17678 Swollen Knee

Overview

A swollen knee happens when extra fluid builds up in or around the knee joint. Doctors call this fluid a joint effusion. The knee may look puffy, feel tight, or be harder to bend. Swelling can come on suddenly after an injury, or slowly over days or weeks. It is a symptom, not a disease on its own, so the cause matters.

Yes. Knee swelling is a common reason people see a doctor or physiotherapist. It can happen at any age.

Anyone can get a swollen knee. It is more common in people who play sports, have had a knee injury, have arthritis, or are older. Children can also have knee swelling, and it should be checked by a doctor.

Symptoms

Common symptoms

• Puffiness or fullness around the knee
• The knee looks larger than the other one
• Tightness or pressure in the knee
• Pain, especially when bending, walking, or standing
• Stiffness or trouble bending the knee fully
• Warmth or redness over the knee
• A feeling that the knee may give way or lock

Symptoms in children

• Limping or not wanting to walk
• Crying with knee pain or when the knee is touched
• Swelling after a fall or sports injury
• Fever with a swollen knee
• Refusing to straighten the leg

Symptoms in older adults

• Swelling that comes and goes with activity
• Stiffness after rest
• Knee pain with walking or climbing stairs
• Swelling after a minor fall
• More bruising if taking blood-thinning medicine

Causes

Main causes

• Injury, such as a sprain, ligament tear, meniscus tear (cushioning cartilage), or fracture
• Osteoarthritis — wear-and-tear arthritis that can cause extra fluid
• Gout or pseudogout — crystal arthritis that can cause sudden, hot swelling
• Infection in the joint — this is serious and needs urgent care
• Bursitis — swelling of a small fluid sac near the knee
• Rheumatoid arthritis or other inflammatory arthritis
• Bleeding into the knee, often after injury or in people who take blood thinners
• Overuse or repeated strain from sports or work
• Being overweight, which puts more stress on the knee
• A knee replacement or knee surgery

Risk factors

• Playing sports that involve running, jumping, or twisting
• A past knee injury or knee surgery
• Older age
• Being overweight
• Family history of arthritis
• Gout or kidney problems
• Taking blood-thinning medicine
• A weakened immune system or recent infection
• Repeated kneeling or heavy lifting at work

When to see a doctor

See a doctor urgently if:

• Your knee is hot, red, and very painful.
• You have a fever with knee swelling.
• You cannot walk or put weight on the leg.
• You had a fall or injury and the knee is very swollen.
• The swelling is getting worse quickly.
• You have calf swelling, chest pain, or shortness of breath — call emergency services now.

Book a routine appointment if:

• Swelling that comes and goes over weeks.
• Knee stiffness or pain that limits your usual activities.
• Mild swelling after exercise that does not settle.
• A knee that clicks, locks, or feels unstable.
• You are unsure what caused the swelling.

Diagnosis

A doctor or physiotherapist will ask what happened, how long the knee has been swollen, and whether you have pain, fever, or other health problems. They will look at both knees, feel for warmth and fluid, and check how well you can move and walk. The cause is found from your story, the exam, and sometimes tests.

Tests that may be done

• Blood tests to look for inflammation or infection
• X-ray to check for fractures or arthritis
• Ultrasound to look for fluid and guide a needle if fluid is drained
• MRI scan (a detailed picture using magnets) for soft-tissue injuries such as ligament or meniscus tears
• Joint aspiration — using a thin needle to remove a small sample of fluid to check for infection or crystals

What to expect at your appointment

You may not need all tests. If the doctor thinks the knee may be infected, they may drain fluid the same day. Some results take a few days. Ask what to do while you wait and when to come back.

Treatment

Treatment depends on the cause. The first goals are to ease pain, reduce swelling, and protect the knee. Many people improve with rest, ice, activity changes, and physiotherapy. If there is an infection, injury, or arthritis, your doctor will talk with you about the right plan.

Self-care at home

• Rest the knee, but do not stop moving it completely. Gentle movement helps.
• Put an ice pack wrapped in a cloth on the knee for up to 15 to 20 minutes at a time.
• Raise the leg on pillows when sitting or lying down.
• Use a soft bandage or sleeve only if a healthcare professional advises it. Do not wrap it tightly.
• Avoid heavy lifting, deep squats, and sports until the swelling settles.
• Ask your pharmacist or doctor about pain relief that is safe for you.
• Try gentle thigh-muscle exercises if they are comfortable.
• Use a walking stick or crutch if it helps you move safely.

Medical treatments

Your healthcare provider will treat the cause. This may include physiotherapy to build strength and movement, draining extra fluid from the knee, medicines for infection or inflammation, or treatment for gout or arthritis. They may also advise weight management, foot care, and activity changes. Always ask your clinician before starting any medicine, and follow their instructions.

When is surgery considered?

Surgery is only needed for some causes. It may be considered for a torn ligament or meniscus, a broken bone, a knee that locks, or severe arthritis that has not improved with other treatment. Your care team will explain the benefits and risks.

Living with this condition

Pace yourself. Break activity into short sessions and rest when the knee feels hot or more swollen. Keep using the knee gently so it does not get stiff. Check the swelling each day and note what makes it worse. Follow any exercise plan from your physiotherapist.

Lifestyle tips

• Keep to a healthy weight to reduce strain on your knees.
• Wear supportive, comfortable shoes.
• Warm up before exercise and cool down after.
• Build strength in the thigh and hip muscles.
• Choose low-impact activities such as swimming or cycling if they are comfortable.
• Avoid sudden increases in training or repeated deep knee bending.
• Use knee pads or safe equipment for sports.

Diet and exercise

A balanced diet with enough protein, calcium, and vitamin D supports muscle and bone health. Exercise helps keep the knee moving and the muscles strong. Ask a physiotherapist or doctor which exercises are safe for you. Start slowly and stop if pain gets much worse.

Mental health and emotional wellbeing

Ongoing knee pain or swelling can be frustrating and can affect sleep, work, and mood. It is normal to feel worried or low at times. Tell your doctor if pain or worry is affecting your daily life. If you feel overwhelmed or in crisis, contact your local emergency services or a local crisis support service right away.

Prevention

Not all knee swelling can be prevented, but you can lower your risk. Strong muscles, safe exercise, and a healthy weight help protect the knee. Treating arthritis and avoiding overuse also matter.

Vaccines

There is no vaccine that prevents a swollen knee. Keep up with routine vaccines recommended by your local health service.

Screening programmes

There is no routine screening test for a swollen knee. If you have ongoing knee pain, stiffness, or swelling, ask your doctor for an assessment.

Complications

If left untreated

• An infection in the knee can damage the joint if it is not treated quickly.
• Ongoing swelling can lead to stiffness and weaker thigh muscles.
• Pain and instability can make walking and daily tasks harder.
• Some injuries, such as ligament or meniscus tears, may cause long-term problems if ignored.
• A blood clot in the leg can be serious if not treated.

Long-term outlook

Most people with a swollen knee improve with the right care. Some need physiotherapy or treatment for an underlying condition. Getting checked early helps rule out serious causes and gives you the best chance of a good recovery.

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