Perthes disease — formative SAQs

SAQ 1 — The boy with a persistent limp (10 marks)

Stem: A six-year-old boy is brought to the clinic 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] [1]}

a) State the most likely diagnosis and explain why the knee pain does not localise the source of the problem. (2 marks) ^[4]

b) Describe the four radiographic phases of this disease and explain why the lateral pillar grade is assigned in the fragmentation phase. (3 marks) ^{[1] [4]}

c) Define the Herring lateral pillar classification, including groups A, B, B/C border, and C. (3 marks) ^[1]

d) Outline the containment principle and the two surgical options by which it is achieved. (2 marks) ^{[1] [6]}