Septic Arthritis
System: Bone and Joint Infections · Parent MOC: MOC - Diseases by System
One-Sentence Definition
Infection of a native joint space — an orthopedic emergency because delayed drainage destroys cartilage.
Clinical Presentation
- Hot, swollen, painful joint; refusal to bear weight; fever common
- Knee most frequent native joint; hip in young children
- Gonococcal triad variant: migratory polyarthralgia, dermatitis, tenosynovitis (TBD GC note)
Pathogens
| Context | Organism | Notes |
|---|---|---|
| All ages dominant | Staphylococcus aureus | |
| Young children | Kingella, strep (TBD) | |
| Sexually active | N. gonorrhoeae (TBD) | Often culture-negative synovial |
| Elderly / comorbid | GNRs including Escherichia coli | |
| Prosthetic | See PJI under Device-Associated Infections | Different algorithms |
Pathogenesis
- Hematogenous seeding of synovium most common in native joints
- Neutrophils + bacterial enzymes → rapid cartilage damage
Diagnosis
| Step | Test | Note |
|---|---|---|
| Arthrocentesis | Cell count, Gram Stain, culture, crystals | Don’t let crystals exclude infection entirely |
| Blood cultures | Concurrent | |
| PCR / NAAT | GC or Lyme in context | Selected |
Treatment Principles
- Urgent drainage (serial aspiration or arthroscopy/OR) + IV antibiotics
- Empiric cover S. aureus (± MRSA) and other risks by age/exposure
- Narrow with Gram/culture; typical duration weeks with ID/ortho input
Prevention
- Treat bacteremia promptly; GC prevention/safer sex; perioperative joint prophylaxis protocols
Differential / Pitfalls
- Gout/pseudogout, reactive arthritis, Lyme, trauma hemarthrosis
- Delaying tap for MRI
Learning Aids
Clinical Example
Example
Case: 70M diabetic, acute monoarthritis of knee, WBC 85k in synovial fluid, Gram+ cocci clusters.
Question: Organism and next procedural step?
Answer: Staphylococcus aureus septic arthritis — urgent drainage + IV antibiotics; blood cultures; AST.
Related
Active Recall
- Why is septic arthritis time-critical?
- Essential diagnostic procedure?
- Most common native-joint bacterium?