What Causes Septic Arthritis?
Septic arthritis is caused primarily by hematogenous (bloodstream) spread of bacteria to the joint space, with Staphylococcus aureus being the most common pathogen across all age groups. 1
Primary Mechanism of Infection
The infection reaches the joint through hematogenous seeding in the vast majority of cases 1. Bacteria circulating in the bloodstream lodge in the highly vascular synovial membrane, proliferate, and invade the joint space. Less commonly, infection spreads by:
- Direct extension from adjacent osteomyelitis (particularly in neonates and infants where infection can cross from bone into the joint) 1
- Direct inoculation from trauma, surgery, or intra-articular injections 2
Causative Organisms by Population
The bacterial etiology varies significantly by patient age and risk factors:
General Population
- Staphylococcus aureus: Most common pathogen overall 1
- Streptococci: Second most common 3
- Gram-negative bacilli: Less frequent but important 3
Age-Specific Pathogens
- Neonates: Group B Streptococcus 1
- Children <4 years: Kingella kingae 1
- All pediatric ages: S. aureus remains most common 1
Special Populations
- Sickle cell disease: Salmonella species 1
- Sexually active adults: Neisseria gonorrhoeae 4
- Immunocompromised: MRSA, atypical organisms 5
Key Risk Factors
Understanding risk factors helps identify patients at highest risk:
- Age >80 years 2, 4
- Diabetes mellitus 2, 4
- Rheumatoid arthritis (increases risk substantially) 2, 4
- Recent joint surgery or prosthesis 2, 4
- Skin infection (provides bacterial source) 4
- Immunosuppressive medications 4, 5
- HIV infection 2
- Pre-existing joint disease (osteoarthritis) 2
Clinical Significance
Septic arthritis is an orthopedic emergency because bacterial proliferation and metabolites rapidly destroy articular cartilage 1. The knee and hip joints are most commonly affected 1. Mortality ranges from 2-15% overall 3, with 90-day mortality reaching 7% in patients ≤79 years and 22-69% in those >79 years 2.
Age Distribution Patterns
- Children <2 years: Septic arthritis more common than osteomyelitis 1
- Children 2-10 years: Osteomyelitis slightly more common 1
- Children 10-18 years: Septic arthritis slightly more common 1
Important Caveats
In neonates and infants, septic arthritis frequently occurs secondary to osteomyelitis spreading into adjacent joints due to transphyseal vessels present until 18 months of age 1. This anatomic feature allows infection to cross the growth plate, a route not available in older children and adults.
MRSA has emerged as a major cause in the United States and appears associated with worse outcomes 6. Empiric antibiotic selection must account for local MRSA prevalence.
Viral and fungal causes exist but are far less common than bacterial etiologies 2. Consider these in appropriate clinical contexts (immunosuppression, endemic exposures).