Characteristics

• Previously known as congenital dislocation of the hip (CDH), but now recognised as a developmental failure of the acetabulum around the femoral head.

• A dysplastic acetabulum is shallow and ‘vertical’ rather than concentrically covering the femoral head. This allows an increased degree of instability in the hip joint which, in extreme cases, presents as frank dislocation of the hip. However, this is a spectrum of disease and DDH often has a bearing on the aetiology of early adult hip OA.

• Female : male ratio of 7:1. Left hip more commonly affected than right; bilateral in 20% cases.

• The reported incidence of neonatal hip instability is 5–20/1000 live births, but following re-examination 6 weeks later, the incidence falls to 1–2/1000 infants.

• Risk factors include family history (including maternal/sibling DDH, joint laxity and shallow acetabulae), breech presentation, oligohydramnios, other developmental anomalies, high levels of oestrogen, progesterone and relaxin in the last few weeks of pregnancy, and cultural post-natal factors (swaddling increases the risk, e.g. North American Indians, whereas encouraging legs to be widely abducted to allow sitting astride the back, as in southern Chinese and certain African peoples, decreases the risk of DDH).

Clinical features

• Neonatal diagnosis is the gold standard. Look for decreased abduction in flexion, unequal gluteal and groin folds (only a ‘soft sign’ though) and an apparently short femur (Galleazzi test), but beware bilateral DDH.

• Ortolani test – the dislocated or subluxed hip (out) is relocated (in flexion and abduction) by ‘lifting up’ with pressure on the greater trochanter.

• Barlow's test looks for instability by pushing back (i.e. posterior) in the flexed hip.

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