It is unnecessary for our present purpose to attempt a comprehensive classification of the numerous varieties of dislocation that have been met with at the hip-joint. It will suffice if we divide them into those in which the head of the femur passes backward, and comes to rest on the dorsum ilii, or in the vicinity of the great sciatic notch; and those in which it passes forward and comes to rest in the obturator foramen, or on the pubes (Fig. 71). [Illustration: FIG. 71.--Diagram of the most common Dislocations of the Hip.] The backward are much more common than the forward dislocations, in contrast to what obtains at the shoulder, where the forward varieties predominate. On account of the great strength of the hip-joint, dislocation is by no means a common injury. It occurs most frequently in strong adults after the epiphyses have ossified, and before the bones have commenced to become brittle; and it is much more common in men than in women. It is invariably the result of severe violence, the limb at the moment being in such a position that the ligaments are on the stretch and the muscles taken at a disadvantage. The head of the femur usually leaves the joint at the lower and back part, where the socket is most shallow and the ligaments weakest. The ligamentum teres is almost always torn from its femoral attachment, and one or more of the muscles inserted in the region of the trochanters may be ruptured. The [inverted Y]-shaped ligament, on the other hand, is seldom torn, and so long as it remains intact the dislocation belongs to one or other of the types above named. All atypical dislocations, such as the supra-cotyloid, infra-cotyloid, ilio-pectineal, are due to rupture of some part of the [inverted Y]-ligament, and are so rare as not to call for individual description. The central dislocation of German authors, in which the head is driven through the floor of the acetabulum, is described on page 126. Like other dislocations, those of the hip may be complicated by laceration of muscles, blood vessels, or nerves, or by fracture of one or other of the bones in the vicinity. #Dislocation on to the Dorsum Ilii.#--This, the commonest form of dislocation of the hip, is usually the result of the patient falling from a height, or receiving a heavy weight on the back while stooping forward with the thigh flexed, slightly adducted, and rotated medially. It is also said to have occurred from muscular action. The shaft of the femur acts as the long limb of a lever of which the neck is the short limb, the femoral attachment of the [inverted Y]-ligament forming the fulcrum. The head, thus brought to bear upon the lower and back part of the capsule, tears it and leaves the socket, passing upwards and coming to rest on the dorsum of the ilium, above and anterior to the tendon of the obturator internus (Fig. 73). The articular surface is directed backward, while the trochanter looks forward.