An ultrasound study in young infants is performed when hip dysplasia is suspected. This is necessary to make the diagnosis or to be sure the hip is normal.
There are angles that are measured during the examination. The one that is used the most to help guide treatment is the alpha angle. This angle is considered normal if it is more than 60 degrees. Mild dysplasia is present when the alpha angle is between 43-60 degrees. Dysplasia is severe when the alpha angle is less than 43 degrees.
During the ultrasound examination, the hip is also examined for stability in the same manner as the Barlow test, but the ultrasound is used to see if the hip is unstable instead of relying on the feel of the doctor. Many babies have slightly loose hips so this can be very difficult to interpret. The images can generally show how far out of the socket the hip will move. Of course, this depends on how hard the examiner pushes.
In general more than 50% of the ball should remain in the socket. When the number is less than 45% then there is instability. The instability is much more common in newborn infants and the ligaments get tighter with age. Some instability in a six-week old infant is not very uncommon.
Sometimes this is treated and sometimes it is observed with a repeated ultrasound study at age three months. Other imaging methods that may be used for babies or children are x-rays and arthrograms.
More information on how ultrasound imaging works
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