Imaging Approach for Septic Arthritis
Initial Imaging: Start with Plain Radiographs
Begin with plain radiographs of the affected joint as the first imaging study, followed immediately by ultrasound to detect joint effusion and guide aspiration for definitive diagnosis. 1, 2
- Radiographs provide an essential anatomic overview and exclude alternative diagnoses such as fractures, tumors, slipped capital femoral epiphysis, and Legg-Calvé-Perthes disease 1, 2
- Early acute septic arthritis (<14 days) often shows normal radiographs or only mild soft tissue swelling, making radiographs insensitive for early diagnosis 1
- Radiographic findings may include joint effusion, soft tissue swelling, and erosions, though these are nonspecific 1
Critical Second Step: Ultrasound
Ultrasound is the most important imaging study for suspected septic arthritis and should be performed immediately after radiographs. 2
- Ultrasound has extremely high sensitivity for detecting joint effusion, with the ability to detect hip effusions as small as 1 mL 1
- The absence of joint effusion on ultrasound virtually excludes septic arthritis, with only a 5% false negative rate (occurring primarily in patients with <1 day symptom duration) 1
- Nearly 80% of pediatric patients with hip septic arthritis require only ultrasound imaging to guide management 1
Definitive Diagnosis: Image-Guided Aspiration
Imaging cannot distinguish infected from noninfected joints—aspiration with culture is mandatory for definitive diagnosis. 1, 3
- Image-guided aspiration (ultrasound, fluoroscopy, or CT) should be performed urgently when effusion is detected to confirm proper needle placement and reduce complications 1, 3
- US-guided hip aspiration is considered highly accurate and prevents unnecessary arthrotomy 1
- Synovial fluid analysis must include culture, Gram stain, cell count with differential, and crystal analysis 1, 3
When to Add MRI
Obtain MRI when there is concern for concurrent osteomyelitis, soft tissue abscess, or when diagnosis remains uncertain despite negative aspiration. 2, 3
- MRI shows synovial enhancement (98%), perisynovial edema (84%), and joint effusion (70%) in septic arthritis 4
- Decreased femoral head enhancement on early post-contrast imaging reliably distinguishes septic arthritis from transient synovitis 1, 2
- MRI detects concomitant osteomyelitis in up to 30% of pediatric septic arthritis cases, with abnormal T1 marrow signal and diffuse involvement being most specific 1, 4
- Consider preoperative MRI if ≥3 high-risk criteria are present: CRP >13.8 mg/L, absolute neutrophil count >8,600 cells/mm³, platelet count <314,000 cells/mm³, symptoms >3 days, or age >3.6 years 2
Modalities to Avoid
Do not use bone scintigraphy or CT as initial imaging for suspected septic arthritis. 1
- Three-phase bone scan has poor spatial resolution and lacks specificity compared to ultrasound, CT, and MRI 1
- CT has decreased sensitivity for bone marrow pathology and inferior soft-tissue contrast compared to MRI 2
- Bone scintigraphy may be considered only for evaluating multifocal infections 1
Special Consideration: Prosthetic Joints
For suspected periprosthetic joint infection, proceed directly to image-guided aspiration after radiographs, as nuclear medicine studies have limited utility. 1
- Joint aspiration under fluoroscopy, ultrasound, or CT guidance is the gold standard for prosthetic joint infection diagnosis 1
- Metal artifact reduction MRI sequences can detect synovial thickening, joint effusion, and soft tissue abscesses despite hardware 1
- FDG-PET/CT has uncertain specificity due to persistent radiotracer accumulation around prosthetic components for up to 2 years post-implantation 1
Critical Pitfall
Never delay aspiration to obtain advanced imaging—septic arthritis is an orthopedic emergency requiring immediate joint drainage. 3, 5, 6