The exact cause or causes of hip dysplasia are not known. It is widely accepted that hip dysplasia develops around the time of birth because the hip socket is shallower at birth than at any time before or after birth. The shallow socket at birth is because of natural fetal growth that increasingly limits hip movement during later stages of pregnancy. The shallow socket may allow more flexibility for the infant to pass through the birth canal. After birth, loose ligaments usually recover, and the hip sockets rapidly become deeper during the first year of life. This deepening process continues for several years, but shallow sockets may be diagnosed during adolescence or early adulthood, and this is the stable type of hip dysplasia that is often called acetabular dysplasia.
It is currently believed that infants are prone to hip dysplasia for the following reasons:
Hip dysplasia is approximately 12 times more likely when there is a family history.
Genetics plays a role but is not a direct cause of hip dysplasia.
If a child has DDH, the risk of another child having it is 6% (1 in 17)
If a parent has DDH, the risk of a child having it is 12% (1 in 8)
If a parent and a child have DDH, the risk of a subsequent child having DDH is 36% (1 in 3)
The baby’s womb position can increase pressure on the hips.
The positioning of the baby in the womb can cause more pressure on the hip joints, stretching the ligaments. It’s thought that babies in a normal position in the womb have more stress on the left hip than on the right hip. This may be why the left hip tends to be more affected.
Babies in the breech position are more likely to have instability than babies in a normal womb position and have an increased risk of DDH.
Babies with fixed foot deformity or stiffness in the neck (torticollis) have slightly increased risk of hip dysplasia. This may partly be due to limited space in the womb from these deformities.
Also, around the time of birth, the mother makes hormones that allow the mother’s ligaments to become lax (stretch easier) so that the baby can pass through the birth canal.
Some infants may be more sensitive to these hormones than others, allowing for excessive ligament laxity in the baby. Girls usually have more ligament laxity than boys and girls are 4-5 times more likely to have hip dysplasia than boys.
The bones of an infant’s hip joint are much softer than an adult hip joint.
It is easier for an infant’s hip to become misaligned (subluxate) or dislocate than an adult hip. This is because an infant hip socket is mostly soft, pliable cartilage, whereas an adult’s hip socket is hard bone.
Infant positioning during the first year of life.
Culture that keep infants’ hips extended on a cradleboard or papoose board have high rates of hip dysplasia in their children. Cultures that hold infants with the hips apart have very low rates of hip dysplasia. For this reason, swaddling with the hips extended during the first few months after birth should be avoided, and a hip-safe method should be used.