What imaging is indicated for suspected septic (infection of the) hip?

Imaging for Suspected Septic Hip

Start with plain radiographs of the hip, followed immediately by ultrasound to detect joint effusion and guide aspiration for definitive diagnosis. 1, 2

Initial Imaging Approach

Plain Radiographs First

• Obtain AP and lateral radiographs of the hip and pelvis as the initial imaging study to exclude fractures, Legg-Calvé-Perthes disease, slipped capital femoral epiphysis, and other bony pathologies 1, 2
• Radiographs have low sensitivity and specificity for septic arthritis, particularly in early disease (<14 days), where they may only show soft tissue swelling or be completely normal 1, 3
• In children >1 year of age, radiographs are negative in approximately 80% of culture-proven septic hip cases 3
• Do not delay further workup if radiographs are normal but clinical suspicion remains high 3

Ultrasound as the Critical Next Step

• Ultrasound of the hip is the most important imaging study for suspected septic arthritis, with extremely high sensitivity for detecting joint effusion 1, 2
• US can detect effusions as small as 1 mL and has a false-negative rate of only 5%, typically occurring when symptoms have been present <24 hours 1
• The absence of hip joint effusion on ultrasound virtually excludes septic arthritis 1
• Nearly 80% of pediatric patients with hip septic arthritis require only US imaging to guide their management 1

Image-Guided Aspiration

Ultrasound-Guided Hip Aspiration

• US-guided hip aspiration is considered highly accurate and should be performed urgently when effusion is detected 1, 4
• Aspiration provides definitive diagnosis through synovial fluid analysis (cell count, Gram stain, culture) and cannot be reliably replaced by imaging alone 1
• Bedside ultrasound-guided aspiration by trained personnel significantly reduces time to diagnosis (median 7.4 hours vs. 15.7 hours for OR aspiration) and avoids unnecessary trips to the operating room in ~50% of cases 5
• Image guidance reduces vascular complications, nerve injury, and risk of contaminating adjacent tissues 1

Advanced Imaging: MRI

When to Consider MRI

• MRI of the hip/pelvis should be obtained when there is concern for concurrent osteomyelitis, soft tissue abscess, or pyomyositis 1
• MRI detects osteomyelitis in approximately 50-70% of children with clinically suspected septic arthritis 1
• Decreased femoral head enhancement on early post-contrast imaging is a reliable MRI feature of septic arthritis not seen in transient synovitis 1, 4

Risk Stratification for MRI

Consider preoperative MRI if the patient has ≥3 of the following high-risk criteria for adjacent infection 1:

• C-reactive protein >13.8 mg/L
• Absolute neutrophil count >8,600 cells/mm³
• Platelet count <314,000 cells/mm³
• Duration of symptoms >3 days
• Age >3.6 years

This combination has 86-90% sensitivity and 80-91% positive predictive value for detecting infection beyond the joint space 1

Imaging Modalities to Avoid

Limited or No Role

• Bone scintigraphy has poor spatial resolution and lacks specificity compared to US, CT, and MRI, though it may be useful for multifocal infections 1
• CT has decreased sensitivity for bone marrow pathology and inferior soft-tissue contrast compared to MRI, but can be considered when MRI is contraindicated 1
• Whole-body MRI has no established role in isolated hip septic arthritis 1

Critical Pitfalls to Avoid

• Never rely on normal radiographs to exclude septic arthritis—immediate joint aspiration is required regardless of radiographic findings when clinical suspicion is high 3
• Do not delay aspiration waiting for MRI, as this can postpone definitive treatment; MRI should be reserved for cases where concurrent osteomyelitis or abscess is suspected based on clinical criteria 1
• Hip pain may be referred to the thigh, knee, or buttock, potentially leading to imaging the wrong anatomic region 2
• A false-negative ultrasound can occur within the first 24 hours of symptom onset 1