Congenital dislocations of the hip are for the most part traumatic. Displacements occur also, however, through deficient development.
Any obstruction at birth, whether through malformation of the pelvis or caused by the position of the child—the breech for instance, presenting, or the child or its head being unusually large, may occasion this dislocation.
In a breech presentation the thighs are flexed upon the abdomen, so that the head of the femur presses on the posterior and inferior portion of the capsule of the joint. But in this position, the application of the slightest force in downward traction will displace the head of the bone and cause it to pass away from the acetabulum and to lie upon its brim. And lying on the border of the cotyloid cavity, extension of the limb at birth is alone sufficient to displace the head beyond the brim. The blunt hook or the finger
BRODHURST BE. OBSERVATIONS ON CONGENITAL DISLOCATION OF THE HIP.. JAMA. 1896;XXVI(3):121-123. doi:10.1001/jama.1896.02430550023002k