The hip is a 'ball and socket' joint. The top of the thigh bone (femur) is shaped like a ball and fits into a matching cup (acetabulum) on the outer side of the pelvis. Various problems can affect the baby's hip as it develops. Sometimes the ball does not lie safely in the socket and is displaced from it: this is what is meant by dislocation. Sometimes, although the ball is in the socket it can slip in and out of place. This is what is meant by the hip being dislocatable. Sometimes although the hip is in the socket it is not deeply in place and we call this hip 'subluxated.' Finally in some children although the hip is in the right place the socket does not grow properly and is too shallow. If the hip socket is shallow this may allow the ball to move from the position it should occupy.

1 to 2 in 1,000 babies born may have a hip that is dislocated at birth. A slightly larger group of children have hips which are not safely in the socket or in whom the socket is shallower than it should be. In general girls are more likely to be affected that boys. The left hip is more often affected than the right.

Most children who have slight instability of the hip at birth will get better on their own without the need for specific treatment. Nonetheless it is important to try to identify these children to ensure that the expected improvement occurs. Babies undergo routine examination of their hips at different ages. At birth they are checked by two tests called the Ortolani and Barlow tests. The baby is laid on his or her back and the hips are gently taken sideways. It is usually possible for a baby's hips to be takenfully out sideways. If the baby's hip does not move as fully as this it may be that the hip is not developing properly and further investigations and checks are necessary. In a young baby further investigation is usually by ultrasound but in an older child x-rays are more commonly helpful in establishing the diagnosis. The GP or clinic doctor will check the hips again at around six to eight weeks.

The Ortolani and Barlow Tests are not one hundred per cent accurate. This means that sometimes there is a false alarm, where the baby appears to have a hip condition. However, further tests may show that in fact she or he does not have the condition.

It also means that sometimes a problem may not be picked up even if it is present. So even if a baby has had a hip check and was found to be OK, there may still be a problem; any concerns should be brought to the attention of a health visitor or GP. Do not assume that because the check was 'normal', there cannot be a problem.

Research has shown that parents are good at detecting hip problems, but often delay seeking advice because of uncertainty. However, treatment is usually less complex the earlier it is started, so if there are concerns parents should talk to the Health Visitor or GP as soon as possible.

The condition is not painful and there are no definite signs that a child may have a problem with hip development, but the following are associated with the condition:

• One leg appears shorter than the other;
• An extra deep crease is present on the inside of the thigh;
• One hip joint moves differently from the other and the knee may appear to face outwards;
• When a baby's nappy is changed one leg does not seem to move outwards as fully as the other one;
• The child crawls with one leg dragging.

After walking age it may be noticed that:

• a child stands and walks with one foot on tiptoes with the heel up off the floor. (The child walks this way in an attempt to accommodate the difference in leg length);
• the child walks with a limp (or waddling gait if both hips are affected).

If developmental dysplasia of the hip is recognised early it can nearly always be treated simply by a splint which may need to be worn for six to twelve weeks. This keeps the baby's hips flexed and out sideways. This is a position in which the hip is most likely to develop satisfactorily.

Sometimes, however, these simple splints do not work and the baby's hip does not become stable and grow normally. Some children's hip problems are not detectable at birth or in early infancy and it is not until they begin to walk that a limp is detected which highlights the problem. For older children treatment is usually a little more difficult. Sometimes it is possible to put the hip safely into joint and hold it in a plaster cast. Sometimes it is necessary to release some slightly tight tendons in the groin through a small incision and occasionally it is necessary for an operation to put the hip safely in the socket. After such an operation it is usual to put the child in a plaster of Paris or fibre glass plaster which extends from the waist down to the ankles or feet.

Whenever children have been treated for Congenital Dislocation of the Hip (CDH) or Developmental Dysplasia of the Hip (DDH) it is very important that they are carefully followed up for a long time to make sure the hip grows properly. Occasionally another operation is necessary as they grow older if the socket fails to grow properly.

In many instances the causes of hip dislocation are not known. It is more common in babies who are born by breech and it is more common if the baby has been a little squashed inside the womb. Special care is taken in the examination of babies when there is a special risk factor. In many centres in the UK children who have risk factors are examined not only clinically but by ultrasound. The ultrasound improves the chance of diagnosing the hip problem and is very helpful in checking to see that hips grow better after treatment. Ultrasound is most useful, however, in the first few weeks of life. Thereafter x-rays are more reliable in seeing how a baby's hip is growing.

Inheritance patterns
In some families there may be an hereditary element but this is unusual.

Prenatal diagnosis
There is no prenatal diagnosis for Congenital Dislocation of the Hip or Developmental Dysplasia of the Hip.

Medical text written May 2000 by Mr M K D Benson. Last updated May 2005 by Mr M K D Benson, Consultant Orthopaedic Surgeon, Nuffield Orthopaedic Hospital, Oxford, UK.

As Congenital Dislocation and Developmental Dysplasia of the Hip is a lower limb disorder, information, support and advice is available from STEPS (see entry, Lower Limb Abnormalities).