A 5-year-old boy is transferred with a 12-hour history of severe left groin pain, fever (38.6°C), and absolute refusal to bear weight. He is holding the hip in extension and external rotation. Ultrasound shows a significant joint effusion. Discuss your immediate diagnostic priorities and how you would apply clinical prediction algorithms to this case.
Candidate: I would immediately suspect septic arthritis. I’d order inflammatory markers (ESR, CRP) and a blood culture. I’d use the Kocher criteria to see if he needs surgery. Since he has a fever, can't bear weight, and has a painful hip, he meets the criteria, so I would plan for an urgent arthrotomy.
Failing to address the atypical "extension" resting position or waiting for an MRI to "confirm" the diagnosis. Candidates often treat the Kocher criteria as a rigid, automatic rule rather than a guide, and neglect to mention the importance of blood cultures BEFORE starting antibiotics.
The candidate must systematically categorize the patient: 1) Clinical assessment (toxic appearance, non-weight bearing); 2) Labs (WBC, ESR, CRP—highlighting that CRP is more sensitive in the acute <24hr phase than ESR); 3) Imaging (Radiographs to rule out malignancy/trauma, Ultrasound for effusion). Crucially, they should justify the departure from "classic" FABER positioning by mentioning early-stage distension or compensatory muscle spasm. They must emphasize that if the suspicion is high (≥3 Kocher/modified criteria), delay for MRI is contraindicated, and the priority is emergent surgical lavage after blood cultures.
The child is now in the operating room. Describe your surgical approach, the key anatomical structures you will encounter, and the specific pitfalls to avoid during the exposure and washout.
Candidate: I would use an anterior (Smith-Petersen) approach. I’d go between the sartorius and the tensor fasciae latae, then retract the rectus femoris medially to reach the capsule. I'd perform a T-capsulotomy, irrigate, and leave a drain.
Forgetting to mention the ascending branch of the lateral femoral circumflex artery. Failing to emphasize the need to wash out the dependent posterior/inferior recesses of the joint, which is the most common cause of persistent infection.
Describe the Smith-Petersen interval (Sartorius/Femoral nerve vs. TFL/Superior Gluteal nerve). Highlight the anatomical danger zone: the ascending branch of the lateral femoral circumflex artery which must be ligated. Detail the importance of a thorough washout using a soft catheter or suction tip to reach the posterior inferior recess. Note the importance of synovectomy for tissue culture and leaving the capsule open for continuous decompression.
Post-operatively, the child remains irritable. How do you monitor progress, and what are the indications for re-operation or further investigation?
Candidate: I would watch his temperature and keep checking his blood work. If his CRP doesn't go down, I would repeat the surgery to wash it out again. I'd transition to oral antibiotics when he looks better.
Being too vague. Failing to distinguish between the kinetics of CRP (rapidly responsive) vs. ESR (delayed). Ignoring the need to rule out other sites of infection or failure to consider late complications like AVN.
Use a structured monitoring plan: Clinical (systemic signs), Laboratory (daily CRP tracking—a failure of CRP to downtrend after 48-72h mandates repeat imaging/washout), and Radiographic (serial follow-up to detect AVN/chondrolysis). Mention the transition to oral antibiotics based on clinical improvement and CRP normalization. Mention that persistent symptoms post-washout should trigger an MRI to rule out synchronous osteomyelitis or inadequately drained sub-capsular collections.
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