Septic arthritis

Overview

•Infection of the synovium and joint space - a true orthopaedic emergency in children

•Most common in children under 4 years; boys > girls

•Most commonly affected joint: knee (~50%), then hip, shoulder, ankle, wrist

•Most common organism: Staphylococcus aureus at any age

•Other organisms by context:

•Streptococcus pyogenes (Group A strep) - second most common in children

•Neisseria gonorrhoeae - sexually active teenagers; may be polyarticular

•Escherichia coli / Gram-negatives - neonates and immunocompromised

Presentation

•Classic triad: fever, joint pain, impaired range of motion

•Hot, erythematous, swollen joint with exquisite pain on active and passive movement

•Refusal to weight bear - key clinical flag for hip/knee involvement

•Hip held in flexion, abduction, and external rotation (position of maximum capsular volume)

•Infants: may present only with pseudoparalysis or irritability on nappy changes

⚠️

Absence of fever does NOT exclude septic arthritis in infants and young children. Always maintain a high index of suspicion in any child with a limp, refusal to weight bear, or joint irritability.

Investigations

🥇 First-line

•FBC (WBC >12,000), ESR (>40 mm/h), CRP, blood cultures (before antibiotics), plain X-ray (usually normal early; excludes fracture/osteomyelitis), ultrasound (detects effusion, guides aspiration)

🏆 Gold standard

•joint aspiration (arthrocentesis) - synovial fluid for Gram stain, culture, WBC; WBC >50,000 cells/mm³ highly suggestive

🥈 Second-line

•MRI - best for early osteomyelitis, extent of infection, soft tissue involvement

📌

Joint aspiration must be performed before starting antibiotics wherever possible - antibiotics significantly reduce culture yield from synovial fluid.

Differential Diagnosis

Septic arthritis vs transient synovitis

Feature	Septic arthritis	Transient synovitis
Onset	Hours to days	Days
Fever	Usually present	Usually absent or low-grade
Passive movement	Exquisitely painful	Relatively preserved
WBC / ESR / CRP	Elevated (Kocher criteria)	Normal or mildly raised
Systemically unwell	Yes	No

•Osteomyelitis - bony tenderness rather than effusion; may co-exist with septic arthritis

•Perthes disease - insidious onset, no systemic features, X-ray shows femoral head flattening/fragmentation

•SUFE - adolescent males, pain on internal rotation, X-ray diagnostic

•Reactive arthritis - sterile, follows GI/genitourinary infection, HLA-B27 associated

Management

•Urgent hospital admission and orthopaedic team involvement

•Two pillars: joint decompression (surgical washout/aspiration) + antibiotic therapy

•Empirical IV antibiotics: flucloxacillin (Gram-positive cover including *S. aureus*) - adjust once sensitivities available

•Total antibiotic course: 4-6 weeks - commence IV, transition to oral when apyrexial and inflammatory markers falling

🚨

Septic arthritis of the hip is a particular surgical emergency - rising intra-articular pressure compresses the lateral epiphyseal vessels, risking avascular necrosis of the femoral head.

Complications

Complications of septic arthritis

Avascular necrosis of femoral head

Irreversible joint destruction

Osteomyelitis (contiguous spread)

Growth disturbance / limb length discrepancy

Ankylosis (fibrous/bony fusion)

Chronic septic arthritis

Systemic sepsis (~10% mortality)

Kocher Criteria

Validated rule to differentiate septic arthritis from transient synovitis of the hip in children. One point each for: history of fever, non-weight-bearing, ESR >40 mm/h, WBC >12,000 cells/mm³.

Score	Probability of septic arthritis
0	<0.2%
1	~3%
2	~40%
3	~93%
4	~99.6%

•Score 3-4: urgent orthopaedic review and operative management

•Score 0-1 in a well child: supports watchful waiting for transient synovitis

•Score 2: clinical grey area

🎯

Kocher criteria apply specifically to the hip and must be used alongside clinical judgement, not as a standalone rule.