Septic Arthritis in Children
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Direct answer
Septic arthritis is a bacterial infection of a joint that can destroy cartilage within hours and needs urgent aspiration and drainage. In children the hip is the joint most often affected, and Staphylococcus aureus is the commonest organism, with Kingella kingae important in toddlers. The Kocher criteria, built on fever, inability to bear weight, ESR and white cell count, help identify children who need urgent joint aspiration.
What you must remember
- Joint of predilection: the hip is the commonest joint affected in children, followed by the knee and ankle.
- Kocher criteria: fever above 38.5 degrees Celsius, inability to bear weight, ESR above 40 mm per hour and white cell count above 12,000 per microlitre; more criteria present mean higher probability.
- Organisms: Staphylococcus aureus is commonest; Kingella kingae in children under about four years; streptococci and Neisseria gonorrhoeae in sexually active adolescents.
- Joint aspirate: cell count, Gram stain and culture, with the aspirate sent before antibiotics where the child is stable enough.
- Imaging: ultrasound shows effusion early, plain films are often normal, and MRI helps in complicated or deep joints.
- Treatment: urgent drainage, either repeated aspiration or surgical washout for hip infection, with IV antibiotics guided by culture.
- Complications: avascular necrosis of the femoral head and joint destruction if drainage is delayed.
A stepwise approach to a toddler whose hip will not bear weight
A 3-year-old has fever and refuses to walk on one leg, with the hip held slightly flexed. The sequence below separates septic arthritis from transient synovitis.
- Score the Kocher criteria. Record fever, weight bearing, ESR and white cell count, since more criteria raise the probability of infection.
- Examine and ultrasound the hip. An effusion on ultrasound with a high-probability picture needs urgent action, and a normal film does not exclude infection.
- Aspirate urgently. Ultrasound-guided hip aspiration sends fluid for cell count, Gram stain and culture, and can itself relieve pressure.
- Start antibiotics after sampling. Give empirical IV cover for Staphylococcus aureus once the aspirate is sent, without waiting for results.
- Drain if not responding. Surgical washout is needed for hip infection, large collections or failure of aspiration, and delay raises the risk of avascular necrosis.
Early aspiration is both diagnostic and therapeutic, because removing pus lowers joint pressure and often relieves pain within hours. Surgical review should be requested as soon as the aspirate is sent, rather than after antibiotics have failed.
Where the exam tries to catch you
Three errors recur on this topic.
- Trap: labelling a febrile child with hip pain as transient synovitis without scoring Correct: Kocher criteria should be applied, because septic arthritis can destroy the femoral head.
- Trap: waiting for MRI before aspiration Correct: an effusion in a suspicious hip needs urgent aspiration, and imaging must not delay drainage.
- Trap: assuming a normal ESR excludes infection Correct: a low ESR and white cell count do not exclude septic arthritis, so clinical suspicion guides management.
Frequently asked questions
What are the Kocher criteria in septic arthritis?
The Kocher criteria for septic arthritis in children are fever above 38.5 degrees Celsius, inability to bear weight, ESR above 40 mm per hour and white cell count above 12,000 per microlitre. The more criteria present, the higher the probability of septic arthritis and the stronger the case for aspiration.
Which joint is most commonly affected by septic arthritis in children?
The hip is the joint most commonly affected by septic arthritis in children, and it carries a high risk of femoral head damage. The knee and ankle are also commonly involved.
Why must a septic hip be drained urgently?
A septic hip must be drained urgently because pus raises intra-articular pressure and damages cartilage and the blood supply to the femoral head. Delay increases the risk of avascular necrosis and permanent joint damage.
Which organism is important in septic arthritis of toddlers?
Kingella kingae is an important cause of septic arthritis in toddlers, typically under about four years, and it is fastidious, so it can be missed on routine culture. Staphylococcus aureus remains the commonest organism overall, so empirical antibiotics must cover it.