Septic arthritis vs osteomyelitis: diagnosis and management
RezzyThis is a classic "orthopedic emergency" comparison that every med student needs to have down cold. While both involve infection of the musculoskeletal system, the urgency and the way we approach them are quite different.
Think of Septic Arthritis as a "joint on fire"—it's a surgical emergency because those proteolytic enzymes can destroy cartilage in hours. Osteomyelitis, on the other hand, is more of a "bone under siege"—it's serious, but usually follows a more subacute or chronic course.
I've put together a comparison table to help you distinguish between the two at a glance.
| Feature | Septic Arthritis | Osteomyelitis |
|---|---|---|
| Primary Site | Joint space (synovium/fluid) | Bone (metaphysis in kids, vertebrae in adults) |
| Clinical Presentation | Acute onset, severe pain, joint swelling, refusal to move joint (pseudoparalysis) | Subacute onset, localized bone pain, fever, swelling/overlying erythema |
| Physical Exam | Pain with passive range of motion | Point tenderness over the bone |
| Gold Standard Dx | Arthrocentesis (Joint aspiration) | Bone Biopsy (for culture/histology) |
| Synovial Fluid | WBC > 50,000/mm³ (>75% PMNs) | N/A (unless secondary joint involvement) |
| Imaging (Early) | Ultrasound (effusion); X-ray often normal | MRI is most sensitive; X-ray takes 10-14 days to show changes |
| Management | Emergency I&D or aspiration + IV Antibiotics | IV Antibiotics (long course) +/- Debridement |
The management of septic arthritis is particularly high-yield for exams. You'll often see the Kocher Criteria mentioned for pediatric septic hip—it helps you decide if that limping child needs an urgent trip to the OR.
Here is a quick flowchart of how we typically manage a suspected case of septic arthritis.
To wrap this up, here are some high-yield "pearls" for your exams:
- Most common organism overall: Staphylococcus aureus (for both).
- Sickle cell patients: Watch out for Salmonella osteomyelitis.
- Sexually active young adults: Think Neisseria gonorrhoeae for septic arthritis (often presents with a triad of tenosynovitis, dermatitis, and polyarthralgia).
- IV drug users: High risk for Pseudomonas in the fibrocartilaginous joints (like the sacroiliac or sternoclavicular joints).
Would you like to see some practice questions on this to see how it's tested, or maybe some flashcards to lock in those synovial fluid values?
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