Septic Arthritis
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Direct answer
Septic arthritis is a bacterial joint infection that destroys cartilage within days and is a true emergency. A single hot, swollen, exquisitely painful joint held immobile — a limp or refusal to walk in a child — with fever and raised markers is septic arthritis until aspirated and proved otherwise. Aspiration precedes antibiotics, Staphylococcus aureus is the commonest organism, and treatment combines prolonged targeted antibiotics with drainage, the paediatric hip decompressed urgently to save its blood supply.
What you must remember
- Organisms: Staphylococcus aureus is the commonest across ages; group B streptococci and gram-negative bacilli matter in neonates, gonococcal infection enters the list in young adults, and salmonella is classic in sickle cell disease.
- Route: haematogenous spread predominates, but the joint may be seeded directly, by penetrating injury or injections, or by extension of adjacent osteomyelitis — the proximal femoral metaphysis lies inside the hip capsule in young children.
- Diagnosis before treatment: aspirate the joint first for Gram stain, culture, cell count and crystal microscopy; a turbid fluid with tens of thousands of neutrophils and no crystals confirms infection; blood cultures and inflammatory markers support.
- Kocher criteria separate septic arthritis from transient synovitis of the child's hip: fever over 38.5 degrees, refusal to bear weight, ESR over 40 and white cell count over 12,000 — more criteria mean higher probability.
- The septic hip is the emergency of emergencies: pus under pressure in the capsule strangulates the femoral head's blood supply, so open or arthroscopic drainage is done urgently.
- Treatment: empirical intravenous antibiotics after aspiration, targeted once cultures return and continued for several weeks, plus arthroscopic or open washout for deep joints such as the hip.
- Gonococcal arthritis behaves differently: migratory polyarthralgia, tenosynovitis and a pustular rash, usually resolving well with antibiotics alone.
Common confusion
Septic arthritis is confused with crystal arthritis when a hot joint appears — the aspirate is examined for crystals and organisms together, and the two can coexist. In children, transient synovitis of the hip is the everyday mimic, and the Kocher criteria separate them, with fever and high inflammatory markers pushing towards infection. A modest white cell count in the fluid and antibiotics given before sampling never override a clinically septic joint, and the irritable hip of a child who refuses to walk is as alarming as the florid adult knee.
Exam-focused takeaway
NEET-PG tests order of steps above all: the single hot joint demands aspiration before antibiotics, and the septic hip in a child needs urgent drainage, with antibiotics alone or observation offered as distractors. Organism one-liners pair Staphylococcus aureus with the commonest cause, salmonella with sickle cell disease and gonococcus with the young adult's rash-tenosynovitis-arthritis triad. Kocher stems list the four variables, image stems show an effusion, and complication questions target cartilage destruction and avascular necrosis of the delayed septic hip.
Frequently asked questions
Which organism most commonly causes septic arthritis?
Staphylococcus aureus in all age groups, with Neisseria gonorrhoeae considered in young adults and salmonella in sickle cell disease.
Why aspirate the joint before starting antibiotics?
Because the aspirate provides Gram stain, culture, cell count and crystal analysis — the definitive diagnosis — and prior antibiotics sharply reduce culture yield.
What are the Kocher criteria?
Fever over 38.5 degrees, non-weight-bearing, ESR over 40 and white cell count over 12,000 in a child's painful hip, each raising the probability of septic arthritis over transient synovitis.
Why is septic arthritis of the hip an emergency?
Pus under tension in the capsule cuts off the femoral head's blood supply, causing avascular necrosis and permanent destruction unless drained urgently.
How is a septic joint treated?
Urgent aspiration or washout with empirical then targeted intravenous antibiotics for several weeks, and immediate drainage for deep joints, especially the hip.