DEAR MAYO CLINIC: What causes hip instability and hip dysplasia in infants? How effective is treatment in correcting this problem?

ANSWER: Hip instability and hip dysplasia are two related but not identical hip conditions in children. The earlier either condition is found and treated, the more successful the long-term outcome is likely to be.

The hip joint is a ball and socket. In a normal hip joint, the ball fits deeply and securely into a well-formed socket. The shape of the ball and socket match closely. Tight ligaments and muscles help hold the ball securely in place.

Hip instability happens when the ball is not securely seated in the socket. Then the ball can slide partially or completely out of the socket and become dislocated. Hip dysplasia occurs when the ball and socket do not closely match each other. In hip dysplasia, the socket is often shallow like a saucer, instead of being deep like a cup or bowl. Hip dysplasia can contribute to hip instability.

Although the exact cause of hip instability is not always clear, a variety of factors can put an infant at risk for developing this disorder. Hip instability and dysplasia are six times more common in girls than in boys. Being a first-born child and having other family members with the disorder also raises the risk.

In addition, babies positioned within the womb in a breech position are at a much greater risk for hip dysplasia than other infants. A breech baby has his or her knees straight, the hips are flexed upward, the feet are near the head, and the baby's bottom enters the birth canal first. Studies have shown the risk of hip instability and dysplasia for these infants to be as high as 25 percent.

Checking for hip instability is a standard part of a newborn exam. If hip instability is detected in a newborn, or within the first six months of life, it's relatively easy to treat and treatment is often successful. If it is found in children older than six months, treatment is much more invasive, often requiring surgery, and the results may not be as favorable.

One reason hip instability occurs in newborns is that the ligaments that hold the hip joint in place are looser than normal in some children immediately after birth. As a child ages, the ligaments tighten around the joint. The earlier treatment can be started to hold the hip joint securely in the correct position while those ligaments tighten, the more effective treatment will be.

Treatment of hip dysplasia for young infants usually involves the child wearing a hip brace or harness that keeps the legs apart. This holds the hip joint in place as the baby grows. In most cases for infants younger than six months, this treatment is all it takes to correct the problem.

For children older than six months, a procedure may be done to gently reposition the femoral head (the ball) inside the socket. After that procedure, a body cast is placed on the patient to keep the hip joint positioned correctly. If that is not successful, surgery often is the next option.

Surgery for hip dysplasia may include releasing or lengthening ligaments, tendons or muscles that are keeping the joint from staying in place, positioning the ball and socket correctly, and then tightening the soft tissues to hold the femoral head within the socket. In some cases, particularly in children older than 18 months, surgery also may involve changes to the bone, so the joint is properly aligned.

Children who are successfully treated for hip dysplasia typically do not have long-term hip problems. If the condition goes untreated or if treatment is delayed for several years, serious problems can occur. These include significant arthritis in the hip joint and eventually deterioration of the joint that may require a hip replacement. -- Anthony Stans, M.D., Orthopedic Surgery, Mayo Clinic, Rochester, Minn.

(Medical Edge from Mayo Clinic is an educational resource and doesn't replace regular medical care. E-mail a question to medicaledge@mayo.edu. For more information, visit http://www.mayoclinic.org.)