A swollen knee is a common problem caused by accumulation of fluid in, or around, the knee joint. Knee swelling is commonly referred to as "water on the knee" or a "knee joint effusion".
Swollen knees can develop 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.
Knee swelling most commonly develops after an knee injury but does sometimes come on for no obvious reason usually due to an underlying knee condition such as gout.
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 and how to tell whether it's serious. Then we'll go on to look at the best swollen knee treatment options.
A swollen knee is a common problem that affects people of all ages. Knee swelling typically indicates a problem somewhere inside the knee joint.
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, think of it 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 joint capsule and is caused by either:
Often, 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, such as with knee bursitis.
Knee swelling usually develops in one of four ways:
Let's have a look at the features of each and then we'll look at the common causes.
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. The blood pools inside the capsule building up pressure in the joint.
Knee swelling after an injury is normally profuse and the knee typically balloons very quickly. It will feel tense and very sore and is often accompanied by bruising, although that may take longer to develop. A sudden, swollen knee caused by a haemarthrosis needs urgent medical attention.
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.
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.
Occasionally, a swollen knee develops rapidly without any injury. In most cases it is due to an infection in the joint and requires urgent medical attention.
Let's have a look at the most common causes of a swollen knee and the features of each.
Ligament tears are a common cause of swollen knees. Ligaments are tough, fibrous bands which connect bone to bone and provide stability. There are two pairs of ligaments in the knee
Ligament tears occur when one or more of the knee ligaments are stretched beyond their elastic limit and tear. The speed and amount of knee swelling with a ligament tear will depend on the extent of the damage.
Another common cause of knee swelling is a tear in the meniscus, the thick layer of cartilage that lines the knee.
Meniscus tears typically fall into two categories:
The amount of swelling depends not only on how large the meniscus tear is, but also which part of the meniscus is damaged. The inner part of the meniscus has a poor blood supply so doesn’t tend to bleed much when damaged and therefore doesn't cause much swelling. However, the outer part does have and a tear in this region is a common cause of a swollen knee.
Common symptoms associated with a meniscus tear include locking (where the knee gets stuck) and pain on stairs and when squatting Learn More >
A break in one of the knee bones, such as a patellar fracture, will cause rapid, extensive knee swelling. It takes a good deal of force to break one of the knee bones and there may be an obvious deformity as well as a severely swollen knee. It will also be very difficult to walk or move the knee Learn More >
Arthritis is the most common cause of gradual knee swelling in the over 60's, often referred to as water on the knee.
Osteoarthritis causes wear and tear in the knee joint. If an arthritic knee has been overworked, or gets knocked or twisted, the joint gets irritated and responds by producing extra synovial fluid to try and protect and heal itself. This results in a swollen knee.
Knee swelling from arthritis tends to come on gradually and often fluctuates. Learn More >
Knee infections are a potentially serious cause of a swollen knee. Infections can contaminate and increase in the amount of fluid produced in the joint. Knee infections usually develop after surgery or a deep cut, but sometimes an infection in your body can spread to your joint.
Knee swelling from an infection is often accompanied by a fever, intense knee pain and redness around the knee and requires immediate medical attention.
It is very difficult for your body to fight an infection within a joint. Treatment for an infected swollen knee may consist of antibiotics, aspiration (where the fluid is drained) and sometimes surgery.
An intensely painful, swollen knee joint that develops rapidly, often at night, is often caused by 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
Gout is usually treated with medication and appropriate diet. Find out more about the causes, symptoms and treatment options in the Gout Knee section.
Pockets of swelling around the knee are often due to bursitis. Bursa are small fluid filled sacs that sit between bones and soft tissues. If there is excessive friction on them, they get inflamed and swell. You tend to get pockets of swelling rather than general swelling of the whole knee.
The location of the pocket of knee swelling depends on which bursa is affected:
Find out more about the causes, treatment and types of Knee Bursitis >
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, known as a haematoma. If there is only a small amount of bleeding it is usually referred to as a contusion/bruise.
The most effective swollen knee treatment will depend on the cause of the knee swelling. Common tools for reducing knee swelling include:
Other things that can help reduce knee swelling include compression bandages e.g. tubigrip, massage, and positioning and it is really important to get the right balance between rest and movement.
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.
Find Out More: Best Swollen Knee Treatments >
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.
Alternatively, the swelling may be more on one side of the knee or above the knee - find out about the possible causes and how to treat them in the Side Knee Swelling and Swelling Above Knee articles.
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.
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 for help working out what is wrong.
Page Last Updated: 12/01/23
Next Review Due: 12/01/25
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