Septic Arthritis of the Hip in Children

Definition & Importance

Septic arthritis of the hip in a child is a surgical emergency: the deep, intracapsular position of the proximal femoral metaphysis means infection readily reaches the joint, and pus rapidly destroys cartilage and can devascularise the femoral head. The main clinical challenge is distinguishing it from the far more common, benign transient synovitis.

Kocher Criteria

The Kocher criteria use four predictors — inability to bear weight, fever > 38.5 °C, ESR > 40 mm/h and WCC > 12 × 10⁹/L. The probability of septic arthritis rises steeply with the number present: roughly <3% with none, but very high (>90%) when all four are positive. A raised CRP is a useful additional predictor.

Clinical Features & Investigations

The child is unwell and febrile, holds the hip flexed, abducted and externally rotated, and resists all movement with severe pain. Investigations: FBC, CRP, ESR and blood cultures; ultrasound confirms an effusion and guides aspiration; radiographs exclude other causes. Ultrasound-guided aspiration of the joint is diagnostic (Gram stain and culture).

Transient Synovitis – the Key Differential

Transient synovitis (irritable hip) is the common benign mimic: a child, often after a viral illness, with a limp and mild hip discomfort but who is systemically well, afebrile and can usually bear weight. Inflammatory markers are normal or only mildly raised. The Kocher criteria and CRP help separate the two, but where doubt remains the safe course is aspiration, because missing a septic hip is far more damaging than a negative tap.

All four criteria give over 99% probability of septic arthritis.

• Emergency; deep intracapsular hip is readily infected and pus damages cartilage/femoral head.
• Kocher criteria (weight-bearing, fever >38.5, ESR >40, WCC >12) grade probability vs synovitis.
• Hip held flexed, abducted, externally rotated; ultrasound + aspiration confirm.
• Confirmed septic hip → emergency arthrotomy and washout + IV antibiotics.