tears. This is particularly common over the age of 60 as the meniscus becomes more brittle with age. Everyday use of the knee can cause the meniscus to fray or tear.

Another common cause of side knee swelling is a tear in one of the collateral ligaments, aka a knee sprain.

The knee collateral ligaments (MCL & LCL) are two strong bands of tissue that sit on either side of the knee joint. They help to keep the knee stable and provide support when you move your leg.

Knee collateral ligament injuries are often caused by sudden, sharp movements like twisting the knee or making a hard landing. These unexpected motions can stretch or tear the ligaments, resulting in instability and decreased range of motion.

Ligament tears can cause swelling because they disrupt the normal flow of fluid in the joint and can cause extra fluid to collect around the joint. This extra pressure can cause the area to become puffy and painful.

The location of the knee swelling will depend on which ligament is torn:

• Medial Collateral Ligament Tear: causes swelling on the inside of the knee
• Lateral Collateral Ligament Tear: causes swelling on the outside of the knee

With ligament tears, the swelling at the side of the knee tends to be fairly localised around the area where the tear is. The larger the tear, the more swelling there is likely to be.

You can find out more about the cause of swelling on the side of the knee from ligament tears and how to treat them in the Knee Ligament Injury section.

The most common cause of swelling on the side of the knee in people over 65 is knee osteoarthritis.

Osteoarthritis is a degenerative joint disease that causes damage and wear to the cartilage and tissue surrounding the knee bones.

This damage leads to knee pain and swelling, as well as joint stiffness, which can make it difficult to bend or straighten the knee.

As knee arthritis progresses, the joint space, the normal gap between the bones, gradually narrows and bone spurs form. Over time, the bones may start grinding against each other, causing increased friction, damage and irritation.

Swelling is a common feature of osteoarthritis for two reasons. Firstly, the tissues lining the joint become inflamed and secondly, excess synovial fluid is produced as the joint tries to protect and heal itself.

In some cases, knee osteoarthritis is confined to one side of the joint and therefore there may only be swelling on that side of the knee, most typically the inner side. Other times, the arthritis extends throughout the joint leading to more diffuse swelling.

You can find out loads more about causes, symptoms, diagnosis and best treatment options in the knee arthritis section.

If you have a small pocket of swelling on the side of the knee, it may be due to knee bursitis.

Bursa are small fluid-filled sacs that sit between bones and soft tissues to allow smooth, friction-free movement.

If one of the knee bursa gets inflamed, it can cause a localised pocket of swelling on the side of the knee.

Bursitis may be caused by repetitive or excess force through the bursa e.g. prolonged kneeling, compression from muscle tightness, injury or arthritis.

Bursitis usually causes a defined, balloon-like pocket of swelling that can vary greatly in size, and may resemble anything from a small grape to a squashy orange.

There are a number of different bursa around the knee which can cause swelling in different places:

• Pes Anserine Bursitis: swelling on inside of knee, 2-3cm below the joint
• Iliotibial Bursitis: swelling on the outside of the knee, just below the joint
• Semimembranosus Bursitis: swelling behind the knee
• Prepatellar Bursitis: swelling in front of knee
• Infrapatellar Bursitis: swelling below knee
• Suprapatellar Bursitis: swelling above knee

You can find out all about the different types of bursitis, how they cause knee swelling and how to treat them in the Knee Bursitis section.

Another possible cause of a lump on the side of the knee is plica syndrome.

Knee plica are small folds in the synovial membrane, the thin sheet of tissue that lines and surrounds the knee joint, providing lubrication and protection.

In plica syndrome, these folds become inflamed, due to irritation from repetitive friction, a knee injury or underlying knee condition.

Plica syndrome typically causes a dull, aching pain in the knee which is often worse at night or when bending and straightening the knee e.g. stairs or squats. There is often a clicking or popping sound as you move the knee.

There may be localised knee swelling over the inflamed area, which may not be obviously visible, but when you press over the area it may feel “boggy”. The area of knee swelling will depend on which part of the plica is inflamed:

• Medial Plica Syndrome: swelling on inside of knee. Most common place to develop plica syndrome
• Lateral Plica Syndrome: swelling on outside of knee. Least common site for plica syndrome
• Suprapatellar Plica Syndrome: lump above knee
• Infrapatellar Plica Syndrome: lump under knee

You can find out lots more about the causes, symptoms, diagnosis and treatment options in the Knee Plica Syndrome section.