Dislocation into the obturator foramen (thyroid) is produced where the head maintains its primary position, or moves slightly forwards. If, on the other hand, extension and external rotation are present, the head may slip forward, and lie on the pubic ramus in front of the ilio-pectineal eminence, producing a dislocation on to the pubis. In these forms the limb is flexed, abducted, and everted. In the obturator form the position is maintained by the ilio-psoas and the* Y-shaped ligament ; in the pubic form the abduction and eversión is chiefly due to the position of the head of the bone, the Y-shaped ligament remaining intact, while the flexion is due chiefly to tension of the ilio-psoas. In these forms the adductors, gracilis, and pectineus muscles are frequently lacerated, while the ilio-psoas, pyriformis, and glutei muscles are stretched. The obturator nerve may be stretched or torn, and in the pubic form the anterior crural nerve may suffer. In the pubic dislocation there is slight shortening of the limb, while in the obturator there is apparent lengthening due to tilting of the pelvis down on the injured side.
In reducing these dislocations, the limb is (1) flexed fully upon the abdomen so as to relax the Y-shaped ligament, and, further, to disengage the head of the bone. This flexion is combined with marked adduction in the first two forms, and with marked abduction in the two latter. In other words, one first increases the deformity in all cases. (2) In order to bring the head of the bone back to the position of the rent in the capsule, the limb is circumducted out in the first two forms, and circumducted in in the two latter. (3) To make the limb re-enter the acetabulum, the limb is extended in all cases. Much depends in all cases in getting the muscles relaxed, and in making the flexion of the thigh on the pelvis as full as possible. In addition to the action described above, this movement hitches the upper end of the femur against the pelvis, and so lifts the head of the femur on to a level with the acetabulum, which, it will be remarked, presents a raised margin externally as well as internally. It is sometimes useful to remember that the internal condyle faces nearly in the same direction as the head of the bone.
Congenital dislocation of the hip is fairly common, especially in females. While the dislocation is generally of the dorsal type, it differs from a traumatic dislocation in certain important particulars : The acetabulum is rudimentary, its surface being covered by fibrous tissue instead of articular cartilage ; the ligamentum teres may be absent, or, when present, is wider and longer than normal ; the capsule is elongated, and its upper part, which bears the weight of the limb, is thickened, resembling fibro-cartilage. The head of the femur is smaller than normal and rather pointed, and the neck is short and directed straight forwards, instead of forwards and inwards as it is normally. The head generally lies to the outer side of the anterior inferior spine of the ilium, while the great trochanter is directed backwards, but, in children at least,the head possesses a considerable range of movement in an up-and-down direction, producing a characteristic gait. The limb is generally quite straight and shortened ; the great trochanter is situated posterior to instead of in front of the head, and above Nelaton's line, forming a prominence in the gluteal region. As the trunk has a tendency to fall forward, owing to gravity now falling in front of the axis of the bone, the shoulders are thrown back, there is marked lordosis, and the hips are prominent.