Knee Pain: Causes and When to See a Doctor

Most knee pain is caused by osteoarthritis, injury, or overuse, and a good deal of it settles with rest and simple self-care, but pain that is severe, follows a clear injury, or fails to improve after a few weeks should be checked by a doctor. Knowing which kind of pain you have, and which warning signs need urgent attention, takes a lot of the worry out of it. This guide, reviewed by a consultant orthopaedic surgeon, sets out the common causes and exactly when to seek help.

I ignored my own knee pain for years, telling myself it was just age. With hindsight I wish I had been assessed sooner, while there were more options on the table. So the section I care about most below is when to actually pick up the phone. If your pain has become a daily limit on your life, it may be worth reading the signs you may need a knee replacement.

What are the common causes of knee pain?

The most common causes of knee pain split into wear, injury, and overuse, and the likeliest culprit depends largely on your age and activity. The main ones are:

• Osteoarthritis: the gradual wearing of the joint cartilage; the most common cause in people over 45 and the leading reason for a knee replacement¹.
• Injuries: ligament sprains and tears (such as the ACL), torn cartilage (meniscus), and fractures, usually from a twist, fall, or sports incident.
• Tendon and overuse problems: patellar tendinopathy and “runner’s knee” from repetitive load.
• Inflammatory arthritis: such as rheumatoid arthritis affecting the knee, where the joint lining itself is inflamed.
• Gout and infection: a sudden, hot, very painful joint, which needs prompt assessment.
• Bursitis: inflammation of the fluid-filled sacs that cushion the knee, often from kneeling.

Knee pain is extremely common: around 1 in 5 adults over 45 have osteoarthritis pain in the knee, and many more have short-lived pain from injury or overuse².

When should you see a doctor about knee pain?

See a doctor if your knee pain is severe, follows a clear injury, or has not improved after a few weeks of rest and simple self-care. It is also worth getting assessed if the pain keeps coming back, disturbs your sleep, or stops you doing everyday things. Book an appointment if you have:

• Pain that lasts more than a few weeks despite rest, ice, and over-the-counter pain relief.
• Significant or persistent swelling.
• The knee locking, catching, or repeatedly giving way.
• Pain that disturbs your sleep night after night.
• Reduced movement, so you cannot fully bend or straighten the knee.

The NHS advises seeing a GP if knee pain does not improve after treating it yourself for a couple of weeks, if you cannot move the knee or put weight on it, or if it is very painful³. There is no prize for stoicism here; earlier assessment usually means more options.

What knee pain is a red flag for urgent care?

Some signs need same-day or urgent care rather than a wait-and-see approach. Seek urgent help if you have:

• A hot, red, swollen joint with a fever, which can signal infection in the joint.
• An inability to bear weight on the leg, or to straighten it.
• An obvious deformity of the knee or leg after an injury.
• Calf pain, warmth, or swelling, which can signal a blood clot.
• Knee pain with unexplained weight loss or feeling generally unwell.

These are uncommon, but they are the ones not to sit on. An infected joint, in particular, is treated as an emergency.

How is the cause of knee pain diagnosed?

The cause is usually worked out from your history and a physical examination, with scans reserved for cases that need them. NICE advises that osteoarthritis can be diagnosed clinically, without imaging, in people aged 45 or over with activity-related joint pain and either no morning stiffness or stiffness lasting no longer than 30 minutes⁴. Where an injury, a mechanical problem such as a locked knee, or inflammatory arthritis is suspected, an X-ray, MRI, or blood tests may be used to confirm the diagnosis and guide treatment.

How is knee pain treated?

Treatment depends on the cause, but for most everyday knee pain it starts with simple self-care: rest, ice, gentle movement, and over-the-counter pain relief. For longer-term pain from osteoarthritis, the core treatments NICE recommends are exercise, weight management, and pain relief, tried before any surgery is considered⁴. The options, broadly in order, are:

• Self-care: relative rest, ice, and over-the-counter pain relief for short-lived pain.
• Exercise and physiotherapy: to strengthen the muscles around the knee.
• Weight management: to reduce the load through the joint.
• Injections: a corticosteroid injection for a painful flare.
• Surgery: considered only when pain limits daily life and the above no longer help enough.

The full range of non-surgical treatments is in the alternatives to knee replacement. When pain reaches the point of limiting everyday life and non-surgical measures have run out, a knee replacement resurfaces the worn joint, and around 90 to 95% are still in place at 10 years¹. That decision, and what the operation involves, is covered in our complete guide to knee replacement surgery.