Infant hip dysplasia, also known as developmental dysplasia of the hip (DDH), is a condition where the hip joint of a baby or young child is not properly formed.  In babies with DDH, the socket of the hip joint is shallower and more saucer-like, making the ball less stable.  When the ball is unstable and not centred within the socket, it prevents normal growth and development of the hip.  If this persists through to adulthood, the shallow bony socket means the cartilage, labrum and muscles around the hip all take on extra strain, which can lead to pain and arthritis.  It is most commonly diagnosed in the first few months of life and can be treated with a variety of methods, including observation, bracing, and surgery. Early diagnosis and treatment is important to prevent long-term complications.

Infant hip dysplasia (DDH) is often a clinically ‘silent’ condition that has no symptoms or signs at all. Children with milder forms of dysplasia will have normal (or increased) range of motion, normal function and generally no pain throughout childhood.  

If the shallowness of the socket persists into adulthood, the patient may eventually begin to experience hip pain due to overload of the soft tissues and cartilage.  This may occur in the teens, young adult years, middle or old age, depending on severity of the dysplasia.  For more information, see article on adult dysplasia.

In more severe cases where the hip joint is actually dislocated, parents may notice:

• Asymmetry in the legs: One leg may appear shorter than the other.
• Uneven skin folds on the thighs or buttocks: if one hip is dislocated, there will usually be deeper or more skin folds present along the inner thigh.
• Limited range of motion in the hip joint: A baby with a dislocated hip will usually have reduced abduction of that hip (ability for the knee to ‘flop out’ to the side in the frog-leg position)
• A clunk may be felt when changing the nappy as the hip moves in and out of the joint.

In older children, a dislocated hip may present as:

• An asymmetric limp or waddle
• Leg length discrepancy
• Reduced abduction of the affected hip

Dislocation of both hips occurs in rare cases and can be difficult to pick up clinically as there are no asymmetrical findings.  In these cases, the child will often have:

• A waddling gait with increased arch in the lower back
• Restricted abduction of both hips

Developmental dysplasia of the hip (DDH), also known as infant hip dysplasia, is typically diagnosed through a physical examination and imaging tests. The specific diagnostic process may vary depending on the age and symptoms of the child, but generally, the following steps are involved:

• Physical examination: A doctor or other health practitioner will examine the child's hips, legs, and pelvis for signs of asymmetry or limited range of motion. They may also check for other signs of DDH, such as uneven skin folds or a limp.
• Ultrasound: Ultrasound is particularly useful for infants below 6 months of age as many of the bones of the hip are still made of cartilage and can’t be seen on x-ray.  There are several findings on ultrasound that a found in dysplasia:
  1. Femoral head coverage by the socket is the most important indicator.  This should generally be over 50% at 6 weeks and increase gradually with age, generally being over 60% by 3 months of age.  Hip stability is also tested with ultrasound by holding the probe over the hip joint as the ultrasonographer places a gentle force on the leg.  The reduction in femoral head coverage is then reported.
  2. The alpha angle is a secondary measure and looks at the acute angle between the lateral wall of the ileum and the bony roof of the acetabulum to measure the ‘steepness’ of the roof.  The alpha angle should be over 60 degrees.
  3. The general shape of the acetabulum is also important, as the edge of the socket is often much more rounded and has a softer angle in dysplasia than in the well-formed hip.

• X-rays: become the gold standard imaging test from 6 months and older.  There are several indicators of dysplasia including:
  1. Increased slope of the acetabular roof - known as the acetabular index.
  2. Asymmetry of the ossific nuclei of the femoral heads
  3. Migration of the ossific nuclei out of the socket can be seen in subluxed or dislocated hips

It's worth noting that DDH is not always easy to diagnose, and some cases may require multiple exams and tests to determine the extent of the condition. Once diagnosed, the treating doctor will recommend the appropriate treatment based on the degree of dysplasia, the age of the child, and other factors.