✅ Introduction:
Septic arthritis is a pyogenic infection of a joint, most commonly caused by hematogenous spread of bacteria.
It is a medical emergency as it can rapidly destroy cartilage and lead to permanent joint damage.
- Most common organisms: Staphylococcus aureus, Streptococci, Gram-negative bacilli
- Commonly affects: knee, hip, shoulder, and sternoclavicular joint
🧪 Clinical Clues:
- Acute onset joint pain, swelling, redness, and restricted movement
- Fever, elevated ESR/CRP, leukocytosis
- Monoarticular involvement is common
- Hip infections in children often present subtly but need urgent attention
🩻 Imaging Modalities & Their Role:
| Modality | Utility |
|---|---|
| X-ray | Initial test, shows late findings |
| Ultrasound | Detects joint effusion, guides aspiration |
| MRI | Most sensitive for early changes, synovitis, marrow edema |
| CT | Useful in deep joints or guiding aspiration/drainage |
| Nuclear scans | Indicated when MRI not available; Tc-99m or Gallium scans |
🩻 X-ray Findings (Late):
- Joint space widening (due to effusion)
- Periarticular osteopenia
- Joint space narrowing in late stages (due to cartilage destruction)
- Erosions at articular margins
- Soft tissue swelling
- Bone ankylosis in chronic cases
📝 Note: Early septic arthritis may show normal X-rays.
🧠 Ultrasound Findings:
- Joint effusion: anechoic or hypoechoic collection
- Synovial thickening
- Increased Doppler flow suggests active inflammation
- Can guide diagnostic aspiration
🚨 In children with hip pain: always perform USG to rule out septic arthritis!
🧲 MRI Findings (Most Sensitive & Specific):
- Joint effusion: high T2/STIR signal
- Synovial thickening and enhancement post-contrast
- Bone marrow edema (indicating adjacent osteomyelitis)
- Articular cartilage destruction
- Soft tissue abscess or sinus tract in chronic cases
- Muscle inflammation (myositis) or pyomyositis
🔎 MRI is crucial for early diagnosis, especially in deep joints like the hip, sacroiliac, or shoulder.
📍 CT Scan Use:
- Useful if MRI is not available
- Shows bone erosions, effusion, and soft tissue abscess
- Helps guide joint aspiration or drainage, especially in deep joints
🧬 Nuclear Medicine:
- Tc-99m bone scan: increased uptake in infected joint
- Gallium scan: helps differentiate infection from inflammation
- Mostly used when MRI is contraindicated or unavailable
🔄 Differential Diagnosis:
- Reactive arthritis
- Inflammatory arthritis (RA)
- Crystal arthropathy (gout/pseudogout)
- Transient synovitis (especially in children)
🧾 Summary Table:
| Feature | Septic Arthritis |
|---|---|
| Early imaging sign | Joint effusion (USG/MRI) |
| Key MRI feature | Synovial thickening + enhancement |
| Adjacent bone changes | Bone marrow edema ± osteomyelitis |
| Best modality | MRI (early), USG (aspiration) |
| X-ray role | Late-stage changes only |
✅ Conclusion:
Imaging plays a key role in early detection, confirmation, and management of septic arthritis.
MRI is the most sensitive tool for early diagnosis, especially for deep joints, while ultrasound is excellent for joint effusion and aspiration.
Prompt diagnosis prevents irreversible joint damage.
