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".
Knee swelling can come on suddenly or gradually, may be mild or severe and may limit the amount you can move your leg making walking very uncomfortable. It may even be a sign of a serious medical problem. But how can you tell?
A swollen knee indicates a problem somewhere inside the knee joint. It most commonly develops after an injury but does sometimes come on for no obvious reason usually due to an underlying knee problem.
Small amounts of knee swelling may not be visible but can still cause problems. In other cases, the swelling may be more obvious and widespread.
Here we will look at common knee swelling causes, the different types of swelling, how to tell whether it's serious, the best ways to treat a swollen knee.
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.
A swollen knee usually develops when excess fluid builds up inside the capsule and is caused by either:
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 knee feel very tight.
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 degrees of severity.
Knee swelling usually develops in one of four ways:
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.
Knee swelling after an injury is normally 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 three main injuries that cause a swollen knee from a haemarthrosis:
1. Ligament Tear: Where a knee ligament ruptures (tears completely). This is the most common cause and usually involves the ACL (Anterior Cruciate Ligament). Other symptoms include instability and pain Learn More >
A swollen knee caused by a haemarthrosis like these needs urgent medical attention.
Choose from the links or visit the Knee Injuries section to find out more about these common causes of knee swelling, including symptoms and treatment options.
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 swollen knee doesn’t usually feel tense.
The most common causes of a knee joint effusion are:
1. Meniscus Irritation: Compression or a small tear in the outer 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 one of the knee ligaments is overstretched, damaging a few fibres, but the ligament 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 (i.e. muscles & ligaments) to heal, but cartilage injuries can take longer, as the cartilage has a very poor blood supply.
Visit the Knee Injuries section to find out more about these common causes of fluid on the knee, including symptoms and treatment options.
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 in these cases.
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. Other symptoms of arthritis include stiffness and crepitus (noisy knees!).
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.
Occasionally, a swollen knee develops rapidly without any injury. The most common causes of this are:
1. Infection: Infections increase in the amount of fluid produced in the joint resulting in a swollen knee. Knee infections 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 before the swelling will go down.
2. Gout Knee: 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.
Gout is usually treated with medication and appropriate diet. Find out more about the causes, symptoms and treatment options in the Gout Knee section.
Usually, knee swelling remains inside the knee joint as the joint capsule acts like a barrier, preventing the fluid from escaping.
However, it can also occur outside the joint capsule, known as extra-articular swelling. The most common types of swelling outside the joint capsule are:
1. Knee 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. Swelling on the inner side of the knee may be due to Pes Anserine Bursitis. Visit the Bursitis section to find out more, including treatment information.
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.
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 misshapened, swollen knee.
2. Patellar Tendonitis: 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. They are often accompanied by fatigue, weight loss and a general feeling of being unwell.
4. DVT: Deep vein thrombosis is a blood clot in one of the deep veins, most common in the calf or thigh. They are normally painful, hot and red and are most common after prolonged bed rest, surgery or air travel. A DVT is a medical emergency - if you suspect you may have one see your doctor immediately.
5. Spontaneous Haemarthrosis: Sudden bleeding into the joint in someone with a blood clotting problem or who is taking blood thinners e.g. warfarin.
The most effective swollen knee treatment will depend on the cause of the knee swelling, but the most common ways to reduce the swelling are:
Fluid on the knee can indicate a serious problem, and you should always see your doctor with any unexplained or persistent knee swelling.
In most cases of a swollen knee, the whole joint swells up, but there are some knee conditions where the swelling forms a pocket at the back of the knee - find out more in the swelling behind the knee section.
If you have other symptoms besides a swollen knee such as instability, pain on the stairs or popping/cracking noises, then visit the knee symptoms guide.
Alternatively if you want more help working out what might be causing your pain, visit the Knee Pain Diagnosis section.
Page Last Updated: February 27, 2020
Next Review Due: February 27, 2022
1. Journal of the Royal Society of Medicine: The acute swollen knee: diagnosis and management. July 2013
2. Physiotherapy Journal: Efficacy of kinesiology taping in reducing knee swelling in patients who have undergone primary anterior cruciate ligament reconstruction. May 2015
3. Biomed Central: Short-stretch inelastic compression bandage in knee swelling following total knee arthroplasty study (STICKS): study protocol for a randomised controlled feasibility study. March 2015