Radiological Signs in Paediatric Orthopaedics

Paediatric radiological signs are among the most frequently tested topics. They test pattern recognition, an understanding of growth plate anatomy and knowledge of specific paediatric conditions. You should be able to draw and interpret the Klein line for SUFE, construct Perkins and Hilgenreiner lines for DDH, classify growth plate injuries by Salter-Harris, stage Perthes disease and recognise the radiographic features of non-accidental injury.

Why the child is different. The developing skeleton has growth plates, ossification centres and developing joint morphology. These create both diagnostic challenges and pathological patterns not seen in adults.

• Physes are cartilaginous and radiolucent, so fractures through them may be invisible on radiographs (Salter-Harris I and V)
• Ossification centres appear sequentially and can be confused with fractures (CRITOE at the elbow)
• The periosteum is thicker and more metabolically active; greenstick and torus (buckle) fractures are unique to children
• Joints are more cartilaginous, so ultrasound is needed before ossification occurs (DDH screening)
• Remodelling potential is greater, but depends on the direction of angulation and proximity to the growth plate

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The differential for the limping or painful paediatric hip is stratified by age, with a discriminating radiological or clinical clue for each diagnosis.

Developmental dysplasia of the hip

Before 6 months: ultrasound. The femoral head has not yet ossified, which makes radiographs unreliable, so hip ultrasound is the primary modality. The Graf classification uses the alpha angle (bony acetabular coverage) and the beta angle (cartilaginous coverage).

After 6 months: the AP pelvis. Four assessments, remembered as PHAS:

• Perkins line: vertical from the lateral edge of the acetabulum, perpendicular to Hilgenreiner's line. The femoral head ossification centre should lie medial to it.
• Hilgenreiner line: horizontal through both triradiate cartilages, the Y-cartilage at the base of each acetabulum. The femoral head should lie below it.
• Acetabular index: the angle between Hilgenreiner's line and the acetabular roof line, drawn from the triradiate cartilage to the lateral acetabular edge. Normal is less than 30 degrees at birth, decreasing with age; more than 30 degrees is dysplastic.
• Shenton line: a smooth arc along the inferior border of the superior pubic ramus and the medial femoral neck. It is disrupted by subluxation or dislocation, and by femoral neck fractures.

The quadrants. Perkins and Hilgenreiner lines together divide the hip into four quadrants, and the femoral head belongs in the inferomedial quadrant. Any other position is abnormal; a head in the superolateral quadrant is subluxed or dislocated.

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Older children and adolescents. The centre-edge angle of Wiberg is normally more than 25 degrees, and less than 20 degrees is dysplasia. The Tonnis classification grades acetabular coverage and subluxation in older patients.

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Slipped upper femoral epiphysis

Drawing Klein's line. On the AP pelvis or hip radiograph, draw a line along the superior cortex of the femoral neck and extend it laterally. In a normal hip it intersects the lateral portion of the femoral head epiphysis. In SUFE the epiphysis has slipped posteriorly, and often inferiorly, so the line passes above or tangential to the epiphysis without intersecting it: the Trethowan sign.

What a normal line is worth. The classic all-or-nothing sign is highly specific but insensitive, identifying only 40.3% of slips in one reliability study and missing roughly three in five. A line that fails to cut the epiphysis is close to diagnostic; a line that still cuts it excludes almost nothing. Treating a normal Klein line as reassurance is a recognised route to a missed or delayed SUFE, and slip severity drives the AVN risk that follows.

The modified method. Measure the width of epiphysis lying lateral to Klein's line on each side and compare them. A side-to-side difference of 2 mm or more indicates a slip, which raises sensitivity to 79%.

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The rest of the AP. Beyond Klein's line, look for physeal widening, blurring or irregularity of the growth plate, and decreased epiphyseal height.

The frog lateral. This is the key view, and it shows the posterior slip far better than any AP sign; the frog lateral and the Southwick angle must always supplement the AP. The slipped epiphysis gives the classic 'ice cream falling off the cone' appearance: the metaphysis is the cone, visible above the displaced epiphysis, the ice cream.

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Grading severity. The Southwick angle is measured on the frog lateral and is normally about 12 degrees. A slip is mild at less than 30 degrees, moderate at 30-50 degrees and severe at more than 50 degrees.

Stability. Loder's classification calls a slip stable if the child can weight-bear, with or without crutches, and unstable if not. It is the classification that determines prognosis: in Loder's original series unstable slips had an AVN rate of 47%, stable slips 0%. The child stays non-weight-bearing until surgical fixation.

Both hips, and the knee. Always compare with the contralateral hip and assess the Klein line on both sides, because SUFE is bilateral in 20-40%. SUFE can present with isolated knee pain, referred via the obturator nerve, so always image the hips in a child with atraumatic knee pain.

Perthes disease

Staging. The radiographic stages of Legg-Calvé-Perthes disease reflect the pathological process:

• Initial (necrosis): subtle sclerosis of the femoral head, with a possible subchondral fracture line (the crescent sign)
• Fragmentation: the head appears fragmented, with mixed lytic and sclerotic areas
• Reossification: new bone progressively replaces the necrotic bone
• Healed: the head reconstitutes but may be aspherical (coxa magna, coxa plana)

Prognosis in active disease. Catterall grades the extent of head involvement in four groups (I-IV). The Herring lateral pillar classification grades how well lateral pillar height is preserved (A, B, C) and is the best prognostic indicator: group A carries a good prognosis, group C a poor one.

Head-at-risk signs. Their presence indicates a worse prognosis and may indicate the need for containment treatment.

• Gage sign: a radiolucent V-shaped defect in the lateral epiphysis
• Lateral subluxation: uncovering of the femoral head
• Lateral calcification: calcification lateral to the epiphysis
• Horizontal growth plate: horizontal rather than its normal oblique orientation
• Metaphyseal cysts: radiolucent areas in the metaphysis

Outcome at maturity. Catterall and Herring stage the active disease; the Stulberg classification grades the final healed femoral head by sphericity and congruency at skeletal maturity, and is the best predictor of later osteoarthritis. The grouping is the high-yield part.

The governing principle from the same data is head-neck-acetabulum congruency: a congruent joint, even a non-spherical one, fares far better than an incongruent one.

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Physeal injuries: Salter-Harris

The five types. Salter-Harris describes the pattern of a growth plate fracture.

• Type I: through the physis only. The radiograph may appear normal, a widened physis being the only sign, so the diagnosis is clinical: tenderness at the physis.
• Type II: through the physis with a metaphyseal fragment, the Thurston-Holland fragment. Usually an excellent prognosis.
• Type III: through the physis into the epiphysis, and so intra-articular. Needs anatomical reduction (ORIF) to restore the articular surface and growth plate alignment.
• Type IV: crosses metaphysis, physis and epiphysis. Most prone to growth disturbance through bone bridge formation, and requires ORIF for anatomical reduction.
• Type V: a crush of the physis without a visible fracture. It may be radiographically occult and is often diagnosed retrospectively, when growth arrest becomes apparent; the worst prognosis for growth.

How common Type II is. Type II is the most common growth plate fracture, commonly quoted as about 75%, and 54% in the population-based Olmsted County series.

Which types threaten growth. Types III to V are intra-articular or growth-arresting and carry the highest risk of premature physeal closure and deformity.

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Non-accidental injury

The signs. The metaphyseal corner fracture (bucket-handle) in an infant is highly specific for non-accidental injury. The other signs are multiple fractures of different ages, posterior rib fractures and complex skull fractures. A skeletal survey is mandatory.

Congenital limb anomalies

What is imaged. Proximal femoral focal deficiency (PFFD), fibular hemimelia and limb length discrepancy. PFFD is classified by Aitken, limb length is measured on a scanogram, and the assessment feeds planning for reconstruction or amputation.

The paediatric elbow has six secondary ossification centres that appear in a predictable order, and knowing it is essential to avoid mistaking a normal centre for a fracture (or a fracture for a normal centre). The mnemonic is CRITOE, and the centres appear roughly at the odd ages 1, 3, 5, 7, 9, 11 (the "rule of odds"; girls ossify earlier than boys, so the ages are approximate).

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Paediatric radiological signs are interpreted within national screening and child-protection frameworks that differ in detail but share the same imaging principles worldwide.

Global Epidemiology

• DDH: clinically significant dysplasia in roughly 1-3 per 1,000 live births, but sonographic immaturity/instability is far more common (Graf type IIa in ~14% of neonates). Risk factors: female sex, breech presentation, family history, firstborn, oligohydramnios, swaddling practices.
• SUFE: incidence ~10 per 100,000 children, rising with the global increase in childhood obesity; peak age 10-16 years; more common in boys and in some Pacific Islander, African and Hispanic populations; bilateral in 20-40%.
• Perthes: incidence varies markedly by region (roughly 1-20 per 100,000 children under 15), higher in Northern Europe and lower in some Asian and Black populations; boys affected 4-5 times more than girls.
• Physeal fractures: account for ~18-30% of paediatric long-bone fractures, peaking around the adolescent growth spurt (Olmsted County incidence 279/100,000 person-years).

Side-by-Side Guideline Comparison

Registry and Practice Variation

• Registries: dedicated paediatric registries are fewer than adult arthroplasty registries, but regional DDH and Perthes datasets (e.g. UK BSCOS studies and Nordic/Australasian DDH cohorts) inform screening policy and late-presentation rates. The debate between universal (Graf, German-speaking Europe) and selective (US/UK) ultrasound screening turns on cost, over-treatment of self-resolving immature hips, and late-diagnosis rates.
• High-resource settings: ready access to ultrasound, MRI for occult physeal/Perthes assessment, and arthrography-guided reduction.
• Limited-resource settings: reliance on clinical examination and plain radiographs; DDH and SUFE often present late, increasing rates of open reduction, osteotomy, and established avascular necrosis. The same radiographic signs (Klein, Perkins/Hilgenreiner, Shenton, Salter-Harris) remain the diagnostic backbone everywhere because they require only a plain film.
• Child protection: suspected non-accidental injury triggers a standardised skeletal survey worldwide; mandatory reporting and exact survey protocols vary by jurisdiction, but the metaphyseal corner fracture, posterior rib fractures and fractures of differing ages are universally recognised red flags.