Initial Management of Hip Joint Effusion
Begin with plain radiographs of the hip and pelvis, followed immediately by ultrasound to detect joint effusion and guide aspiration for definitive diagnosis, as septic arthritis must be ruled out urgently to prevent permanent joint damage. 1, 2
Immediate Diagnostic Algorithm
Step 1: Plain Radiographs First
- Obtain AP and lateral views of the hip and pelvis as the initial imaging study 1, 3
- Radiographs exclude fractures, Legg-Calvé-Perthes disease, slipped capital femoral epiphysis, and other bony pathologies 1, 3
- Critical caveat: Radiographs have low sensitivity (20-73%) for detecting hip joint effusions and are often completely normal in early septic arthritis 4, 2
- Radiographic changes in osteomyelitis don't appear until 7-10 days and require >30% bone destruction 4
Step 2: Ultrasound Immediately After Radiographs
- Ultrasound is the most important imaging study for hip effusion, with extremely high sensitivity for detecting fluid as small as 1 mL 4
- The absence of hip joint effusion on ultrasound virtually excludes septic arthritis, with only a 5% false-negative rate (occurring in patients with symptoms <1 day) 4, 1
- Ultrasound reduces the need for early radiographs by 75% and can detect effusions in 71% of cases versus only 15% by radiography 5
Step 3: Risk Stratification for Septic Arthritis
Septic arthritis is the diagnosis until proven otherwise when pain localizes to the hip. 4 Assess these clinical criteria:
- Fever >101.3°F 2, 3
- ESR ≥40 mm/hr 2, 3
- WBC count ≥12,000 cells/mm³ 2
- Inability to bear weight 2
- CRP >2.0 mg/dL (or >15 mg/L in pediatrics) 2, 6
Meeting all criteria approaches 100% likelihood of septic arthritis 2
Step 4: Ultrasound-Guided Aspiration
- Perform urgent US-guided hip aspiration when effusion is detected 1, 2, 7
- Joint aspiration is the definitive diagnostic procedure and cannot be reliably replaced by imaging alone 1, 2
- Synovial fluid WBC count ≥50,000 cells/mm³ suggests septic arthritis 2
- Synovial fluid culture has 76% sensitivity and 96% specificity for septic arthritis 2
Conservative management exception: In pediatric patients with effusion volume <0.5 cc in the anterior recess AND normal inflammatory markers (ESR <40 mm/hr OR CRP <15 mg/L), close follow-up without aspiration is reasonable, as septic arthritis was not observed in this subset 6
When to Obtain MRI
Order MRI of the hip/pelvis when:
- Concern exists for concurrent osteomyelitis, soft tissue abscess, or pyomyositis 1
- Patient has ≥3 high-risk criteria: CRP >13.8 mg/L, absolute neutrophil count >8,600 cells/mm³, platelet count <314,000 cells/mm³, symptoms >3 days, age >3.6 years 1
- Joint aspiration is negative but clinical suspicion remains high 2
- Symptoms persist or worsen despite treatment (changes management in 21% of cases) 2
MRI distinguishes septic arthritis from transient synovitis: Decreased femoral head enhancement on early post-contrast imaging is reliable for septic arthritis and not seen in transient synovitis 4, 1, 3
Immediate Treatment if Septic Arthritis Confirmed
- Surgical drainage combined with immediate IV antibiotics after cultures are obtained 2
- Empiric IV vancomycin 15 mg/kg every 6 hours for MRSA coverage in adults 2
- Pediatric dosing: IV vancomycin 15 mg/kg/dose every 6 hours 2
- Bacterial proliferation causes irreversible cartilage damage within hours to days 2
Common Pitfalls to Avoid
- Never rely on radiographs alone to exclude septic arthritis—they are normal in early infection 4, 2
- Don't delay aspiration waiting for MRI—aspiration provides definitive diagnosis and MRI is adjunctive 1, 2
- False-negative ultrasound can occur if performed within 24 hours of symptom onset 4, 3
- Negative synovial fluid culture does not exclude infection—consider percutaneous bone biopsy if clinical suspicion remains high 2
- Over 50% of pediatric patients with septic arthritis have infection beyond the joint space on MRI, including concurrent osteomyelitis 4, 2