ABSTRACT
Bone, cartilage, muscle, and connective tissue all arise from the
primitive mesoderm. All the elements of the hip joint differentiate
from a single mass of mesoderm, and in 50 to 60 postovulation days,
it resembles its final form. The joint cavity appears after 45 to 50
days in the central portion of the interzone and spreads peripherally
around the head of the femur, which is initially crossed by cellular
strands. The concavity of the acetabulum appears at an early age
and the congruence of the hip is established as soon as the joint
is formed. Hip formation begins in the seventh week of gestation.
It develops as a cartilaginous analogue at 4 to 6 weeks of birth. At
around 7 weeks, a cleft develops in the precartilagenous cells, which
thereafter develops into the femoral head and the acetabulum, and
this development is complete by 11 weeks to form the hip joint. The
femoral head is completely enclosed by the acetabular cartilage, and
in the later stages of gestation, the femoral head grows more rapidly
than the acetabular cartilage, so much so that at birth, the femoral
head is less than 50% covered. The acetabulum is most shallow and
lax, thereby facilitating the delivery process, and the hip is uncon-
tained in extension and adduction, reflecting on the hip shallowness.
Several weeks after birth, the acetabular cartilage grows faster than
the femoral head, which allows more coverage. If the head of the
femur is not positioned properly in the acetabulum, or if movement
of the femoral head is reduced, normal bonemodelling and a shallow
hip socket may develop. By 11 weeks of gestation, hip formation
is complete. The pelvis and femoral head are composed primarily
of cartilage rather than bone. Hence the normal factors such as
shallowness and laxity are the main initial factors for the develop-
ment of developmental dysplasia of the hip (DDH). The first critical
period for hip dislocation occurs at 12 weeks’ gestation when the
foetal lower limbs rotate nearly 90◦ medially, so that the knees point anteriorly and the hips assume their normal position in the pelvis.
