CIM Case: Transient Synovitis vs Septic Hip

Clinical Scenario

Patient: 4-year-old boy Presentation: Sudden onset right hip pain and limp since this morning, crying with movement, refusing to walk Relevant history: Mild upper respiratory tract infection 1 week ago, no trauma, no recent vaccinations, previously well, fully immunised, no sick contacts Examination findings:

• Temperature 37.8°C
• Right hip held in flexion, abduction, external rotation (position of comfort)
• Refuses to weight-bear on right leg
• Log roll positive (pain with any hip rotation)
• Severely limited internal rotation and extension
• Mild fullness in groin
• No skin changes, no erythema
• Left hip and knee examination normal
• Abdomen soft, non-tender

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Investigations Provided

Laboratory Results

Imaging

Image 1: AP Pelvis Radiograph

Radiological features:

• No bony abnormality
• Widening of right hip joint space (suggests effusion)
• Soft tissue swelling lateral to right hip
• No femoral head abnormality (no Perthes changes)
• Physes normal
• Left hip normal

Image 2: Ultrasound of Right Hip

Ultrasound findings:

• Moderate hip effusion present (8mm fluid collection)
• Synovial thickening
• No organised collection
• No bony erosion visible

Questions & Model Answers

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Key Teaching Points

Pattern Recognition

This pattern suggests Septic Arthritis:

• Acute onset, unwell child
• Refuses to weight-bear
• Fever (even low-grade)
• Elevated CRP/ESR/WCC
• Hip held in flexion, abduction, external rotation
• Effusion on ultrasound

Distinguish Septic Arthritis from Transient Synovitis:

Kocher Criteria Quick Reference:

• 0 criteria: <1% risk
• 1 criterion: 3% risk
• 2 criteria: 40% risk
• 3 criteria: 93% risk
• 4 criteria: 99.6% risk

Critical Management Points

1. Never miss septic arthritis - assume it until proven otherwise
2. Kocher ≥2 = aspirate - err on the side of aspiration
3. Aspiration is diagnostic AND therapeutic - decompresses the joint
4. Surgery within 6 hours - delays cause permanent cartilage damage
5. Staph aureus is commonest organism - flucloxacillin empirically
6. Follow up transient synovitis - X-ray at 6-8 weeks to exclude Perthes

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Common Examiner Follow-ups

Q: "What is Kingella kingae and why is it important?"

Kingella kingae:

• Gram-negative coccobacillus
• Increasingly recognised as cause of paediatric septic arthritis (age 6 months - 4 years)
• May be culture-negative on standard media (needs special techniques)
• Often preceded by URI or stomatitis
• Usually good prognosis
• Treated with cephalosporins or penicillin

Consider Kingella if: young child, culture-negative septic arthritis, less unwell than expected for Staph.

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Q: "When would you get an MRI?"

MRI indications in paediatric irritable hip:

• Diagnostic uncertainty after aspiration
• Poor response to treatment (to look for abscess, osteomyelitis)
• Adjacent osteomyelitis suspected (proximal femoral metaphysis is intra-capsular)
• Multifocal disease suspected
• Iliopsoas abscess suspected

MRI is NOT first-line but helpful when diagnosis unclear or complications suspected.

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Q: "This child has a draining sinus at follow-up. What now?"

A draining sinus indicates:

• Chronic infection - inadequately treated
• Osteomyelitis - may have developed
• Sequestrum - dead bone harbouring infection

Management:

1. MRI to assess extent
2. Inflammatory markers
3. Surgical debridement (possibly multiple)
4. Prolonged antibiotics (6-12 weeks)
5. Consider bone biopsy/culture
6. MDT discussion (paediatric ID, orthopaedics)

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Q: "How do you manage neonatal septic hip?"

Neonatal septic arthritis is different:

• Organisms: Group B Strep, Staph, Gram-negatives
• Presentation: May be subtle - pseudoparesis, irritability, poor feeding
• Kocher criteria don't apply - neonates often afebrile
• High index of suspicion needed
• Urgent aspiration and washout if suspected
• Antibiotics: Flucloxacillin + gentamicin (cover Gram-negatives)
• Prognosis: Worse than older children due to physeal damage

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Related Topics

• Paediatric Osteomyelitis
• Perthes Disease
• Hip Aspiration Technique
• Paediatric Sepsis
• Antibiotic Therapy in Bone and Joint Infection