A girl with an abnormal gait

Answer

Bilateral developmental dysplasia of the hip.

Discussion

The image shows changes typical of developmental dysplasia of the hip (DDH). Bilateral delayed ossification at the centre of the femoral head is visible in the lower outer quadrant (fig 2⇓, arrows A and B), which indicates bilateral developmental subluxation of the hip. The acetabular index (AI) was calculated using the angle formed by the acetabular roof line and the Hilgenreiner’s line (fig 2⇓, angles 1 and 2).1 A large AI (>30°) suggests a tendency towards hip dislocation. Also visible were bilateral disrupted Shenton’s lines, which are defined by the superior border of the obturator foramen and the medial border of the femoral neck (fig 2⇓, arrow C). This finding indicates cranial displacement of the femoral head from the acetabulum. Two shallow acetabulums and asymmetrical skinfolds were evident (fig 2⇓, arrow D).

Fig 2 Anteroposterior radiograph of pelvis showing changes typical of developmental dysplasia of the hip. A Hilgenreiner’s line was drawn through the triradiate cartilages, and a Perkin’s line was drawn perpendicular to the Hilgenreiner’s line at the lateral edge of each acetabulum. Bilateral delayed ossification at the centre of the femoral head is identified by arrows A and B. The left acetabular index was 39.5° and the right was 40.4° (angle 1, angle 2, respectively; normal is <30°). Arrow C identifies bilateral disrupted Shenton’s lines, and arrow D asymmetrical skinfolds.

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DDH refers to a continuum of abnormalities in the immature hip, ranging from mild instability or subluxation to permanent dislocation. About 20% of cases are bilateral, which causes the legs to be non-discrepant in length. It is unusual for parents to identify the deformity at an early stage. To facilitate early diagnosis of DDH, ultrasonography is routinely performed for infants aged less than 6 months who are at risk (breech presentation, or a strong family history of DDH or breech birth). Multiple characteristics on plain radiographs of the pelvis are useful to diagnose the condition in infants older than 6 months. In addition, asymmetrical skin creases may be a useful indicator for unilateral or bilateral DDH.