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Knees that swell

Medical Professionals

Professional Reference articles are designed for health professionals to use. They are written by UK doctors and based on research evidence, UK and European Guidelines. You may find the Housemaid's knee article more useful, or one of our other health articles.

This article refers to swelling of the patellofemoral and tibiofemoral joints. For swelling in front of the knee, see separate Prepatellar Bursitis article.

The knee is susceptible to trauma and is often the site of systemic disease. A thorough knee assessment is essential in determining the cause and, therefore, appropriate management. See also separate Knee Assessment article.

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Causes of a swollen knee (aetiology)1





See also separate Bone Tumours article.

  • Malignant:

    • Leukaemia.

    • Chondroblastoma.

    • Eosinophilic granuloma.

    • Giant cell tumour.

    • Ewing's sarcoma.

    • Osteosarcoma.

    • Synovial sarcoma.

  • Benign:

    • Aneurysmal bone cyst.

    • Fibrous dysplasia.

    • Osteochondroma.

    • Osteoid osteoma.

    • Pigmented villonodular synovitis.



Localised swellings

Signs and symptoms of a swollen knee (presentation)1 2


Specific information is required regarding the following: any trauma; the speed of onset; whether unwell in any other way; any problems with other joints, including the back; and whether or not it has happened before.

  • Sudden onset of pain and swelling (within 12 hours) with a history of trauma:

    • Suggests a traumatic haemarthrosis, and damage within the joint.

    • Haemarthrosis is increasingly seen in patients on warfarin.

    • Haemarthrosis may also be associated with clotting disorders - eg, haemophilia.

  • Findings suggesting a fracture include:

    • Mechanism of injury involving a high-velocity collision.

    • An audible/palpable 'pop' at the time of injury.

    • Age greater than 55 years.

    • Inability to bear weight immediately after the injury.

  • Anterior cruciate ligament (ACL) injuries usually occur after non-contact deceleration. There may be hyperextension, often accompanied by a 'pop'. They are unable to continue sport participation and there is associated knee instability.

  • The posterior cruciate ligament (PCL) is injured far less often than the ACL. The usual mechanism of injury is a blow to the anterior proximal tibia with the knee flexed - eg, tripping over a hurdle.

  • More gradual accumulation of fluid might suggest an effusion secondary to traumatic synovitis (eg, cartilage injury), or to an exacerbation of osteoarthritis.

  • Sudden onset of pain without trauma - acute gout typically starts with a sudden onset of pain.

  • Less dramatic onset might imply pseudogout, an exacerbation of osteoarthritis, reactive arthritis or septic arthritis.

The pain of most of these conditions is severe.

Other symptoms:

  • Septic arthritis: systemic symptoms are likely, although they are not always present. Risk factors include:

    • Old age.

    • Immunocompromised.

    • Diabetes.

    • Rheumatoid arthritis.

    • Recent joint surgery.

    • Prosthetic joint.

    • Intravenous drug use.

    • Lack of traumatic injury.

    • Recent sexual encounter.

    • History of abnormal joint.

    Any patient who is systemically unwell and who has a painful, hot joint should be admitted.

  • Reactive arthritis: systemic symptoms, especially a low-grade fever, conjunctivitis and urethritis. There may be a history of recent diarrhoeal illness or symptoms or risk of sexually transmitted disease.

  • Connective tissue disease: other joints and systems affected - eg, iritis, rashes, proteinuria.

  • Medications: diuretics and aspirin may provoke attacks of gout.

  • Tumour is suggested by a history of night pain, fevers, night sweats and unintentional weight loss.


Careful examination of both the affected knee and the unaffected one, to identify the structure causing the swelling.

  • Effusion: a large effusion will be obvious. Smaller effusions may be missed but synovial fluid can often be 'milked' from one patellar groove to the other. However, intra-observer variation is high.3

  • Thickened synovium is non-fluctuant.

  • Osteophytes may be palpable.

  • Crepitus may be apparent on moving the joint.

  • There may be limitation of movement or locking of the joint.

Continue reading below


  • If history of trauma, laboratory testing is unnecessary.

  • ESR, CRP, WCC: normal results may be useful in excluding inflammatory joint disease or septic arthritis.

  • Other tests may be indicated:

    • FBC and differential: chronic inflammation may cause anaemia.

    • Renal function: can be impaired in gout.

    • Uric acid: may be raised in gout, although is often normal during the acute phase. Also asymptomatic hyperuricaemia is common.

    • Rheumatoid factor.

  • X-ray: AP and lateral views (preferably weight bearing):

    • Criteria for X-ray include the inability to bear weight, acute or recent onset of effusion.

    • Performed to identify a fracture or, as in arthropathy, erosive disease, cartilage calcification and joint space narrowing.

  • Joint aspiration:

    • For diagnosis of septic arthritis, or diagnosis of crystal or inflammatory arthropathy. Fluid examined for leukocytes, bacteria and crystals.

    • For relief of symptoms when significant haemarthrosis and no fracture on X-ray.

    • Not to be undertaken if a tumour is suspected.

  • Ultrasound scan: may be helpful in detecting joint effusion and inflammation when there is clinical uncertainty.

  • MRI scan:

    • Is highly accurate in diagnosing meniscal and ACL tears; diagnostic arthroscopy is largely redundant.4

    • The DAMASK trial looked at the use of direct access MRI and reported that, whilst it increased GPs' confidence in the management of knee problems, it did not reduce the rate of referral to orthopaedic services.5

    • A further study using patient questionnaires found that GP access resulted in a small but significant increase in patients' knee-related quality of life but not in functional improvement.6

    • It should be noted that sensitive imaging may lead to overdiagnosis, as abnormal findings have been reported in healthy individuals with no knee symptoms: 16% have evidence of meniscal tears, increasing to 36% for people aged over 45.4

Management of a swollen knee3

The management will depend on:

  • The nature of the cause of the swelling.

  • The degree of swelling.

  • The medical history and social circumstances of the patient.

General measures may include simple analgesics or non-steroidal anti-inflammatory drugs (NSAIDs) and partial or non-weight-bearing. Antibiotics should not be started before appropriate sampling has been undertaken. Intra-articular steroids should not be administered until there is a confirmed diagnosis.

Continue reading below


Refer immediately any patient with a suspicion of septic arthritis or trauma, with an onset of swelling within 12 hours, to a doctor experienced in musculoskeletal diseases. Admission to hospital is likely to be indicated.

Refer to an orthopaedic surgeon, any patient in whom a bone tumour is suspected, within one week.

Refer to a rheumatologist, any patient with suspected inflammatory arthritis, within six weeks.

Further reading and references

  1. Johnson MW; Acute Knee Effusions: A Systematic Approach to Diagnosis. American Family Physician, April 2000.
  2. Jackson JL, O'Malley PG, Kroenke K; Evaluation of acute knee pain in primary care. Ann Intern Med. 2003 Oct 7;139(7):575-88.
  3. Landewe RB, Gunther KP, Lukas C, et al; EULAR/EFORT recommendations for the diagnosis and initial management of patients with acute or recent onset swelling of the knee. Ann Rheum Dis. 2010 Jan;69(1):12-9. Epub .
  4. Crawford R, Walley G, Bridgman S, et al; Magnetic resonance imaging versus arthroscopy in the diagnosis of knee pathology, concentrating on meniscal lesions and ACL tears: a systematic review. Br Med Bull. 2007;84:5-23. Epub 2007 Sep 3.
  5. Brealey SD; Influence of magnetic resonance of the knee on GPs' decisions: a randomised trial. Br J Gen Pract. 2007 Aug;57(541):622-9.
  6. No authors listed; Effectiveness of GP access to magnetic resonance imaging of the knee: a randomised trial. Br J Gen Pract. 2008 Nov;58(556):e1-8; discussion 774.

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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