Swollen Knee

A swollen knee is a common problem caused by accumulation of fluid in or around the knee joint. It is commonly referred to as "water on the knee" or a "knee joint effusion".

A swollen knee can come on suddenly or gradually, and may limit the amount you can move your leg making walking very uncomfortable.

Knee swelling most commonly develops after an knee injury but does sometimes come on for no obvious reason or due to an ongoing knee problem.

Here we will look at what causes knee swelling to develop, the different ways in which the knee swells, what is serious and what isn't, and the best ways to treat the swelling.

What Causes Knee Swelling?

The knee has a joint capsule, which is like a sac that surrounds the whole joint. The capsule contains synovial fluid which nourishes and lubricates the joint, so that it can move smoothly (a bit like the oil in your car). The joint capsule acts as container, keeping the fluid within the knee joint. Knee swelling usually happens when excess fluid builds up inside the capsule and is caused by either:

A swollen knee. Arrow points to smwelling just above patella

A) Bleeding in the Joint: aka Haemarthrosis. This is normally caused by an injury and the knee swelling comes on rapidly (within minutes). The swelling can be intense making the leg feel very tense

B) An accumulation of Synovial Fluid: aka knee joint effusion or water on the knee. This type of swollen knee tends to come on gradually and may come and go, varying in amounts. It is most commonly associated with Arthritis.

Knee swelling most commonly develops in one of four ways:

1) Rapid swelling that comes up immediately after an injury
2) Delayed swelling that appears a few hours after an injury
3) Gradual swelling that develops and often fluctuates without an injury
4) Rapid swelling that comes on very suddenly without an injury

1) Rapid Swelling After An Injury

A swollen knee that develops immediately after an injury, within minutes, is usually due to haemarthrosis, where blood accumulates in the joint. Essentially what happens is that a structure inside the knee gets damaged and starts to bleed, building up pressure in the joint. The swelling is normal profuse and the knee balloons up. It will feel tense and very sore and is often accompanied by bruising, although that may take longer to develop. There are 3 main causes of Haemarthrosis:

Ligament tears are a common cause of knee swelling.

1) Ligament Tear: Where a ligament ruptures (tears completely). This is the most common cause and usually involves the ACL (Anterior Cruciate Ligament)

2) Meniscus Tear: A tear in the outer rim of the cartilage lining the knee

3) Bone Fracture: A break in one of the knee bones

A swollen knee like this needs urgent medical attention. Visit the Knee Injuries section to find out more about these common causes of knee swelling, including symptoms and treatment options.

2) Delayed Swelling After An Injury

If a swollen knee develops anything from a few hours to a few days after an injury, it is most likely due to an increase in the synovial fluid in the joint – a knee effusion. This happens when something inside the knee is damaged slightly causing irritation and a resultant increase in synovial fluid. The amount of swelling varies but it tends not to be as much as with a haemarthrosis and the knee doesn’t usually feel tense.

The most common causes of a knee joint effusion are a:

1) Meniscus Tear: A tear in the inner part of the cartilage lining the joint (the inner part of the meniscus has a poor blood supply so doesn’t tend to bleed much when damaged)

2) Knee Sprain: Where a ligament is overstretched, damaging a few fibres, but remains intact

The amount of knee swelling may vary day to day and it may feel like it comes and goes as the injury is healing. It usually takes 6-12 weeks for soft tissues (eg muscles & ligaments) to heal, but cartilage injuries can take longer, as the cartilage has a very poor blood supply (except at the edges).

Visit the Knee Injuries section to find out more about these common causes of fluid on the knee, including symptoms and treatment options.

3) Gradual Swelling Without An Injury

A swollen knee that develops gradually is usual a sign of an underlying knee condition rather than an injury. The fluid on the knee tends to come and go, and varies in amount. There is usually only mild to moderate amounts of swelling.

Arthritis is the most common cause of gradual knee swelling, often referred to as water on the knee. Arthritis is the wear and tear of the cartilage and bones. It causes the body to produce extra fluid in the knee, which fluctuates in amounts.

Sometimes if the leg has been overworked, or gets knocked or twisted, the joint gets irritated, and responds by producing more fluid to try and protect and heal itself, hence the term water on the knee.

Visit the Arthritis section to find out more including causes, symptoms and treatment options.

4) Rapid Swelling Without An Injury

Occasionally, knee swelling comes on rapidly without any injury. The most common causes of this are an infection or gout:

1) Infection: Infections increase in the amount of fluid produced resulting in a swollen knee. They usually develop after surgery or a deep cut, but sometimes an infection in your body can spread to your joint. It is very difficult for your body to fight an infection within a joint and sometimes surgery is required

2) Gout: High levels of uric acid (produced as part of the digestive process) cause sharp, needle like crystals to form in your joints leading to inflammation and water on the knee. It is usually treated with medication and proper diet.  Find out more in the Gout Knee section

Treatment for Knee Swelling

Treatment will depend on the cause of the swollen knee, but the most common ways to reduce the swelling are:

Knee braces/supports are a good way to reduce knee swelling

1) Ice: can be used to slow down the blood flow and therefore reduce swelling and pain. It is important to use it properly otherwise it can make things worse – see the Ice Therapy section for more details and the ice wraps section for the best ways to apply ice

2) Compression: Tubigrip and Knee Braces can be used to provide compression to the knee which helps reduce swelling. Click the links to find out more

3) Medication: Non-steroidal anti-inflammatories eg ibuprofen may be prescribed to reduce the knee swelling

4) Aspiration: Water on the knee can be drained by your doctor with a needle, but it does sometimes come back

5) Cortisone Injections: Cortisone is a steroid hormone that suppresses the immune system, reducing inflammation and pain

Other Causes of Swelling

Usually, knee swelling remains inside the knee joint as the joint capsule acts like a barrier, preventing the fluid from escaping. There are however, a few other causes of knee swelling.

Swelling Around The Knee

Swelling can also occur outside the joint capsule, know as extra-articular swelling. It is usually due to either inflammation in one of the sacs outside the joint (bursitis), or from a forceful injury to the muscles around the knee (Haematoma). The most common types of swelling outside the joint capsule are:

Bursitis commonly causes a swollen knee.

1) Bursitis: Bursa are small fluid filled sacs that sit between bones and soft tissues to reduce friction. If there is excessive friction on them, they get inflamed. You tend to get pockets of swelling rather than general swelling of the whole knee. Swelling in front of the knee cap of usually caused by Pre-Patellar Bursitis, aka Housemaids Knee. Swelling behind the knee, often like a squashy orange, is usually due to Popliteal Bursitis, aka Bakers Cyst. Visit the Bursitis section to find out more, including treatment info

2) Haematoma: Blunt trauma to the soft tissues around the knee can cause bleeding. The blood collects around the muscles and can build up into a hard lump. If there is only a small amount of bleeding it is usually referred to as a contusion/bruise.

Rare Causes of a Swollen Knee

1) Knee Cap Dislocation: the patella usually glides in a groove at the front of the knee but a forceful injury can push it out to the side, resulting in a swollen knee

2) Runners Knee: irritation of the patellar tendon can lead to mild swelling at the front of the knee

3) Tumour: there are various types of tumour that can cause knee swelling

4) DVT: Deep vein thrombosis is a blood clot in one of the deep veins, most commonly in the leg. They are normally painful and red and are most common after prolonged bed rest, surgery or air travel

5) Spontaneous Haemarthrosis: Sudden bleeding into the joint in someone with a blood clotting problem or who is taking blood thinners eg warfarin

Safety Advice

Fluid on the knee can indicate a serious problem, and you should always see your doctor with any unexplained or persistent knee swelling.

What Next?

If you have other symptoms or want more help working out what might be causing your pain, visit the Diagnose Your Pain section.

Go to Common Knee Injuries or Homepage


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