Aspirate a suspected septic hip in a child — OSCE

Candidate brief

You have this station to assess a 3-year-old febrile child with a suspected septic hip, score the prediction rule, exclude the relative contraindications, and outline the ultrasound-guided aspiration, the synovial fluid handling, the complications and the disposition. Treat this as a time-critical procedure in a high-risk child, with an explicit safety check and an ultrasound-guided single pass. [1] [3]

Key teaching and management objectives

Begin by scoring the prediction rule at the bedside. The Kocher predictors are a fever above 38.5°C, refusal to bear weight, an erythrocyte sedimentation rate above 40 mm per hour and a peripheral white cell count above 12,000 per microlitre; the Caird refinement adds a C-reactive protein above 20 mg per litre. This child meets four of the five, so the probability of septic arthritis is high and the rule dictates aspiration, empirical antibiotics and orthopaedic washout, not observation. [1]

Next, perform the structured pre-aspiration check and exclude the relative contraindications. Examine the joint for an effusion, warmth, erythema and severe pain on passive movement, and assess the child for toxicity and shock and the distal neurovascular status. Inspect the overlying skin for cellulitis, because passing the needle through infected skin seeds the joint. Exclude a bleeding risk by asking about haemophilia, anticoagulants and thrombocytopenia. Confirm this is a deep hip that must be aspirated under ultrasound, and obtain and document consent. [3] [4]

Outline the technique under ultrasound guidance. Position the child supine with the leg neutral, apply the topical anaesthetic early, infiltrate 1% lidocaine at a maximum of 3 mg/kg without adrenaline, prepare a sterile field, and perform an anterior ultrasound-guided approach entering the joint lateral to the femoral vessels. Explain that the hip is aspirated under ultrasound because the effusion cannot be felt, the landmarks are cartilaginous, and the femoral neurovascular bundle lies medial to the joint. [2] [4]

Handle the synovial fluid correctly. Send the aspirate for a white cell count and differential, a Gram stain and culture, and in the young child a Kingella kingae-specific culture or polymerase chain reaction. State the categories: normal fewer than 200 white cells per microlitre, non-inflammatory 200 to 2,000, inflammatory 2,000 to 75,000, and septic above 50,000 with a neutrophil predominance — and emphasise that a count below 50,000 does not exclude sepsis in a partially treated child or in Kingella disease. [4]

Close with complications, aftercare and disposition. The common complications are local pain, a transient haemarthrosis and a vasovagal response; the serious ones are iatrogenic septic arthritis from seeding, a tense haemarthrosis in the uncorrected coagulopathic child, and cartilage or neurovascular injury. The disposition for confirmed septic arthritis is admission for intravenous antibiotics, analgesia and an urgent orthopaedic washout, with serial C-reactive protein to track the response. Obtain the fluid before the first antibiotic dose whenever possible, because culture is the test most affected by prior antibiotics. [3] [4]

Marking domains

• Risk scoring and the decision (3 marks). Quotes the Kocher and Caird predictors with their thresholds; scores the child as high-risk; states that this dictates aspiration, antibiotics and washout rather than observation.
• Pre-aspiration safety check (3 marks). Examines the joint and the child; excludes overlying cellulitis, uncorrected coagulopathy and the haemophilic child without factor cover; confirms ultrasound for the deep hip; documents consent.
• Procedural knowledge (4 marks). Supine position; topical anaesthesia early; 1% lidocaine at 3 mg/kg without adrenaline; anterior ultrasound-guided approach lateral to the femoral vessels; single sterile pass.
• Fluid handling and interpretation (2 marks). Sends cell count, Gram stain and culture with Kingella-specific testing; quotes the synovial cell-count categories and the septic threshold above 50,000; notes that a lower count does not exclude sepsis.
• Complications and disposition (3 marks). Names the common and serious complications; arranges admission, antibiotics and orthopaedic washout for confirmed sepsis; states the rule of obtaining fluid before antibiotics where feasible. [2] [3]