The dislocation takes place in the early months of intra-uterine life, and may be associated with deficiency of the liquor amnii.
#Pathological Anatomy.#--_In the infant_, the anatomical changes in the joint are less marked than they are after the child has borne its weight on the limb. The acetabulum, never having been occupied by the head of the femur, is imperfectly developed; it remains flat and shallow, is partly filled with fibro-fatty tissue derived from the synovial membrane, and is always too small for the head of the femur. The cotyloid ligament being broader and thicker than usual, makes the osseous portion of the socket appear deeper than it really is. In unilateral cases the affected half of the pelvis is contracted, so that the pelvic basin is narrowed and oblique. The head of the femur is small, flattened, and, in some cases, conical; and the angle formed by the neck with the shaft is altered, sometimes diminished, it may be to a right angle--_coxa vara_ (Fig. 129); sometimes increased--_coxa valga_. There is also a variable degree of torsion of the neck, ante-torsion being of practical importance as it increases the difficulty of retaining the head in the socket. The capsule is lax and admits of the head passing upwards for a variable distance on to the dorsum ilii. In unilateral cases the ligamentum teres is elongated and thickened; in bilateral cases it is frequently absent.
[Illustration: FIG. 128.--Radiogram of Double Congenital Dislocation of Hip in a girl æt. 4.]
[Illustration: FIG. 129.--Innominate Bone and upper end of Femur from a case of Congenital Dislocation of Hip.]
In _children who have walked_, the head of the femur is pushed farther upwards on the dorsum ilii; the capsule becomes lengthened by supporting the weight of the body. That part of the capsule which arises from the lower margin of the acetabulum stretches across the socket and partly shuts it off from the rest of the joint cavity. In course of time the capsule becomes greatly thickened, and may present an hour-glass constriction about its middle, which may prove a serious obstacle to reduction. The socket becomes small and triangular, and there is almost no ledge against which the head of the femur can rest. A superficial depression may form on the ilium where it is pressed upon by the head of the femur, covered by the capsule; and in the course of years, as the head changes its position, several secondary sockets may be formed. No proper new bony socket forms like that in traumatic dislocations that remain unreduced because in the congenital variety the thickened capsule intervenes between the head of the bone and the dorsum ilii. The displacement of the head is most frequently backwards (dorsal luxation), and as the point of support thus falls behind the acetabulum the pelvis tilts forwards, and the lumbar spine becomes unduly concave (lordosis). The muscles of the hip and thigh alter in consequence of the changed relations; the gemelli, obturators, and piriformis are lengthened, the adductors, hamstrings, and ilio-psoas are shortened, while the glutei and quadriceps are but little altered. In rare cases the head is displaced upwards and lies immediately above the acetabulum.