Lump on knee

Definition - What is a knee bump?

Colloquially, a bulge is a bulge on a part of the body that is caused by tissue swelling, fluid retention or tissue proliferation.
A bump on the knee is harmless in most cases. Most of the time it resolves itself in a short time without the need for special treatment. A doctor should be examined if there is severe pain or restricted mobility in the knee, the lump continues to grow or it does not go away on its own after a few days. In some cases, treatment with medication or a minor surgical procedure must then be carried out.


The possible causes of a bump on the knee are diverse and can be subdivided according to trigger and frequency of occurrence.
Most knee bumps are caused by injury to the knee joint. This can happen, for example, if you bump your knee or fall on your knees. There is an increased accumulation of tissue fluid in the affected area, which leads to the formation of the bump. It is usually painful and can also be red and slightly overheated. In the event of a serious injury such as a traffic accident or as part of a sporting activity, ligaments, tendons or bones of the knee joint can also be injured, which often leads to swelling of the entire knee and restriction of movement.

Another possible cause of a bump on the knee, which can arise even without a previous injury, is the so-called Baker cyst. This is a bulging out of the knee joint capsule that surrounds the knee from the inside like a stocking. The bump occurs in the hollow of the knee and can be larger and smaller or even temporarily disappear again.

A large group of possible causes of a bump on the knee is also inflammation. Various structures on the knee can become inflamed or the entire joint can be generally affected. For example, bursitis, which has a cushioning function when moving the knee, is quite common. On the one hand, inflammation can occur due to the penetration of germs into the knee joint (for example after a medical intervention).

On the other hand, it can lead to inflammation in the context of a chronic disease involving the joints, such as a chronic inflammatory bowel disease. In addition, a previous gastrointestinal infection can lead to reactive inflammation with bulging on the knee. Another cause is the acute attack of gout, in which an increase in uric acid levels leads to the precipitation of crystals in the joints. An exclusive involvement of the knee is rare but possible.

Bursitis on the knee

Many large joints have so-called bursa, which take on a cushioning function when stressed. There are several of them in the knee joint.
These bursa are prone to inflammation, which, in addition to pain, can manifest itself in a bump on the knee. The trigger is often improper or excessive stress. People with a job that puts a lot of strain on their knees, such as tiler or roofer, are particularly affected.
Treatment by protecting, cooling and relieving the strain is often sufficient. If necessary, the doctor will prescribe painkillers and possibly antibiotics. If an inflammation of a bursae occurs again and again, it may have to be surgically removed.

Also read: Bursitis on the knee

Lump on side knee

If you have a bump on your side knee, some of the possible causes of swelling are more likely than others.
Often it is a bursitis, in which pain, redness and overheating appear as symptoms. However, if the bump on the lateral knee shows no further signs of inflammation, it may also be a so-called meniscus ganglion. The outer and inner meniscus act as a kind of shock absorber in the knee joint.

If it occurs, for example, as a result of a twisting injury (often in sports with rapid changes of direction), the meniscus can tear. As a result, a fluid-filled protuberance can form on the lateral knee. A meniscus gangilion more often starts from the outer meniscus. Ultimately, what the bump on the lateral knee is in individual cases can only be determined by a medical examination and, if necessary, diagnostic imaging.

Read more on the topic: Meniscal ganglion

Bump after a fall

A bump on the knee that occurs as a result of a fall is in most cases harmless. By storing it up, cooling it and protecting it, the bump usually disappears within a few days.
A doctor should be consulted if the whole knee swells badly, the pain is very great and the knee can no longer be moved properly. If the bump does not go away after a few days, a doctor should also be seen.

You might also be interested in this topic: Joint swelling in the knee

Lump in the back of the knee

In many cases, a bump on the knee occurs in the hollow of the knee.
In the vast majority of cases, it is a so-called Baker cyst, which is a bulging out of the joint capsule. This capsule surrounds the knee joint and all parts involved, such as ligaments, bones and menisci. The joint capsule is filled with synovial fluid. An increase in fluid (for example due to an inflammatory process) or a heavy load on the knee can cause the capsule to bulge. Since the weakest point of the capsule is in the hollow of the knee, this is where the Baker's cyst, which is often visible as a bump, develops.

In addition to a feeling of pressure and a restriction of movement, there can be discomfort in the lower leg due to pinched nerves. In addition, the risk of clots forming in the veins (thrombosis) is increased, as the blood vessels can also be squeezed off by the cyst. In some cases the Baker's cyst ruptures and the fluid is distributed in the leg. This is usually noticed by pain and the fact that the bump on the knee is suddenly no longer there.
Often, however, a Baker's cyst never causes discomfort, so that the bump in the hollow of the knee does not need treatment. Otherwise surgical removal is indicated.

Read more on the topic: Baker's cyst in the knee

Concomitant symptoms

A bump on the knee can have various accompanying symptoms that can provide an indication of the cause of the bump.
Inflammation usually leads to reddening and overheating. In addition, there is usually pain, which often has a throbbing character. It makes no difference in terms of symptoms which cause the inflammation is based.
If a fever occurs and general well-being deteriorates, a doctor should be contacted as soon as possible. A bump in the back of the knee due to a Baker's cyst can be a concomitant symptom of restricted mobility. The knee joint can often no longer bend normally, so that you can no longer kneel or crouch, for example. In addition, some people experience tingling or numbness in the lower leg.

Bump on knee with no pain

If you have a bump on your knee that is not painful, the most common causes of a bump, such as inflammation or the result of an injury, are unlikely.
Instead, other causes are more likely to come into question in this constellation. Most commonly, a Baker's cyst is responsible for a painful bump on the knee. This bulging of the joint capsule occurs in the hollow of the knee and only needs treatment if it causes discomfort.
In patients who suffer from a bleeding disorder or who are taking blood-thinning medication, bleeding can also occur, which is manifested, among other things, by a bluish bump on the knee, which does not have to cause pain either.
An extremely rare cause of a bump on the knee is also a benign or malignant soft tissue or bone tumor. Again, there is often no pain. Therefore, a bump on the knee should be examined by a doctor, especially if it is steadily increasing in size, even if it does not cause pain or other discomfort.


The most important way to diagnose a bump on the knee is to get a physical exam by a doctor.
Based on the localization, size and texture as well as other factors such as reddening or overheating, it can already be determined in many cases what the cause of the bump is and what measures are indicated for treatment. The doctor also collects important information for the diagnosis by asking specific questions about the possible course of the accident, accompanying illnesses and long-term medication of the patient.

A more detailed examination is only required in certain cases to establish a diagnosis. For example, an ultrasound image of the bump on the knee can be performed. In addition, if an inflammatory event is suspected, the doctor may take blood to determine the inflammation values. In the case of severe bacterial inflammation, the knee joint sometimes has to be relieved by a prick with a hollow needle. A sample of the synovial fluid can then also be sent to a laboratory for examination. Only in rare cases, for example if there is a suspicion of an injury to ligament structures, further diagnostics such as MRI (magnetic resonance imaging) can be useful as additional imaging.


Whether or which treatment is required for a bump on the knee is determined by the triggering cause.
Dents resulting from the effects of force from a shock or accident should be regularly cooled and stored up. In addition, it is important to protect the knee joint as much as possible until the bump has subsided and there are no more symptoms. As a further supportive measure, an ointment bandage, for example with arnica ointment, can be applied overnight. In addition, short-term anti-inflammatory and analgesic tablets can be prescribed by the doctor.

If the cause of the bump is inflammation in the knee joint, the measures mentioned are often sufficient for a successful therapy. In many cases, a simple bursitis can heal without extensive treatment. If necessary, antibiotic therapy is indicated for a few days.

In the case of an inflammation of the knee joint caused by bacteria with heavy effusion, in some cases a hollow needle has to be inserted into the joint under local anesthesia in order to relieve it. If there is repeated inflammation of the bursa on the knee, the affected bursa may have to be removed by a minor surgical procedure.

Surgical removal of the bump in the hollow of the knee may also be necessary with a Baker's cyst if it causes severe discomfort or if it has burst. In most cases, however, no intervention is required and symptomatic treatment can be carried out by cooling and relieving.

Read more on the topic: Treatment of a Baker's cyst

How long has a knee bump been there?

The length of time a knee bump persists depends primarily on the cause.
If the bump was the result of an injury or an accident, the bump will usually go away within a few days. If the joint is protected, regularly raised and cooled, the bump can subside more quickly. However, if the bump does not get smaller or even bigger after several days, a doctor should be consulted early on. There may then be an injury to the tendons, ligaments, or a meniscus.

The duration of a bump caused by a bulge in the joint capsule (Baker's cyst) varies widely.In some patients this disappears on its own after a few days, in others it remains for a lifetime or until surgical treatment. A bump due to inflammation, for example a bursa, usually disappears within a few days with appropriate treatment. However, inflammation in the knee joint can also lead to a delayed course with recurring deterioration, so that the lump can last for several weeks or even months.