Developmental dysplasia of the hip

Congenital hip dysplasia, or developmental dysplasia of the hip, is a problem where the socket or acetabulum - and the femoral head are misaligned, resulting in an unstable hip joint.

Typically, the problem is present at birth, but sometimes it appears later as the bones develop over time.

The hip joint is a ball and socket type because the ball-shaped head of the femur sits and rotates within the acetabulum which is a cup-shaped socket.

The hip joint is supported by a tough fibrous joint capsule, which is made up of three main ligaments, the iliofemoral, the pubofemoral, and the ischiofemoral.

The main job of the joint capsule is to hold articulating bones together and make sure the joint stays stable when the hip is moving.

Now, the acetabulum itself is a combination of parts of three pelvic bones that join together - the ischium, the ileum, and the pubis.

At the bottom of the acetabulum known as the acetabular fossa, arises a ligament, called the ligamentum teres that attaches to the fovea capitis, which is a depression found on the tip of the femoral head.

This ligament helps with joint stability especially during hip flexion and abduction.

Now, the edge of the acetabulum has a thick bony circular rim covered by a ring of cartilage known as the acetabular labrum.

At its lower end, there’s a depression called the acetabular notch, which is covered by the transverse ligament which fills the gap within the circumference of the acetabulum.

Now, the normal development of a hip joint requires that the femoral head stays fitted within the acetabulum so that they both grow together keeping their sizes and shapes proportional.

In congenital hip dysplasia, the femoral head dislocates out of its acetabulum during development, and as a result, the ball and the socket grow out of proportion to one another, so that they’re unable to form a normal stable joint.

The cause of the dislocation isn’t always known. But one situation that can give rise to the problem is when too much mechanical force is applied against a fetal thigh, it can cause the femoral head to slip out of the acetabulum.

This can happen when a baby is lying in a breech position within the mother’s uterus, resulting in the baby’s buttocks being near the cervix at the time of delivery.

This sort of mechanical force is most likely to occur with firstborns, because the mother’s uterus is not as stretched out, putting a lot of pressure on a baby’s thighs.

This can also happen when there is not enough amniotic fluid to expand the uterine cavity so that the fetus can have enough room for its legs.

When the femoral head spends a lot of time outside of the acetabulum, the acetabular structures specifically the labrum, ligamentum teres, and the transverse ligament start hypertrophying within the acetabular fossa and occupy the space for the femoral head.