Septic Arthritis
Septic arthritis (infectious arthritis) is an infection of a native or prosthetic joint caused by microorganisms leading to inflammation, rapid cartilage destruction, joint damage, disability, and potentially death if treatment is delayed.
It is considered an orthopedic and rheumatologic emergency because irreversible cartilage damage can begin within hours to days.
Pathogenesis
Routes of Infection
Route of Infection | Sources / Examples |
Hematogenous Spread (Most Common, ~70%) | Skin infection, Cellulitis, Endocarditis, Urinary tract infection, Pneumonia, Intravenous drug use |
Direct Inoculation | Joint injection, Arthroscopy, Surgery, Trauma, Animal bite |
Contiguous Spread | Osteomyelitis, Soft tissue infection, Abscess |
Why Joints Are Vulnerable
Synovium:
- Highly vascular
- No limiting basement membrane
Therefore bacteria enter easily.
After bacterial invasion:
- Synovial inflammation develops
- Neutrophils infiltrate
- Cytokines released:TNF-α—,IL-1,IL-6
- Proteolytic enzymes destroy cartilage
- Increased intra-articular pressure
- Ischemic cartilage necrosis
Permanent joint destruction may occur within days.
Risk Factors
Patient Factors | Joint Factors |
Age >60 years | Prosthetic joint |
Diabetes mellitus | Pre-existing arthritis |
Rheumatoid arthritis | Crystal arthropathy |
Chronic kidney disease | Osteoarthritis |
Cirrhosis | Recent intra-articular injection |
Malignancy | Joint surgery |
HIV infection | — |
Immunosuppressive therapy | — |
Neutropenia | — |
Alcoholism | — |
Intravenous drug use | — |
Microbiology
Clinical Setting | Common Organisms |
Adults (Most Common Overall) | Staphylococcus aureus (most common worldwide, including MRSA) |
Other Organisms in Adults | Streptococcus species, Pneumococcus, Gram-negative bacilli (Escherichia coli, Klebsiella, Pseudomonas) |
Young Sexually Active Adults | Neisseria gonorrhoeae (often associated with migratory arthritis, tenosynovitis, and dermatitis syndrome) |
Intravenous Drug Users | MRSA, Pseudomonas aeruginosa, Serratia |
Immunocompromised Patients | Gram-negative bacilli, Fungi, Mycobacteria |
Animal Bites | Dog/Cat bites – Pasteurella multocida; Human bites – Eikenella corrodens |
Prosthetic Joint Infection | Staphylococcus aureus, Coagulase-negative staphylococci, Cutibacterium acnes |
Commonly Affected Joints
Adults
Most common:
- Knee (≈50%)
- Hip
- Shoulder
- Ankle
- Wrist
Intravenous Drug Users
Common:
- Sternoclavicular joint
- Sacroiliac joint
Clinical Features
Classical Presentation | Systemic Symptoms | Physical Examination Findings |
Acute onset over hours to days • Joint pain • Swelling • Warmth • Erythema • Restricted movement | • Fever • Chills • Malaise • Rigors
Important: Fever may be absent in up to one-third of patients | • Tender joint • Effusion • Painful range of motion • Inability to bear weight • Joint held in position of comfort
Passive movement: Extremely painful |
Pattern of Involvement
Pattern of Joint Involvement | Features / Associated Conditions |
Monoarthritis (Most Common, 80–90%) | Usually involves the knee, hip, or shoulder |
Oligoarthritis | More commonly seen in immunosuppressed patients and those with bacteremia |
Polyarthritis (5–20%) | Associated with rheumatoid arthritis, endocarditis, and gonococcal disease; associated with a poor prognosis |
Differential Diagnosis
Infectious Causes | Noninfectious Causes |
Osteomyelitis | Gout |
Cellulitis | Pseudogout |
Lyme disease | Reactive arthritis |
Tuberculous arthritis | Rheumatoid arthritis flare |
Viral arthritis | Hemarthrosis |
— | Osteoarthritis flare |
Diagnostic Approach
Any acute hot swollen joint is septic arthritis until proven otherwise.
Arthrocentesis should not be delayed.
Laboratory Investigations
Investigation | Findings / Significance |
CBC | Usually leukocytosis; may be normal despite septic arthritis |
ESR | Usually elevated; sensitivity >90%; not specific |
CRP | Most useful inflammatory marker; high sensitivity; useful for monitoring treatment response |
Procalcitonin | May support bacterial infection; not sufficiently sensitive to exclude septic arthritis |
Blood Cultures | Obtain at least two sets; positive in 30–60% of cases; should be drawn before antibiotics whenever possible |
Arthrocentesis (Gold Standard)
Mandatory Investigation
Perform urgently before antibiotics if feasible.
Synovial fluid should be sent for:
- Cell count
- Differential count
- Gram stain
- Culture
- Crystal analysis
Parameter | Septic Arthritis |
Appearance | Purulent |
WBC count | Usually >50,000/µL |
Often | >100,000/µL |
Neutrophils | >75–90% |
Glucose | Low |
Protein | High |
Important:
- Septic arthritis can occur with counts <50,000/µL.
- Crystal arthritis and septic arthritis may coexist.
Gram Stain
Positive rates:
Organism | Yield |
Gram-positive cocci | 60–80% |
Gonococcus | <30% |
Gram-negative bacilli | 40–60% |
Synovial Fluid Culture
Diagnostic yield:70–90%
Remains the diagnostic standard.
Molecular Testing
PCR useful for:
- Gonococcal arthritis
- Kingella kingae
- Mycobacterial disease
- Culture-negative infections
Imaging
Imaging Modality | Findings / Utility |
Plain X-ray | Initially often normal; may show effusion and soft tissue swelling. Late findings include joint space narrowing, erosions, and joint destruction. |
Ultrasound | Useful for detecting joint effusion and guiding aspiration; particularly useful for the hip joint. |
CT Scan | Useful for evaluating deep joints, sternoclavicular joint infections, and sacroiliac joint infections. |
MRI | Most sensitive imaging modality; detects early synovitis, osteomyelitis, soft tissue abscess, and cartilage destruction. |
Diagnostic Criteria Suggestive of Septic Arthritis
Strongly suspect when:
- Acute monoarthritis
- Fever
- Elevated CRP
- Synovial WBC >50,000/µL
- Positive Gram stain
- Positive blood culture
Diagnosis ultimately relies on:
- Synovial fluid analysis
- Culture results
Management
Clinical Scenario | Empiric Antibiotic Regimen | Dose / Coverage |
Gram-Positive Coverage (MRSA/MSSA) | Vancomycin | Loading dose: 20–30 mg/kg IV Maintenance: 15–20 mg/kg IV q8–12h Target trough: 15–20 mcg/mL Coverage: MRSA, MSSA, Streptococci |
Gram-Negative Risk Factors (Elderly, Immunocompromised, Urinary source, Healthcare exposure) | Vancomycin + Ceftriaxone or Vancomycin + Cefepime | Vancomycin 15–20 mg/kg IV q8–12h PLUS Ceftriaxone 2 g IV q24h or Cefepime 2 g IV q8h |
Pseudomonas Risk Factors (IV drug use, Neutropenia, Healthcare exposure, Recent hospitalization) | Vancomycin + Cefepime Alternative: Vancomycin + Piperacillin–Tazobactam | Vancomycin 15–20 mg/kg IV q8–12h PLUS Cefepime 2 g IV q8h
Alternative: Piperacillin–Tazobactam 4.5 g IV q6h |
Severe Sepsis / Septic Shock | Vancomycin PLUS one of the following: • Cefepime • Piperacillin–Tazobactam • Meropenem | Vancomycin 15–20 mg/kg IV q8–12h PLUS Cefepime 2 g IV q8h or Piperacillin–Tazobactam 4.5 g IV q6h or Meropenem 1 g IV q8h Provides broadest initial coverage |
Gram-Stain Directed Targeted Therapy
Gram Stain Finding | Likely Organism(s) | Recommended Therapy |
Gram-Positive Cocci in Clusters | Staphylococcus aureus (including MRSA) | Vancomycin (continue until culture and susceptibility results are available) |
Gram-Positive Cocci in Chains | Streptococcus spp., Enterococcus spp. | Ceftriaxone 2 g IV daily or Penicillin G 18–24 million units/day IV(continuous infusion or divided q4–6h) or Ampicillin 2 g IV q4h |
Gram-Negative Bacilli | Enterobacterales, Pseudomonas spp. and other Gram-negative organisms | Cefepime 2 g IV q8h or Piperacillin–Tazobactam 4.5 g IV q6h |
Gonococcal Arthritis
Ceftriaxone
1 g IV/IM daily
Treat concurrent STI:Chlamydia coverage if not excluded
Duration: 7–14 days
Doxycycline-100 mg orally twice daily for 7 days
or
Azithromycin-1 g single dose
Definitive
Organism | Preferred / Alternative Antibiotics | Dose & Duration |
MSSA | Preferred: Cefazolin Alternatives: Nafcillin or Oxacillin | Cefazolin 2 g IV q8h Nafcillin 2 g IV q4h Oxacillin 2 g IV q4h Duration: 3–4 weeks (4–6 weeks if complicated) |
MRSA | Preferred: Vancomycin Alternative: Daptomycin | Vancomycin (AUC-guided dosing; target AUC/MIC 400–600) Daptomycin 8–10 mg/kg IV daily Duration: 4–6 weeks |
Streptococci | Penicillin G or Ceftriaxone | Penicillin G 18–24 million units/day IV (continuous infusion or divided q4–6h) OR Ceftriaxone 2 g IV daily Duration: 2–4 weeks |
Enterococci | Preferred: Ampicillin If resistant: Vancomycin or Daptomycin | Ampicillin 2 g IV q4h If resistant: Vancomycin or Daptomycin Duration: Usually 4–6 weeks (organism and response dependent) |
Gram-negative bacilli (E. coli, Klebsiella) | Ceftriaxone | Ceftriaxone 2 g IV daily Duration: 4–6 weeks |
Pseudomonas aeruginosa | Cefepime or Ceftazidime | Cefepime 2 g IV q8h OR Ceftazidime 2 g IV q8h Duration: 4–6 weeks |
Salmonella | Ceftriaxone or Ciprofloxacin | Ceftriaxone 2 g IV daily OR Ciprofloxacin 400 mg IV q12h Common in sickle cell disease and immunocompromised patients; duration usually 4–6 weeks |
Duration of Therapy(Native Joint Septic Arthritis)
Organism | Duration |
Gonococcus | 7–14 days |
Streptococci | 2–4 weeks |
MSSA | 3–4 weeks |
MRSA | 4–6 weeks |
Gram-negative bacilli | 4–6 weeks |
Prosthetic joint infection | 6–12 weeks |
Joint Drainage
Fundamental Principle
Antibiotics alone are often insufficient.
Drainage is essential.
Repeated Needle Aspiration
Useful for:
- Knee
- Easily accessible joints
Advantages:
- Minimally invasive
Arthroscopic Drainage
Increasingly preferred.
Advantages:
- Better washout
- Less morbidity
Open Surgical Drainage
Indications:
- Hip infection
- Failure of aspiration
- Large purulent collections
- Osteomyelitis
- Prosthetic infection
Indications for Urgent Surgical Drainage
- Hip septic arthritis
- Shoulder septic arthritis
- Failure of aspiration
- Persistent bacteremia
- Loculated pus
- Osteomyelitis
- Prosthetic joint infection
Monitoring Response
Clinical
- Pain improvement
- Reduced swelling
- Improved mobility
- Defervescence
Laboratory
- CRP (best markerMonitor every:2–3 days initially)
- ESR
- WBC count
Complications
Local Complications | Systemic Complications |
Cartilage destruction (most important complication) | Sepsis |
Osteomyelitis (especially hip and shoulder) | Septic shock |
Joint instability | Endocarditis |
Ankylosis | Metastatic abscesses |
Chronic pain | Death |
Reduced range of motion | — |
Prosthetic Joint Septic Arthritis
Suspect When
- Painful prosthetic joint
- New effusion
- Fever
- Reduced function
Common pathogens:
- Staphylococcus aureus
- Coagulase-negative staphylococci
Management:
- Debridement + implant retention (selected cases)
- One-stage revision
- Two-stage revision (gold standard in many chronic infections)
- Prolonged antibiotics
DVT Prophylaxis
Enoxaparin-40 mg SC daily unless contraindicated.
Especially important with:
- Lower-limb involvement
- Reduced mobility
Corticosteroids?
Routine corticosteroids are not recommended in adults.
May mask treatment failure.
No major adult guideline recommends routine use.
Pain Management
Acetaminophen-1 g every 6 hr
Maximum:-4 g/day
Opioids
If severe pain.
Examples:
- Morphine
- Fentanyl
NSAIDs
Can be used cautiously if:
- Renal function adequate
- No bleeding risk
Physiotherapy
Acute Phase
- Rest
- Joint splinting briefly if needed.
- Avoid prolonged immobilization.
After Infection Control
Begin:
- Passive ROM
- Active ROM
- Strengthening exercises
Early mobilization reduces:
- Contractures
- Stiffness
- Functional disability