Perthes disease — branching viva

Candidate brief

You are the general paediatric registrar in the orthopaedic clinic. You have eight minutes to answer the examiner's questions on the child below. The viva branches through diagnosis, investigation, classification, management, and then to a second scenario. Be prepared to justify each answer with the underlying pathophysiology and the relevant evidence. [1] [4]

Opening scenario

A six-year-old boy presents with a three-week history of a limp that comes and goes, worse after activity. He is afebrile and well. He points to his right knee when asked where it hurts. On examination he has a mild antalgic gait and his right hip loses abduction and internal rotation compared with the left. His anteroposterior pelvis radiograph shows a slightly smaller, denser right femoral head with a faint subchondral line. [4] [3]

Branch 1 — Diagnosis and pathophysiology. What is your working diagnosis? Explain why the knee pain does not localise the source, and describe the pathophysiology of the disease from the ischaemic insult through the four phases to the healed head. [4] [3]

Branch 2 — Investigation and differentiation. What is the role of the plain radiograph and of magnetic resonance imaging? How do you distinguish this from transient synovitis and from septic arthritis? Why must a normal early radiograph not reassure you? [4]

Branch 3 — Classification. Define the Herring lateral pillar classification, including groups A, B, B/C border, and C. Why is the grade assigned in the fragmentation phase, and why has this system overtaken the Catterall and Salter-Thompson classifications? [1]

Branch 4 — The containment decision. State the containment principle and the two surgical options. How do age and lateral pillar grade together drive the decision, and what does the Herring multicentre study show? [1]

Branch 5 — Follow-up and prognosis. How is the child followed, and what is the Stulberg outcome classification? What is the role of the general paediatrician versus the orthopaedic surgeon? [4] [9]

Pivot scenario — The nine-year-old girl with a collapsing head

The examiner now introduces a nine-year-old girl with a four-week limp. Her radiograph shows the right femoral head in the fragmentation phase with the lateral third of the head collapsed to less than half of its original height. [2]

Branch 6 — The high-risk child. State the lateral pillar grade and the prognosis. List the independent poor prognostic factors from the Wiig prospective study and explain why older age at onset worsens the outcome. [2]

Branch 7 — Containment and salvage. How does the management change for this older, high-risk child? What is hinge abduction, and why is it a contraindication to a simple containment osteotomy? [1] [9]

Examiner's marking key

Strong answers will:

• Name Legg-Calve-Perthes disease and explain the shared nerve supply that refers hip pain to the knee [4]
• Describe the four phases and explain the tenuous retinacular blood supply as the basis for the ischaemic insult [3]
• Define the Herring lateral pillar classification precisely, including the B/C border as the decision point [1]
• Apply age and lateral pillar grade together to the containment decision, citing the Herring multicentre evidence [1]
• Identify age over six, lateral pillar C, and female sex as the poor prognostic factors and recognise the Stulberg classification [2] [9]

Weak answers will:

• Accept the limp as transient synovitis despite the three-week duration [4]
• Confuse the lateral pillar grade with the extent of head involvement or assign it in the wrong phase [1]
• Treat every Perthes case the same, without the age-and-pillar-guided decision [1]
• Miss the high-risk features of the older child and the lateral pillar C head [2]