Aetiology^[1]

Trauma

See also article on Sports Injuries.

• Ligament injury (see also separate article Knee Ligament Injuries).
• Fractures and dislocations: see also separate articles Knee Fractures and Dislocations, Femoral Fractures and Tibial and Fibular Fractures (including Horse Rider's Knee).
• Meniscus injury (see also separate article Knee Cartilage Injuries).

Arthritis

• Reactive arthritis (Reiter's syndrome).
• Juvenile chronic 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

Localised swellings

• Baker's cyst

Presentation^[1][2][3]

Symptoms

• Sudden onset (within four hours of trauma) of pain and swelling - suggests a traumatic haemarthrosis, and damage within the joint, requiring referral. Traumatic 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 noncontact 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:
  • 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.

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.

Investigations^[1]

• ESR, 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.
• FBC 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 finding does not exclude gout.
• 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 identify articular cartilage lesions clearly.^[4] The Direct Access Magnetic resonance imaging: Assessment for Suspect Knees (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]

Management^[7]

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.