Septic Arthritis

Septic arthritis refers to the infection of a joint. It requires a high index of suspicion and can affect both native and prosthetic joints.

The main causative organisms that lead to septic arthritis are:

  • Staph aureus – most common in adults
  • Streptococcus spp.
  • N. gonorrhoea – common in young, sexually active
  • Salmonella – especially in those with sickle cell disease

Bacteria will ‘seed’ to the joint from a bacteraemia, a direct inoculation, or spreading from an adjacent osteomyelitis. Such an infection can cause irreversible articular cartilage damage leading to severe osteoarthritis.

In this article, we shall look at the risk factors, clinical features and management of septic arthritis.


Risk Factors

The main risk factors for developing septic arthritis are:

  • Age >80yrs
  • Any pre-existing joint disease (e.g. rheumatoid arthritis)
  • Diabetes mellitus or immunosuppression
  • Chronic renal failure
  • Hip or knee joint prosthesis
  • Intravenous drug use

Clinical Features

Patients will most commonly present with a single swollen joint causing severe pain. Pyrexia will be in around 60% of affected individuals (although its absence should not rule out septic arthritis).

On examination, the joint will appear red, swollen, and warm, causing pain on active and passive movements. An effusion may also be evident.

Often the joint is rigid and the patient will not tolerate any passive movement at all, and will be unable to weight bear.

Figure 1 – An acutely swollen knee


Differential Diagnoses

Septic arthritis should be the main differential diagnosis in anyone presenting with a single, painful, swollen joint. However, other differentials to consider are:

  • Crystal arthropathies (gout and pseudogout)
  • Rheumatoid arthritis
  • Reactive arthritis
  • Lyme disease (infection with Borrelia burgdoferi)

Investigations

Routine bloods including FBC and CRP, should be sent, alongside ESR and urate levels. Blood cultures should also be sent (at least two separate samples), especially in those with pyrexia.

For those suspected, a joint aspiration should also be performed. Joint fluid analysis should also be sent for Gram stain (Fig. 2), leucocyte count, polarising microscopy, and fluid culture.

It is important that cultures and joint aspiration are sent where possible prior to starting antibiotics (unless the patient is overtly septic).

Figure 2 – S. Aureus, as seen on Gram stain

Imaging

A plain radiograph of the affected joint is often performed; whilst in early stages, no evidence of the disease may be present, it may eventually progress to demonstrate capsule and soft tissue swelling, fat pad shift, and joint space widening.

Further imaging is rarely performed*. Ultrasound imaging can be useful to guide joint aspiration and for drainage (particularly for the hip), whilst CT/MRI imaging is a very sensitive method in assessing joint damage, typically used if there is a difficulty in the diagnosis.

CT/MRI may also be useful in specific joint infections (such as the sternoclavicular or sacroiliac) to assess any extension into the mediastinum or pelvis

*Radionulcide scans can be useful in identifying septic arthritis in isolated joints, such as the sacroiliac joint.


Management

Any septic patient should be managed as appropriate, with early resuscitation and investigation provided.

Empirical antibiotic treatment should be started as soon as possible, after appropriate investigations have been completed. As a guide, antibiotics are typically administered for long term (often 4-6 weeks), initially intravenously.

Most wounds will require surgical irrigation and drainage (‘washout’) in theatre to aid in source control.

Figure 3 – Septic arthritis, as seen on arthroscopy


Complications

The main complications of septic arthritis are osteoarthritis and osteomyelitis. Early intervention and management will reduce the risk of developing these complications.

Key Points

  • Septic arthritis is infection of a joint and is an important orthopaedic diagnosis to make
  • Patients present with acute painful, swollen, and tender joints, and importantly are unable to weight bear or passively move
  • Blood cultures and joint aspiration is essential, prior to any antibiotic management
  • Patients are at increased risk of developing osteoarthritis and osteomyelitis

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