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

Investigations Provided

Laboratory Results

Test	Result	Normal Range	Interpretation
WCC	14.5 ×10⁹/L	5-15 ×10⁹/L	Upper normal
CRP	45 mg/L	<5 mg/L	↑ Elevated
ESR	55 mm/hr	<10 mm/hr	↑ Elevated
Blood culture	Pending	-	Sent
Hb	118 g/L	110-140 g/L	Normal
Platelets	350 ×10⁹/L	150-400 ×10⁹/L	Normal

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

What is your differential diagnosis and which diagnosis concerns you most?

How do you use the Kocher criteria and what are their limitations?

This patient has a Kocher score of 3. What is your management?

The aspirate is purulent with WCC of 80,000/μL. Describe your surgical management.

What if the aspirate was clear with WCC of 5,000/μL? How would you manage?

What are the complications of septic arthritis and what is the prognosis?

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:

Feature	Septic Arthritis	Transient Synovitis
Onset	Acute, hours	Gradual, 1-2 days
Fever	Usually present	Absent or low-grade
General health	Unwell, toxic	Well-appearing
CRP	>20-40 mg/L	<20 mg/L
ESR	>40 mm/hr	<40 mm/hr
Aspirate WCC	>50,000	<50,000
Treatment	Surgical washout	Conservative

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

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

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.

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.

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:

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

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

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

Transient Synovitis vs Septic Hip