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OR3.1-3 | Musculoskeletal Infection — PBL Case

CLINICAL SETTING

Rajan, a 10-year-old boy from a semi-urban area, is brought to the orthopaedic outpatient department by his mother. She reports that for the past 5 days he has had a high fever (measured at home as 39.2°C), refused to walk, and has been crying whenever his left thigh is touched. She recalls that 3 weeks ago Rajan had a small boil on his left knee which seemed to resolve on its own without treatment. On examination, Rajan appears unwell and febrile (temperature 38.9°C). He is reluctant to move his left lower limb. There is exquisite point tenderness over the distal femur metaphysis, warmth, and soft tissue swelling. There is no joint line tenderness of the knee itself. He is unable to bear weight. Blood tests show WBC 21,500/mm³ (90% neutrophils), ESR 82 mm/hr, CRP 32 mg/dL. Blood cultures are collected. A plain radiograph of the left femur shows soft tissue swelling and a subtle periosteal elevation at the distal femoral metaphysis.

Trigger 1: The Febrile Child Who Won't Walk

The admitting resident reviews the history and examination. He notes the antecedent skin boil, metaphyseal point tenderness, high fever, inability to bear weight, and periosteal elevation on X-ray. He considers whether this is osteomyelitis, septic arthritis, or another cause. The paediatric registrar asks him to apply the available clinical and laboratory findings to guide urgency of management.

DISCUSSION POINTS

What is the most likely diagnosis and why? What is the significance of periosteal elevation on the plain X-ray at this stage of presentation?
What is the pathophysiological mechanism by which bacteria from the skin boil could cause osteomyelitis of the distal femur metaphysis? Why is the metaphysis particularly vulnerable?
How would you differentiate acute haematogenous osteomyelitis from septic arthritis of the knee in this patient? What examination and investigation findings would help?
Which organism is most likely responsible, and what host and bacterial factors favour this organism in this clinical context?

Click to reveal Trigger 2: Antibiotics Started — Is Surgery Needed? (discuss previous trigger first!)

Trigger 2: Antibiotics Started — Is Surgery Needed?

Rajan is admitted and started on IV cloxacillin (50 mg/kg/day in divided doses). Blood cultures subsequently grow Staphylococcus aureus (MSSA). After 48 hours of antibiotics, he remains febrile (38.5°C), WBC is 19,000/mm³, and he continues to refuse weight-bearing. A repeat X-ray shows no change. The team requests an urgent MRI which reveals: bone marrow oedema in the distal femoral metaphysis, a 2 cm subperiosteal abscess along the medial cortex, and no joint involvement. The attending orthopaedic surgeon reviews the case and states: 'We need to take this child to theatre tonight.'

DISCUSSION POINTS

What are the indications for surgical intervention in acute osteomyelitis? Does this child meet those criteria? Justify your answer.
Describe the surgical procedure that would be performed: access, decompression technique, what would be obtained intraoperatively, and how the wound would be managed.
Why is a subperiosteal abscess a critical finding? What is the risk to the femoral head if the abscess is adjacent to the hip joint (in other scenarios)?
What is the role of intraoperative cultures even though blood cultures are already positive? How would the culture result guide antibiotic management?

Click to reveal Trigger 3: Chronicity, Complications, and Long-Term Follow-Up (discuss previous trigger first!)

Trigger 3: Chronicity, Complications, and Long-Term Follow-Up

Surgery is performed — a subperiosteal abscess is drained, 20 mL of pus is evacuated, and cortical drill holes are made for medullary decompression. Intraoperative cultures grow the same MSSA. Antibiotics are continued. After 5 days Rajan is afebrile, eating well, and WBC has dropped to 8,500/mm³ with CRP normalising to 4 mg/dL. He is switched to oral cloxacillin to complete a total 6-week course. At 6-week follow-up, however, the family reports that the wound site has developed a small sinus tract with intermittent purulent discharge. A plain radiograph shows a dense bony fragment within the metaphysis surrounded by a sclerotic rim of new bone.

DISCUSSION POINTS

Interpret the radiological findings at 6-week follow-up. Define sequestrum and involucrum, explain how they form, and describe their radiological appearance.
What is the diagnosis now? What surgical procedure(s) are required to achieve cure, and what are the goals of each?
Rajan's mother asks if this could have been prevented. What factors in the initial management (or prior to presentation) contributed to the development of chronic osteomyelitis, and what could have been done differently?
What rare but important long-term complication must the family be warned about if the sinus tract persists for years, and what is the pathological basis of this complication?

Learning Issues

Research these questions and bring your findings to the discussion.

[OR3.1] What are the aetiopathogenesis, clinical features, investigation findings, and management principles of acute, subacute, and chronic osteomyelitis? How does the presentation differ across age groups (neonate, child, adult)?
[OR3.1] What are the Kocher criteria and how are they used to differentiate septic arthritis from transient synovitis of the hip? What is the probability of septic arthritis with 1, 2, 3, and 4 criteria positive?
[OR3.1] What are the pathological features of chronic osteomyelitis — define sequestrum, involucrum, and cloaca, and describe their radiological appearances and surgical significance?
[OR3.2] What is the procedure for joint aspiration, what findings on synovial fluid analysis differentiate septic arthritis from crystal arthropathy and transient synovitis, and what are the indications for aspiration versus surgical drainage?
[OR3.3] What are the surgical procedures used in source control for musculoskeletal infection — describe the indications, technique, and goals of drainage/arthrotomy, sequestrectomy, and saucerisation?
[OR3.1] How does spinal tuberculosis (Pott's disease) differ from pyogenic spondylodiscitis in terms of onset, pathology, imaging features, and treatment? What are the classic features of a cold abscess and what is a gibbus deformity?