Introduction

Septic arthritis is one of the important differentials for a red, hot and swollen joint.

It is an inflammatory condition of the joint caused by bacterial infection. Septic arthritis can be caused by direct inoculation of the joint or by haematogenous spread of bacteria from another site. The most common causative organism in adults is Staphylococcus aureus.

Typically patients will present with a fever and an acutely swollen, red, painful joint with a limited range of movement.

Suspicion of septic arthritis should warrant prompt joint aspiration for synovial fluid analysis, gram staining and culture.

Once samples have been acquired from the joint space, initial management should begin immediately with empiric antibiotic therapy.

Epidemiology

  • Incidence: 13.00 cases per 100,000 person-years
  • Peak incidence: 70+ years
  • Sex ratio: 1:1
Condition Relative
incidence
Osteoarthritis of the knee57.69
Gout11.92
Septic arthritis in adults1
Pseudogout0.77
Reactive arthritis0.38
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+

Aetiology

The most common causative organisms for septic arthritis are:
  • Staphylococcus aureus and streptococci which make up 91% of infections.
  • Neisseria gonorrhoeae is more common in young, sexually active individuals.

Risk factors include:
  • Rheumatoid arthritis and SLE
    • Previous inflammation can disrupt the normal joint space, resulting in neovascularisation and increased expression of adhesion factors which facilitates bacterial invasion.
    • Additionally, inflammatory activities within the space reduce the natural bactericidal defences of the synovial fluid and dampen down the phagocytic properties of synovial cells.
  • Prosthetic joints
    • Delayed wound healing can disrupt the normal joint defences, promoting bacterial infection of the joint space.
    • If septic invasion of the joint space occurs shortly after surgery it may be the result of intra-operative contamination of the joint.
  • Invasive joint procedures (e.g. steroid injections, arthroscopy)
  • IV drug use
  • Diabetes mellitus
  • Immunosuppression
  • Chronic skin infections

Pathophysiology

Septic arthritis result from the introduction of pathogens into the joint space, either directly or via the haematogenous route (most commonly).

The normal, healthy joint has a number of protective elements that resists pathogenic invasion. Healthy synovial cells possess phagocytic activity and the synovial fluid itself has bactericidal properties.

In vulnerable joints, such as in patients with rheumatoid arthritis and SLE, these defences are compromised:

Pre-existing inflammation → neovascularisation of the joint space → bacteraemic spread from distant sites → ↑ adhesion factor expression in inflamed space → ↑ colonisation of the synovial space → ↓ joint defences → rapid bacterial growth.

When septic arthritis is caused by haematogenous spread it may be a result of:
  • Bacterial migration from a distant site
    • Abscesses and wounds
    • Septicaemia
  • Disseminated infection
    • Gonorrhoea

When septic arthritis is caused by direct inoculation it may be a result of:
  • Iatrogenic procedures
    • Joint injections (e.g. steroid injections)
    • Joint arthrocentesis
    • Athroscopy
  • Traumatic injuries
    • Infected wounds around the joint
    • Penetrating injuries from foreign objects

Clinical features

Adults with septic arthritis will often present with a fever (40-60% of cases) and a joint which is:
  • Erythematous, hot, swollen and painful.
  • Restricted in movement.
    • If the affected joint is weight bearing, the patient will be unable to walk.
    • Pain in the joint will restrict both active and passive movement, a hallmark of intra-articular pathology.
    • The patient may be holding the joint in a position to maximise intra-articular space such as fully extending the knee.
  • Usually only a single joint will be affected (80%).
    • This may not be the case if the infection is disseminated, such as in gonococcal infection.
  • An intra-articular effusion may be present.

In patients with underlying joint disease such as rheumatoid arthritis, septic arthritis should be suspected if a joint has symptoms which are disproportionate to the rest of the joints.

Diagnosis of septic arthritis can be complicated in elderly patients, who commonly have atypical presentations.
  • Elderly patients tend to present as afebrile and systemically well.
  • WCC may be normal in 50%.
  • More likely to present with non-specific symptoms such as worsening cognitive impairment, confusion and more frequent falls.

Any joint can be affected by septic arthritis, with the knee being the most common (53%). Other frequently affected joints include:
  • Hips
  • Wrists
  • Shoulders
  • Ankles

Infections of axial joints such as the sternoclavicular or sacroiliac joints are rare, but present more frequently in patients with a history of IV drug use.

Investigations

There should be a low threshold for suspecting and investigating septic arthritis in a patient with an acutely hot, swollen and tender joint.

Joint arthrocentesis should be conducted promptly for synovial fluid analysis, gram staining and culture prior to beginning empirical antibiotic therapy.
  • Typically aspiration is based on anatomical landmarks, but the role of ultrasound guidance is becoming more utilised where available (particularly when the hip joint is involved).
  • Aspirating the synovial fluid also provides therapeutic effects by decompressing the joint space.

Synovial fluid appearance: often yellow/green and turbid on aspiration compared to uninfected fluid which is clear and usually colourless.

Synovial fluid gram stain and culture: positive in 70% of cases of non-gonococcal septic arthritis, and can reveal the causative pathogen and antibiotic sensitivities.
  • Cultures of synovial fluid in cases of gonococcal septic arthritis yield positive results in only 25% of cases.
  • A negative synovial culture does not exclude septic arthritis.
  • Mycobacterium tuberculosis joint infection is a rare but important differential for a patient where there is high clinical suspicion of septic arthritis but negative culture using standard medium. Typically these cases will have marked leucocytosis and acid-fast bacteria (AFB) culture can be useful.

Synovial fluid WCC: is often raised in cases of septic arthritis. Whilst neither 100% sensitive nor specific, a synovial fluid WCC >100,000 cells/µl suggests the diagnosis is highly likely. There is usually neutrophil predominance.

Other useful tests include:
  • Blood cultures
  • CRP
  • ESR

Importantly, whilst investigations can be extremely useful in supporting the diagnosis of and guiding antibiotic therapy, patients with high clinical suspicion of septic arthritis should be managed as such unless an alternative diagnosis is more likely.

Diagnosis

The Kocher criteria for the diagnosis of septic arthritis:
  • fever >38.5 degrees C
  • non-weight bearing
  • raised ESR
  • raised WCC

Differential diagnosis

It is important that septic arthritis is not confused with another cause of an acutely red, hot and swollen joint. The diagnosis can be made using careful synovial fluid analysis.

Gout: often presents as an acutely hot, tender and swollen joint in a patient >40 years.
  • Usually patients have a known history of gout, with the most commonly affected joint being the 1st MTP (podagra).
  • Joint aspiration will show negative birefringent needle shaped-crystals under polarised light.
  • Common risk factors include chronic kidney disease, alcoholism, myeloproliferative disorders and the use of thiazide diuretics.

Pseudogout: usually presents with an acutely hot, tender and swollen joint in patients >40 years, but may present in younger patients with metabolic conditions such as haemochromatosis and hyperparathyroidism.
  • Commonly affected joints include the knees, wrists, shoulders, ankles and hands.
  • Joint aspiration will show positively birefringent rhomboid shaped crystals under polarised light.

Osteoarthritis: most commonly affects the weight bearing joints such as the hips, knees and lumbar or cervical spine.
  • Pain is often bilateral and rarely associated with systemic symptoms.
  • Patients are typically >60 and present with joint pain and stiffness that worsens with activity

Psoriatic arthritis: usually polyarticular and associated with cutaneous manifestations of psoriasis in a young/middle aged patient.
  • Dactylitis is common (distinguishing psoriatic arthritis from rheumatoid arthritis) and the DIP joints are commonly involved.

Reactive arthritis: an immune mediated polyarthritis.
  • Typically there is asymmetrical involvement of the larger joints in the lower extremity.
  • There is often a HLA-B27 genetic association.
    • It usually involves systemic symptoms and a history of GI or GU infection roughly 4 weeks before the onset of joint involvement.
    • Synovial WCC are usually raised with negative cultures.

Management

Septic arthritis is a joint-threatening and life-threatening condition.

The goals of therapy are to:
  • Avoid progression of infection to systemic sepsis
  • Preserve the joint function and prevent complications

Initial assessment should first rule out systemic bacteraemia, which would warrant the initiation of the septic six protocol.

Patients with suspected septic arthritis should be admitted to hospital, for IV antibiotics and joint drainage.

The next step in management depends on whether the infection exists in a native or prosthetic joint. The British Society of Rheumatology recommend the following:
  • Infection in a prosthetic joint warrants urgent referral to orthopaedics, and should be managed in theatre.
    • This includes surgical arthrocentesis and washout.
  • Infection in a native joint can be treated initially with joint aspiration and empirical antibiotic management
    • Consult local guidelines regarding empirical antibiotic therapy.
    • Flucloxacillin 2g QDS IV is a commonly used regime before culture and sensitivity results are returned.
    • If gonococcal arthritis is suspected, ceftriaxone is more suitable.
    • Antibiotics are typically continued IV for 2 weeks, before switching to PO if the patient is improving.

Complications

The complications associated with septic arthritis can be localised or systemic, and may be temporary or long term.

The most common complications are:

Joint damage: up to 50% of adults who develop septic arthritis may develop joint damage.
  • This is mainly a result of inflammatory obliteration of the articular cartilage.
    • Long term outcomes are therefore worse in patients with previously damaged articular cartilage, such as in patients with pre-existing arthritic disease.
  • Damage can be minor or may result in areas of permanent destruction following the infection.

Osteomyelitis: if the joint infection is not controlled, it may spread into the surrounding bone.
  • If osteomyelitis is suspected, the patient should undergo an MRI.

Sepsis: if the bacteria colonising the joint space reach the bloodstream then sepsis can develop.