By Mr Philip Dobson, Consultant Orthopaedic Surgeon specialising in knee and hip conditions in Newcastle (Newcastle Hospitals NHS and Nuffield Health).

A swollen knee is a common problem and can develop following an injury, increased activity or irritation within the joint. In some cases, swelling appears gradually without any obvious cause.

A small amount of temporary swelling may settle with simple measures. However, persistent, recurrent or significant swelling can indicate an underlying problem that requires assessment.

The timing of the swelling, associated symptoms and whether an injury occurred can all help identify the likely cause.

What causes swelling in the knee?

Swelling usually develops because excess fluid collects within or around the knee joint.

This may be caused by:

• Osteoarthritis
• A meniscal tear
• A ligament or cartilage injury
• Overuse or increased activity
• Bursitis
• Gout or another inflammatory condition
• Infection, although this is less common

The appropriate treatment depends on identifying what is causing the swelling rather than treating the swelling alone.

Swelling after a knee injury

A knee may swell after a twisting injury, fall, sporting incident or direct impact.

Rapid swelling within the first few hours can sometimes indicate bleeding within the joint following a significant injury, such as damage to a ligament, cartilage surface or the kneecap.

Swelling that develops more gradually over several hours or days may occur with a meniscal injury or a less severe soft-tissue injury.

Associated clicking, catching or locking may suggest damage to the meniscus or cartilage within the knee.

Osteoarthritis and knee swelling

Osteoarthritis can cause intermittent swelling, stiffness and aching within the knee.

Symptoms may become more noticeable after:

• A long walk
• Increased exercise
• Prolonged standing
• Repeated use of stairs
• A period of unusually high activity

Patients with arthritis may also experience reduced movement, grinding sensations and knee pain at night.

The amount of swelling can fluctuate. A knee may feel relatively comfortable on some days and become swollen and painful after increased activity on others.

Meniscal tears

The meniscus is a shock-absorbing structure within the knee.

A meniscal tear can occur following a twisting injury or develop gradually as the tissue becomes less resilient with age.

Symptoms may include:

• Pain along the joint line
• Recurrent swelling
• Pain when twisting or squatting
• Clicking or catching
• Difficulty fully straightening the knee

Not every meniscal tear requires surgery. Treatment depends on the type of tear, the severity of symptoms and whether the knee is genuinely locking.

Swelling at the front of the knee

Swelling directly over the kneecap may be caused by bursitis.

A bursa is a small fluid-filled sac that reduces friction between tissues. The bursa in front of the kneecap can become irritated following prolonged kneeling, repeated pressure or a direct impact.

This often causes a visible, localised swelling rather than swelling throughout the whole knee joint.

Could it be gout or another inflammatory condition?

A knee that becomes suddenly very painful, hot and swollen may be affected by gout, pseudogout or another inflammatory joint condition.

These conditions can sometimes resemble an infection. A new, acutely painful and swollen knee therefore requires prompt medical assessment, particularly when there has been no injury.

When is a swollen knee urgent?

Seek urgent medical advice when:

• The knee is hot, red and very painful
• You have a fever or feel generally unwell
• Severe pain and swelling developed suddenly
• You cannot put weight through the leg
• The knee has changed shape following an injury
• You cannot move or straighten the knee after an injury

A hot, painful and swollen joint accompanied by fever or illness can indicate an infection and should not be left to settle without assessment.

When should you arrange a specialist assessment?

A specialist assessment may be appropriate if:

• Swelling repeatedly returns
• Symptoms are progressively worsening
• The swelling persists despite rest and simple measures
• The knee catches, locks or gives way
• Pain is limiting work, exercise or everyday activities
• You are uncertain about the diagnosis

An examination, together with an X-ray or MRI scan where appropriate, can help establish the cause and guide treatment.

What can you do initially?

Following a minor injury or temporary flare, it may help to:

• Reduce aggravating activities
• Apply ice for short periods with the skin protected
• Elevate the leg when resting
• Continue gentle movement within comfortable limits
• Use suitable pain relief where medically appropriate

Complete inactivity is not usually necessary. However, exercise should be reduced if it is repeatedly increasing pain or swelling.

What treatments are available?

Treatment depends on the cause of the swelling.

Options may include:

• Physiotherapy and strengthening
• Activity modification
• Appropriate pain relief
• Injection treatments
• Keyhole surgery for selected mechanical problems
• Surgery for advanced arthritis where appropriate

For arthritis, corticosteroid, hyaluronic acid or platelet-rich plasma injections may be considered in selected patients. These treatments aim to reduce symptoms but do not reverse established arthritis.

In some patients, non-surgical treatment may improve pain and function sufficiently to delay knee replacement surgery.

When might knee replacement be appropriate?

A swollen knee does not automatically mean that surgery is required.

However, where swelling is caused by advanced osteoarthritis and is accompanied by persistent pain, stiffness and loss of function, knee replacement surgery may eventually become the most reliable treatment.

The decision should be based on the overall effect of the condition on your quality of life, rather than swelling or an X-ray finding alone.

Specialist assessment

If your knee remains swollen, repeatedly fills with fluid or is affecting your mobility, a specialist assessment can help establish the underlying cause.

The aim is to provide an accurate diagnosis and a personalised treatment plan that helps reduce symptoms, restore function and allow you to remain active.