Ultrasonography of the infant hip: the significance of provokable instability with normal morphology.

The hips of infants 2- to 4-weeks-old were investigated with ultrasonography. In all cases, ultrasonography verified morphologically normal hips; however, either a bilateral or unilateral instability of the hip could be provoked as demonstrated by ultrasonography. The first series of 27 infants (41 hips) were treated with abduction splint (Frejkas pillow) for 16 weeks. The second series of 28 infants (44 hips) were left untreated. All infants had repeat ultrasonography at 2 and 16 weeks and radiographs at 16 weeks. At 2 weeks, provokable instability was still seen in 10 hips in the treated group and 12 hips in the untreated group. At 16 weeks, all hips were ultrasonographically stable. The morphology was evaluated by alpha and beta angles by ultrasonography and also acetabular index by radiographs at 16 weeks. In all cases, normal hip development was noted with no differences between the two groups. Therefore, sonographically, instability in morphologically normal or immature hips has no clinical significance.