Summary

Septic Arthritis in children is an orthopaedic emergency requiring immediate diagnosis and surgical washout. It is a bacterial infection of the joint space that causes rapid destruction of the articular cartilage (Chondrolysis) via proteolytic enzymes. The hip is the most common site. The diagnostic challenge is distinguishing it from Transient Synovitis (Irritable Hip), which is benign. The Kocher Criteria (Fever >38.5, NWB, WBC >12, ESR >40) are the gold standard for risk stratification. Staphylococcus aureus is the most common pathogen, but Kingella kingae is increasingly recognized in children <4 years. [1,2,3]

Key Facts

• Time is Cartilage: Proteolytic enzymes from bacteria and white blood cells destroy cartilage within 8 hours. Urgent washout is mandatory.
• The Neonatal Trap: Neonates do NOT mount a fever or raise WBC. Signs are subtle: "Pseudoparalysis" (Not moving the leg) or pain on nappy change.
• Kingella kingae: A fastidious organism often missed on standard culture. Requires PCR. Common in throat infections.

Clinical Pearls

"Don't let the sun set on a septic hip": If you suspect it, aspirate it. If you confirm it, wash it out NOW.

"The Position of Comfort": A septic hip is held in Flexion, Abduction, and External Rotation (FABER). This maximizes the joint capsule volume to accommodate the pus and reduce pressure.

"Antibiotics Later": Do NOT give antibiotics until after the cultures (blood and fluid) are taken, otherwise you sterilize the result. (Unless sepsis is life-threatening).

Differentiating Septic Arthritis vs Transient Synovitis

Kocher et al (1999) identified 4 independent predictors:

1. Fever > 38.5°C
2. Non-Weight Bearing
3. ESR > 40 mm/hr (or CRP > 20 mg/L in updated criteria/Caird)
4. WBC > 12,000

Probability of Septic Arthritis

• 1 predictor: 3%
• 2 predictors: 40%
• 3 predictors: 93%
• 4 predictors: 99.6%

Note: Caird et al added CRP > 20 as a 5th predictor, which is more sensitive than ESR.

                 CHILD WITH PAINFUL HIP
                        ↓
                 KOCHER CRITERIA
              (Fever, NWB, WBC, CRP)
             ┌──────────┴──────────┐
          LOW PROBABILITY    HIGH PROBABILITY (>2)
         (Transient Synovitis)     ↓
             ↓                 ULTRASOUND
          OBSERVE                  ↓
       (NSAIDs + Rest)          EFFUSION?
                               ┌───┴───┐
                              NO      YES
                              ↓        ↓
                          OBSERVE   ASPIRATE
                                       ↓
                                    PURULENT?
                                   ┌───┴───┐
                                  NO      YES
                                  ↓        ↓
                            ANTIBIOTICS  EMERGENCY WASHOUT
                                       (Arthrotomy)

1. Emergency Washout (Arthrotomy)

• Hip: Open anterior arthrotomy covers the gold standard. Arthroscopic washout is an alternative in older children.
• Knee: Arthroscopic washout is standard.
• Goal: Remove pus, disrupt loculations, reduce bacterial load.
• Repeats: May need multiple washouts (every 48h) until fluid is clear.

2. Antibiotics

• Start IV immediately AFTER cultures taken.
• Empiric: Flucloxacillin (covers Staph) + Ceftriaxone (covers Gram -ve/Kingella) depending on local guidelines.
• Duration: Typically 2-3 weeks IV, then 3-4 weeks Oral. Total 4-6 weeks. Monitor CRP response.

3. Transient Synovitis (The Differential)

• If aspiration is clear (straw coloured):
• Diagnosis is Transient Synovitis.
• Rx: NSAIDs (Ibuprofen), Rest, Mobilize as tolerated.
• Safety net: If fever spikes or pain worsens -> Return.

Kocher et al (1999) / Caird et al (2006)

• The Diagnostic Criteria.
• Takeaway: Fever and CRP are the strongest individual predictors.

Lyon Classification

• For Kingella kingae.
• PCR increases detection rate from <10% to >90% in "Culture Negative" septic arthritis.

What is it?

There is a bacterial infection inside the hip joint. It is like an abscess, but inside the joint space.

Why is it an emergency?

The pus produces chemicals that dissolve the smooth cartilage lining of the joint. If we leave it for even a day, it can cause permanent arthritis.

What is the operation?

We make a small cut to open the hip joint and wash out the pus with saline. We leave a drain in to let any new fluid out. Your child will also need strong antibiotics through a drip.

How did it happen?

Usually, bacteria from a minor illness (like a sore throat or scraped knee) travel through the blood and get stuck in the hip. It is bad luck, not bad parenting.

Q1: What are the 4 Kocher Criteria? A: 1. Fever >38.5°C. 2. Non-Weight Bearing. 3. WBC >12,000. 4. ESR >40. (Probability 99.6% if all 4 present).

Q2: Which organism is common in children <4 but hard to culture? A: Kingella kingae. It requires inoculation into blood culture bottles (or PCR) to detect. It is often responsible for "Culture Negative" sepsis.

Q3: Why is the proximal femur metaphysis unique regarding infection spread? A: The metaphysis of the proximal femur is Intracapsular (inside the joint capsule). Therefore, osteomyelitis of the femur can rupture directly into the hip joint, causing septic arthritis.

Q4: What is the empirical antibiotic choice? A: Depends on local guidelines, but typically an anti-staphylococcal penicillin (Flucloxacillin) or a Cephalosporin (Ceftriaxone) to cover Staph, Strep, and Kingella.

(End of Topic)