Developmental Dysplasia of the Hip

What is developmental dysplasia of the hip (DDH)?

Developmental dysplasia of the hip, or DDH, is a musculoskeletal condition in children in which the bones of the hip joint, which is a ball-and-socket joint, do not form or grow together properly. This deviation from the normal growth pattern in the hip generally occurs while the child is still a developing fetus, but can occur, more rarely, in early childhood as well. As a result, the joint may undergo subluxation, in which the head (the “ball”) of the femur (the thigh bone) does not maintain its normal, central position in the pelvis (the “socket”). If untreated, the joint may become dislocated, in which the ball sits completely outside of the socket. If DDH goes undetected, a child may end up with an unstable joint in late childhood or adulthood, which can lead to problems with walking and other activities. This instability may cause osteoarthritis in the joint, leading to a painful limp at a relatively young age.

The term DDH actually denotes a wide spectrum of conditions, from a mild change in the shape of the bones of the hip to a major dislocation that can only be corrected with surgery. Some patients are born with this problem and others develop dysplasia as young children. Not all patients with DDH have dislocations, and not all cases are congenital (i.e. presenting at birth). Though the congenital form of the condition is by far the most common subset of DDH, it may also develop after birth and be associated with neurologic conditions (such as spinal cord defects), connective tissue disorders, or other syndromes of the musculoskeletal system (such as Larsen’s syndrome or arthrogryposis).

There are usually no major symptoms of DDH, because it is often detected in infants soon after birth and treated without complications. However, in some instances the condition may be first identified in older infants through several cardinal signs, such as a difference in the length of the two legs or difficulty moving the leg away from the body on one side.

Some basic facts about DDH

  • DDH is generally completely painless in children
  • Approximately 1 out of every 100 children is born with DDH (in some form), and around 1 out of 1000 children has a dislocated hip at birth
  • The incidence of the DDH is higher than normal for children with the following characteristics, known as risk factors:
    • First-born children
    • Positive family history (i.e., a relative of the family had DDH)
    • Female sex (it is approximately 4 times more common in girls than boys)
    • Premature birth (it is more common in babies born prior to 37 weeks in term)
    • Breech position (children with their legs presenting first in the womb, just before birth)
    • Oligohydramnios at the time of labor (children who were surrounded by less amniotic fluid than normal in the womb, when examined by a fetal ultrasound)
    • Children born with specific other musculoskeletal conditions, such as torticolis (in which the neck muscles are tight on one side) or foot deformities
  • DDH affects the left hip more often than the right hip (around 60% of cases affect just the left hip, 20% just the right hip, and 20% both hips)

The normal hip

The hip has a unique combination of structural features that permit the transmission of tremendous forces (including the weight of the body) between the pelvis and the thigh, while still allowing for an extensive range of motion. The head of the femur normally sits in an area of the pelvis bone called the acetabulum, which acts as the socket. There is a joint capsule that sits around the joint and is made up of a series of ligaments, thick fibrous structures found throughout the musculoskeletal system that attach bones to other bones. This capsule surrounding the joint insures that the ball sits snugly in the socket and remains there, throughout the movements of the upper legs. Smooth, slippery cartilage at the center of the socket and at the tip of the ball, as well as a thin layer of the body’s natural lubricating material, called synovial fluid, allow the two bones to slide past each other, much like a well-greased ball bearing.

While most of the development of the hip occurs during fetal life, there is still a critical period of development in the bony structures that occurs in early childhood. (This explains why some children are born with seemingly normal hips, but later develop dislocations.) Also, at the time of birth, the bones still consist mostly of relatively soft cartilage, and have not filled in completely with hard bone. Normal growth requires that the bones of the joint be in contact with one another. Therefore, the ball and the socket help mold each other, so to speak, and any disruption in this process can compromise the healthy development of the joint.

What causes hip dysplasia?

Several factors are thought to contribute to the development of DDH, and certain factors may be more or less prominent in different individuals. One common finding in infants with DDH is that the ligaments that surround the joint show excessive laxity, which means they are looser than normal. Because the capsule does not hold the bones in close contact, as mentioned above, they do not grow completely or properly, leading to joint instability. The more time that the joint spends in an unstable position, the more its normal growth will be affected. It has been hypothesized that DDH is more common among girls because the female hormones circulating in the womb of a female fetus may contribute to increased joint laxity.

Another contributing factor may be the position of the child while in the uterus, or the combination of pressures to which a fetus may be subject, while developing in the womb. For example, one explanation for why first-born children are more commonly affected is because the mother’s uterus has not been stretched out yet by the process of labor, creating greater pressures in the womb that may place the hip in a more unstable position. Though other, similar theories relate to the other risk factors as well, the exact causes and pathophysiology of DDH remain largely unproven, and ongoing research at several medical centers in the U.S. is devoted to identifying these causes and improving the treatment and prevention of the condition.

How is hip dysplasia diagnosed?

In young infants, DDH is usually diagnosed by a simple physical exam, in which a physician tests the movements of the hip joint. These exams are routinely performed on infants by their primary pediatrician. The thighs are gently maneuvered backwards in the Barlow test, which may demonstrate that the hip is unstable and can be dislocated. Another part of the physical exam, called the Ortolani test, in which the thighs are placed to the side and maneuvered forward slightly, can show that the hip is already dislocated, and the head of the femur may or may not slide back into the socket. Ultrasound is a useful diagnostic tool that is often used following the physical exam and involves placing a non-invasive flattened probe on the joint to show a picture of the deep tissues and bones. There are also physical signs that may suggest the presence of DDH, and parents may be helpful in noticing these signs in their child. For example, the skin folds on the back of the thigh may be uneven on the two sides, or the legs may appear to be different lengths. Infants with DDH may have limited movement or flexibility in their affected hip, and older children my show a limp, appear to waddle slightly, or walk on their toes when beginning to walk. X-rays of the hip joint can confirm the diagnosis of DDH in these children.

How is hip dysplasia treated?

There are a variety of treatments available for DDH, and the most important determinant in selection of the appropriate treatment is age of the child.

If DDH is detected soon after birth, the newborn should be placed in a Pavlik harness, a simple device that is strapped around the child’s legs and shoulders. It is designed to keep the legs in the proper position for the hip joint to be in close contact (with the “ball” snugly in the “socket"), so that it can grow together properly with tight ligament support structures. The Pavlik harness is usually worn for 1-2 months, and is surprisingly soft and comfortable.

In some dislocated hips, especially in children greater than 6 months of age, the hip does not immediately normalize, even with a Pavlik Harness. In some instances, a technique known as a closed reduction may have to be performed, which involves manipulating the hip in the operating room under general anesthesia. If the dislocation is more stable and cannot be easily manipulated, a surgical operation known as an open reduction is necessary to tighten the ligaments of the joint capsule around the hip after the joint has been successfully reduced. After the joint is properly positioned, a plaster cast is applied to the leg and lower abdomen to hold the joint in place.

As children age and as the severity of the problem increases, children may require other procedures, including operations that reorient either the femur or the pelvic bones. All methods are designed to position the hip together more appropriately and enhance the stability of the joint in the future. Following surgery for DDH, casting is often necessary to maintain the correction. These are commonly performed operations that can help get the hip back on track towards normal development.

Coping with DDH

DDH in a child can be a difficult condition for some parents to deal with, particularly because their child may seem completely healthy, with often normal-looking legs and hips. Therefore, for children who must wear a cast or receive surgery, the treatment may seem somewhat extreme. However, it is important to remember that such corrective measures greatly enhance a child’s future health and prevent what could otherwise be a debilitating condition. Moreover, the vast majority of children with DDH do tremendously well with treatment and have full walking function at a normal age. Most are not at all limited in the activities in which they participate, such as games or competitive sports. Therefore, although the condition may present some hurdles early in a child’s life, early detection and adherence to the proper treatment regimen will allow your child to be active and healthy through childhood and into adolescence and adulthood.