Knee pain: causes, red flags and when to see a doctor

The knee is the meeting point of the thigh bone, shin bone and kneecap, with ligaments providing stability, cartilage and menisci absorbing load, tendons transmitting muscle force and fluid-filled bursae cushioning friction points. A problem in any of these shows up as pain, swelling, stiffness or instability. NICE CKS also notes that pain felt in the knee can be referred from the hip or the lower spine.

CKS lists the broad causes as osteoarthritis, injuries such as muscle strain, ligament damage and fractures, inflammatory conditions, infection, tumours, referred pain and bursitis. Which of these is likely depends heavily on age and on whether there was an injury, which is why the NHS organises its list that way.

The NHS gives a table of symptoms and possible causes with the warning not to self-diagnose. The version below follows it and adds NICE CKS detail.

The NHS says to get advice from 111 now if your knee is very painful, you cannot move it or put any weight on it, it is badly swollen or has changed shape, it locks, gives way or painfully clicks, or you have a very high temperature or feel hot, cold or shivery with redness or heat around the knee. It also mentions urgent treatment centres, which may see you faster than A&E.

NICE CKS describes the red flags in clinical terms. It lists infection (septic arthritis or osteomyelitis), tumours, inflammatory polyarthritis and significant bony or soft-tissue injury. Immediate assessment in hospital is recommended for septic arthritis, fracture, neurovascular damage, rupture of the quadriceps or patellar tendon, severe soft-tissue injury with gross instability, a first-time traumatic dislocation of the kneecap, or a recurrent dislocation with moderate or severe swelling. Signs of joint infection such as fever, redness and swelling, or severe pain, swelling, instability or inability to bear weight after an acute injury, call for immediate referral. In children a limp calls for assessment.

The NHS gout page adds that sudden pain and swelling in a joint that is getting worse, with a very high temperature or feeling sick or unable to eat, could mean an infection inside the joint and needs urgent medical help.

• Hot, red, swollen knee with fever or shivering.

• Inability to put weight on the knee after an injury.

• A knee that has changed shape, or a kneecap that has dislocated.

• Locking, giving way or painful clicking.

• Severe pain or swelling that is getting worse quickly.

• A child with a limp or a knee that hurts without explanation.

CKS says a person with knee pain should be asked about trauma, the type and duration of pain, what makes it better or worse and any history of swelling. The knee is examined by inspection, feeling for tenderness and warmth, and assessing range of movement. After an injury, the assessment looks for fracture, tendon rupture and ligament tears, together with a check of nerves and circulation. CKS also lists an Ottawa knee rule for excluding a fracture, a clinical decision tool used in practice.

Risk factors are part of the picture. CKS names increasing age, obesity, knee-straining work and sport. A doctor will also ask about gout, anticoagulant medicine, previous knee problems and whether other joints are affected.

The NHS says a doctor can suggest treatment based on the cause, which might include medicine or physiotherapy, or referral to hospital for a scan or specialist treatment, for example surgery. Imaging, such as an X-ray or an MRI scan, is chosen to fit the suspected problem.

The NHS gout page says a GP may check uric acid in the blood and refer to a rheumatologist, who may take a sample of fluid from the joint. CKS says referral to a musculoskeletal service, physiotherapist or orthopaedic surgeon may be needed where symptoms persist or the diagnosis is uncertain, and urgent referral to a rheumatologist if inflammatory polyarthritis is suspected.

For the first days the NHS suggests putting as little weight as possible on the knee, for example avoiding standing for long, using an ice pack or a bag of frozen peas wrapped in a tea towel for up to 20 minutes every 2 to 3 hours, and painkillers such as paracetamol or ibuprofen gel or tablets.

The NHS says to see a GP if knee pain does not improve within a few weeks. In many areas people can refer themselves to NHS community musculoskeletal services, including physiotherapy, without a GP referral.

A pharmacist can advise on painkillers and gels, a GP or self-referral musculoskeletal service on pain that persists, and 111 or an urgent treatment centre on pain that is severe, or comes with inability to bear weight, locking or signs of infection. A&E is for severe injury, a deformed knee, or any suspicion of a broken bone.

People with repeated swelling, gout attacks or morning stiffness affecting several joints should mention these patterns, because CKS lists inflammatory polyarthritis among the conditions needing urgent specialist input.