Septic (infectious) arthritis is a bacterial infection of the joint space. Contamination occurs either via the bloodstream, iatrogenically, or by local extension (e.g., penetrating trauma). Patients with damaged (e.g., patients with rheumatoid arthritis) or prosthetic joints have an increased risk. Patients usually present with an acutely swollen, painful joint, limited range of motion, and a fever. Suspected infectious arthritis requires prompt arthrocentesis for diagnosis. In addition to the immediate broad-spectrum antibiotic therapy, surgical drainage and debridement may be necessary to prevent cartilage destruction and sepsis.
Mechanism of infection
- Hematogenous spread (most common)
- Direct contamination
- Contiguous spread (e.g., septic bursitis, osteomyelitis)
- Risk factors
- Most common in adults and children > 2 years
- Frequently found in patients with arthritis following invasive joint procedures 
- N. gonorrhea
- Gram-negative rods esp. E. coli and P. aeruginosa
- S. epidermidis
- H. influenzae
- M. tuberculosis and atypical mycobacteria
- B. burgdorferi (Lyme disease)
- Staphylococcus aureus
Subtypes and variants
Prosthetic joint infection 
- Usually prolonged, low-grade course
- Minimal swelling, with or without a sinus that drains pus
- Can present acutely (see “Clinical features” above)
In order to avoid infection, strict sterile techniques should be ensured in any procedure that involves penetration of the joint space.
Bacterial coxitis (septic arthritis of the hip) 
- Description: a rare condition that requires urgent treatment to avoid destruction of the joint
- Etiology: S. aureus and group A streptococcus account for the majority of cases
- Clinical findings
- Diagnostics: For diagnostics and therapy, see respective sections below.
Bacterial coxitis is an orthopedic emergency!
- Gonococcal arthritis is the most common form of arthritis in sexually active young adults.
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- If septic arthritis is suspected, arthrocentesis should be conducted for synovial fluid analysis.
- Definitive diagnosis requires detection of bacteria in the synovial fluid, therefore, synovial fluid obtained is to be sent for analysis (culture and gram staining).
- Imaging (e.g., x-ray, MRI) may be indicated to assess potential underlying diseases or differential diagnoses.
Arthrocentesis: a diagnostic and/or therapeutic procedure in which synovial fluid from a joint is aspirated using a sterile needle to determine the etiology of joint effusions and/or to relieve pressure from a joint
- Ultrasound-guided arthrocentesis: Definitive diagnosis requires detection of bacteria in the synovial fluid.
- Culture and gram staining
- Laboratory tests: ↑ CRP, ESR, and leukocyte count (nonspecific, but may be useful for monitoring response to treatment) 
Imaging: to look for signs of underlying osteomyelitis and concurrent joint disease and rule out possible differential diagnoses (see “Differential diagnosis” below)
- MRI or scintigraphy for early detection
Differential diagnosis based on synovial fluid analysis findings
- Synovial fluid analysis: comprises a group of tests that examine joint fluid (synovial fluid) to aid in definitively establishing specific types of arthritis.
|Interpretation of synovial fluid analysis|
|Appearance||WBCs/μl (PMN %)||Glucose levels||Culture||Crystals|
|Normal synovial fluid|| || || || || |
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E.g., bacterial infections
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Further differential diagnoses to consider
- Etiology: parvovirus B19, hepatitis B virus, hepatitis C virus, rubella virus, HIV
- Clinical features
- History and clinical findings are the mainstays of establishing a diagnosis
- Serology: antibodies against the suspected virus
Synovial joint analysis
- Very variable (can be normal or inflammatory)
- Not routinely used since viral isolation is usually not successful
- For other diagnostic tests, see “Diagnostics” above.
Fungal arthritis 
- Etiology: Histoplasma species, Sporothrix schenckii, Blastomyces species, Coccidioides species
- Very variable with acute and chronic courses
- Often with symptoms of disseminated infection (e.g., pulmonary symptoms)
See “Overview of fungal infections” in the “.”
- Legg-Calvé-Perthes disease in children
The differential diagnoses listed here are not exhaustive.
Empiric antibiotic regimens 
- Gram-positive cocci: vancomycin
- Gram-negative cocci: ceftriaxone
- Gram-negative bacilli: 3rd generation cephalosporin (e.g., ceftazidime), cefepime, piperacillin-tazobactam, OR carbapenem
- Suspected pseudomonas infection (e.g., IV drug users): IV ceftazidime PLUS IV aminoglycoside (e.g., gentamicin)
- No organism on gram stain but strong suspicion for bacterial septic arthritis: IV vancomycin PLUS either ceftazidime, cefepime, OR aminoglycoside
- Start serial drainage with lavage
- Sometimes debridement (arthroscopic or open approach) is necessary 
- Tailor antibiotics to gram stain, culture, and susceptibility results when available (see table below)
- Continue antibiotic therapy for at least ≥ 2 weeks 
- Continue serial drainage as needed
- Immobilization and NSAIDs for pain relief and to reduce inflammation
- Follow-up: Physiotherapy should be initiated early to prevent contracture of both the joint and its capsule.
Treatment of adults after culture has returned 
|S. aureus and other gram-positive cocci|
|N. gonorrhea|| |
Treatment of children 
- ≤ 3 months of age: oxacillin PLUS gentamicin
- > 3 months of age: nafcillin PLUS cefazolin
We list the most important complications. The selection is not exhaustive.