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]

SAQ 2 — The older child with a collapsing head (10 marks)

Stem: A nine-year-old girl presents with a four-week limp. Her radiograph shows a right femoral head in the fragmentation phase with the lateral third of the head collapsed to less than half of its original height. She is referred to the orthopaedic service for a containment decision. [2] [1]

a) State the Herring lateral pillar grade and the Stulberg outcome classification it predicts. (2 marks) [1] [9]

b) List the independent poor prognostic factors for Perthes disease as defined by the Wiig prospective study, and explain why older age at onset worsens the prognosis. (4 marks) [2]

c) Outline how age and lateral pillar grade together drive the containment decision, citing the Herring multicentre evidence. (2 marks) [1]

d) Describe what hinge abduction is and why it is a contraindication to a simple containment osteotomy. (2 marks) [4] [9]

Model answer — SAQ 1

a) The most likely diagnosis is Legg-Calve-Perthes disease. The hip and the knee share the obturator and femoral nerve supply, so hip pain is frequently referred to the knee or the thigh. The fellowship rule is that any child with knee pain receives a hip examination, because the source is the hip in a child who localises pain distally. The persistent limp, the stiff hip with loss of abduction and internal rotation, and the dense femoral head with the subchondral fracture line confirm the diagnosis. [4]

b) The four radiographic phases are the initial or necrosis phase, the fragmentation phase, the reossification phase, and the remodeling or healed phase. The lateral pillar grade is assigned in the fragmentation phase because that is when the height of the lateral third of the femoral head can be read most reliably, as the head resorbs and loses height. In the necrosis phase the head has not yet collapsed, and in the reossification phase the new bone has begun to obscure the pillar height. [1] [4]

c) The Herring lateral pillar classification grades the height of the lateral pillar of the femoral head, the outer third of the head, on the anteroposterior radiograph during fragmentation. Group A means the lateral pillar is fully preserved with no lucency and full height. Group B means the lateral pillar shows lucency but maintains more than fifty per cent of its original height. The B/C border describes a head whose lateral pillar is at exactly fifty per cent, or that shows a narrow notch or a small osteolytic fragment at its outer edge. Group C means the lateral pillar has collapsed to less than fifty per cent of its height. [1]

d) Containment means keeping the soft, mouldable femoral head within the acetabulum during the fragmentation and reossification phases, so that the acetabulum acts as a mould and the head heals spherical rather than flat. It is achieved on the femoral side by a proximal femoral varus osteotomy that medialises the greater trochanter and deepens the head beneath the acetabulum, or on the acetabular side by a Salter innominate osteotomy that redirects the acetabular roof over the head. [1] [6]

Model answer — SAQ 2

a) The lateral pillar has collapsed to less than fifty per cent of its height, which is Herring group C. The Stulberg classification is the outcome classification applied at skeletal maturity: Stulberg I and II are spherical or ovoid congruent heads with a low osteoarthritis risk, while Stulberg III, IV, and V are progressively flatter and more incongruent heads with a rising risk. A lateral pillar C head in an older child predicts a Stulberg III, IV, or V outcome. [1] [9]

b) The Wiig prospective study of three hundred and sixty-eight children found the independent poor prognostic factors to be age over six at onset, a lateral pillar C grade, and female sex. Older age at onset worsens the prognosis because the reduced remodeling potential of the older skeleton leaves the deformed head less able to reshape itself during the healing phase, and a child with all three factors carries the highest risk of a poor Stulberg outcome. [2]

c) The Herring multicentre prospective study found that children eight years and older at onset, by bone age, with lateral pillar B and C involvement, had better Stulberg outcomes with surgical containment than without, while children under six at onset did well regardless of treatment. The B/C border in the six-to-eight-year band is the critical decision point. The decision is therefore driven by age and lateral pillar grade together, not by either alone. [1]

d) Hinge abduction occurs when the femoral head has collapsed and broadened beyond the acetabular margin, so that the abducted hip hinges the deformed head against the acetabular rim and the head is levered laterally out of the joint. It is a contraindication to a simple containment osteotomy because the levering is worsened, and it redirects the management toward salvage options such as a shelf or a Chiari procedure. [4] [9]