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Septic arthritis

Last updated: September 15, 2021

Summarytoggle arrow icon

Septic (infectious) arthritis is an infection of the joint space, which can occur in a native joint or a prosthetic joint. Patients with underlying joint diseases (e.g., rheumatoid arthritis) are at an increased risk of septic arthritis. Routes of infection include hematogenous spread (most common), direct inoculation (e.g., iatrogenic, penetrating trauma), and contiguous spread. Patients with native joint infections usually present with an acutely swollen, painful joint, limited range of motion, and fever, whereas patients with prosthetic joint infections (PJIs) usually have a milder, chronic course, which often makes diagnosis more challenging. All patients with suspected septic arthritis should undergo prompt arthrocentesis for synovial fluid analysis. Early administration of empiric antibiotic therapy and therapeutic arthrocentesis is indicated for native joint infections to prevent cartilage destruction. PJIs typically necessitate surgical debridement, including removal of the prosthesis in some cases; empiric antibiotics are not recommended unless the patient is critically ill. Targeted antibiotics should be initiated in all patients once culture and sensitivity results are available.

Routes of spread

Risk factors for septic arthritis

Causative organisms

Prosthetic joint infection (PJI) [4][5]

  • Etiology
  • Clinical features
    • Usually prolonged, low-grade course
    • Minimal swelling, with or without a sinus that drains pus
    • Can present acutely (see “Clinical features” above)
  • Management: See “Diagnostics” and “Treatment” sections.

In order to avoid infection, strict aseptic precautions should be ensured in any procedure that involves penetration of the joint space.

Bacterial coxitis [6]

Bacterial coxitis is an orthopedic emergency that requires urgent management to avoid joint destruction.

Gonococcal arthritis

In a young, sexually active adult presenting with classic symptoms of septic arthritis, gonococcal infection must be ruled out.

Lyme arthritis

Approach

Any red, painful joint with a reduced range of motion should be considered infectious until proven otherwise. The absence of fever does not rule out a diagnosis of septic arthritis. [7]

Maintain a high index of suspicion for septic arthritis in patients with underlying joint diseases (e.g., osteoarthritis, rheumatoid arthritis) who present with joint pain, as the signs and symptoms of an acute flare and an infection often overlap. [7]

Arthrocentesis [12]

Infection of the skin overlying the affected joint is an absolute contraindication to arthrocentesis due to the risk of introducing pathogens into the joint. [7]

Do not delay joint aspiration in suspected septic arthritis as early detection and treatment are imperative to prevent cartilage destruction. [13]

Synovial fluid analysis (SFA) in septic arthritis

Synovial fluid WBC count may be much lower in PJI than septic arthritis in a native joint. WBC count > 1100/mm3 (≥ 64%) should raise suspicion for PJI. [11]

Neutrophil predominant leukocytosis on SFA may also be present in crystal arthropathy (e.g., acute gout flare). Interpret SFA in close conjunction with clinical features and risk factors.

Laboratory studies [7][9][11]

Routine studies

Additional studies

Uric acid levels have no diagnostic value in the evaluation of swollen joints. [9]

Inflammatory markers may be normal in septic arthritis. [11]

Imaging of the affected joint [7][11][15]

X-ray

  • Indication: preferred initial imaging modality (prosthetic and native joints) [7][15]
  • Supportive findings

Ultrasound

  • Indication: Consider in patients with suspected foreign body (e.g., history of penetrating trauma) and negative x-rays. [15]
  • Supportive findings [15][16][17]
    • Joint effusion with or without synovial thickening
    • Inflammatory changes in periarticular soft tissue
    • Effusions and small collections
    • Can detect radiolucent foreign bodies, if present.

CT or MRI

Nuclear medicine studies (e.g., scintigraphy, PET-CT) [8][18]

  • Indication: Consider in the workup of PJIs (not routinely recommended).
  • Findings: focal increase in radiotracer uptake in the affected joint

Nuclear medicine studies can not distinguish between septic arthritis and inflammatory arthritis and are hence not recommended in the diagnostic workup of septic arthritis in a native joint. [15]

Diagnostic criteria for prosthetic joint infections

There are several diagnostic criteria for PJI that can be used alongside clinical features to establish a diagnosis. Clinical judgment plays an important role in determining the likelihood of PJI even in patients who do not fulfill the diagnostic criteria. [8][10]

Infectious Diseases Society of America (IDSA) diagnostic criteria for PJI (for all joints) [8]

2013 IDSA diagnostic criteria for PJI [8]
Definitive PJI
  • Presence of any of the following:
    • Sinus tract that communicates with the prosthesis
    • Purulence surrounding the prosthesis with no other identified cause
    • ≥ 2 positive cultures from periprosthetic tissue isolating the same organism
High likelihood of PJI

Musculoskeletal Infection Society (MSIS) for hip and/or knee PJI [10]

MSIS diagnostic criteria for prosthetic hip and/or knee infections [10]
Major criteria
  • Presence of a sinus tract communicating with the prosthesis
  • ≥ 2 positive cultures from periprosthetic tissue isolating the same organism
Minor criteria
  • Definitive PJI: ≥ 1 major criterion OR ≥ 4 minor criteria
  • PJI possible: < 4 minor criteria

A diagnosis of PJI can be made even if the IDSA and/or MSIS criteria are not fulfilled. Clinical judgment based on clinical features and diagnostic findings plays an important role in determining the diagnosis. [8]

Based on clinical features and imaging

See also “Differential diagnoses of inflammatory arthritis.

Viral arthritis

Fungal arthritis [19]

Miscellaneous

Based on synovial fluid analysis [11]

Synovial fluid analysis comprises a group of tests that examine synovial fluid to help differentiate between subtypes of arthritis.

Interpretation of synovial fluid analysis [11]
Type of arthritis Appearance WBC count (PMN) Gram stain Crystals Glucose levels (compared to blood glucose levels)
No arthritis
  • Transparent
  • Clear and viscous
  • < 200/mm3 (< 25%)
  • Negative
  • None
  • Nearly equal

Noninflammatory arthritis

E.g., osteoarthritis

  • Transparent
  • Yellow and viscous
  • 200–2000/mm3 (< 25%)
  • Negative
  • Nearly equal

Inflammatory arthritis

E.g., rheumatoid arthritis, SLE, gout, pseudogout

  • Translucent-opaque
  • Yellow and watery
  • > 2000/mm3 (≥ 50%)
  • Negative

Septic arthritis [11]

E.g., caused by bacterial infections

  • Cloudy-opaque
  • Yellow or green with variable viscosity
  • > 50,000/mm3 (≥ 75%)
  • Prosthetic joints: > 1100/mm3 (≥ 64%)
  • None, unless the patient has concurrent gout [23]
  • ↓↓

Hemarthrosis [24]

E.g., caused by trauma

  • Red with variable viscosity
  • Synovial Hct can be compared to simultaneous serum Hct to determine the amount of blood in the synovial fluid
  • Variable
  • Difficult to rule out a concurrent inflammatory process based on SFA alone.
  • Negative [24]
  • None
  • Nearly equal

The differential diagnoses listed here are not exhaustive.

Approach

Evacuation of purulent material from the joint and systemic antibiotic therapy are the mainstays of treatment in septic arthritis.

Joint drainage

Native joints [9]

If effusion persists beyond 7 days of arthrocentesis, arthroscopic or open drainage is indicated. [17]

Prosthetic joints [8]

Surgery to remove pus and infected tissue from the affected joint is typically required; examples include:

Consult orthopedic surgery early in patients with suspected prosthetic joint infection.

Empiric antibiotic therapy

Empiric antibiotic therapy is not routinely recommended for PJIs.

Adult patients

Empiric antibiotic therapy for adults with septic arthritis of native joints [3][11]
Causative pathogen Suggested regimens
Gram-positive cocci
Gram-negative cocci
Gram-negative bacilli
Gram stain negative Risk factors for a sexually transmitted disease (STD)
No risk factors for an STD

Intraarticular injection of antibiotics is not recommended. [3]

Pediatric patients [3]

Empiric antibiotic therapy for pediatric patients with septic arthritis of native joints [3]
MRSA unlikely MRSA possible
Infants < 3 months of age
Infants > 3 months and children up to 14 years of age

Targeted antibiotic therapy

Switch from empiric antibiotic therapy to culture-specific antibiotics once the antibiogram is available in patients with native joint infections. Initiation of targeted antibiotic therapy directly (i.e., without preceding empiric therapy) is sufficient for PJI. Specialist consultation is advised especially in patients with PJIs.

Agents

Targeted antimicrobial therapy for adults with septic arthritis (native joint infection and PJI) [8][9]
Microorganism Example regimens
Staphylococcus aureus MSSA
MRSA
Enterococci Penicillin-susceptible
Penicillin-resistant
Pseudomonas aeruginosa
Enterobacter spp.
Enterobacteriaceae
Beta-hemolytic streptococci
Propionibacterium acnes
N. gonorrhoeae [3]

Duration of therapy [9]

The total duration of antibiotic therapy is variable and should be adjusted according to signs of clinical and laboratory evidence of improvement.

Supportive therapy

Native joint infections

Prosthetic joint infections

Empiric antibiotic therapy for septic arthritis is not routinely recommended for PJIs unless the patient is critically ill.

We list the most important complications. The selection is not exhaustive.

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