See also separate article Knee Assessment (History and Examination).

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.

Aetiology¹

Trauma

• Ligament injury
• Intra-articular fracture
• Patellar dislocation
• Meniscus injury

Arthritis

• Reiter's syndrome
• Juvenile rheumatoid arthritis
• Rheumatoid arthritis
• Gout
• Pseudogout (calcium pyrophosphate deposition disease)
• Osteoarthritis and overuse syndrome

Infection

• Septic arthritis
• Osteomyelitis
• Gonorrhoea
• Lyme disease
• Tuberculosis
• Brucellosis

Tumour

• 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

Vascular

• Oedema
• Venous insufficiency
• Lymphatic obstruction
• Deep vein thrombosis

Presentation^1,2,3

Symptoms

• Sudden onset (within 4 hours of trauma) of pain and swelling, suggests a traumatic haemarthrosis, and damage within the joint that requires referral. Traumatic haemarthrosis is increasingly seen in patients on warfarin. Haemarthrosis may also be associated with clotting disorders, e.g. 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 noncontact deceleration. There may be hyperextension, often accompanied by a "pop,". They are unable to continue playing, 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, e.g. tripping over a hurdle.
• More gradual accumulation of fluid might suggest an effusion secondary to traumatic synovitis (e.g. 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, Reiter's syndrome 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:
  • 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.
• Reiter's syndrome: 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, e.g. 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.

Signs

Careful examination of the knee, to identify the structure causing the swelling.³

• 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.
• 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.

Investigations¹

• ESR, C- reactive protein (CRP): may be useful in excluding inflammatory joint disease or septic arthritis.
• X-ray: AP and lateral views. Criteria for X-ray include the inability to bear weight, presence of effusion and ecchymosis. If all these clinical criteria are absent, the sensitivity for excluding fracture is 100%.⁴
• Full blood count and differential: chronic inflammation may cause anaemia.
• Renal function: can be impaired in gout.
• Uric acid: may be raised in gout; however, a normal uric acid does not exclude gout.⁵
• Joint aspiration: for diagnosis of septic arthritis, or diagnosis of crystal arthropathy or for comfort.
• MRI scan: a useful tool for evaluating meniscal and ligamentous knee injuries but it is unable to clearly identify articular cartilage lesions.⁶The Direct Access Magnetic resonance imaging: Assessment for Suspect Knees (DAMASK) trial looked at the use of direct access MRI and recently reported that, whilst it increased GPs' confidence in the management of knee problems, it did not reduce the rate of referral to orthopaedic services.⁷ 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.⁸

Management⁹

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

Referral is indicated if there is any doubt as to the underlying cause.
Admit immediately if septic arthritis is suspected or after significant trauma.

Document 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. [abstract]
3. Wheeless on line; Examination of the knee
4. Bauer SJ, Hollander JE, Fuchs SH, et al; A clinical decision rule in the evaluation of acute knee injuries. J Emerg Med. 1995 Sep-Oct;13(5):611-5. [abstract]
5. Gout, Clinical Knowledge Summaries (2007)
6. Vaz CE, Camargo OP, Santana PJ, et al; Accuracy of magnetic resonance in identifying traumatic intraarticular knee lesions. Clinics. 2005 Dec;60(6):445-50. Epub 2005 Dec 12. [abstract]
7. 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. [abstract]
8. No authors listed; Effectiveness of GP access to magnetic resonance imaging of the knee: a Br J Gen Pract. 2008 Nov;58(556):e1-8; discussion 774. [abstract]
9. Weston V, Coakley G; The British Society for Rheumatology (BSR) Standards, Guidelines and Audit Working Group; British Society for Antimicrobial Chemotherapy; British Orthopaedic Association; Royal College of General Practitioners; British Health Professionals in Rheumatology. Guideline for the management of the hot swollen joint in adults with a particular focus on septic arthritis. J Antimicrob Chemother. 2006 Sep;58(3):492-3. Epub 2006 Jul 19.

Internet and further reading

• Knee Joint - Anatomy and Function, arthroscopy.com
• Rheumatoid arthritis, Clinical Knowledge Summaries (June 2009)

Acknowledgements