DISLOCATION. This term is applied in surgery to the displacement from each other of the cartilaginous or articular surfaces of the bones entering into the formation of a joint. In a normal joint these surfaces are in contact and held together by ligaments and muscles; in a dislocated joint they are separated more or less completely—in the great majority of cases by external violence; in some instances, however, by powerful muscular exertion. The ease with which a joint is dislocated varies with the form and structure of the joint and with the position in which the joint is when the force is applied. The relative frequency of fracture and dislocation depends on the strength of the bones above and below the joint relatively to the strength of the joint. These points may be illustrated by examples from the joints of the arm and leg, because, with perhaps the exception of the joint between the lower jaw and the skull, it is in these situations that dislocation is most frequently observed. The strength of the different joints in the body is dependent on either ligament, muscle, or the shape of the bones. In the hip, for instance, all three sources of strength are present; therefore, considering the great leverage of the long thigh bone, the hip is rarely dislocated. The shoulder, in order to allow of extensive movement, has no osseous or ligamentous strength; its strength is muscular, therefore it is frequently dislocated, because the muscular strength varies in power, the muscles may be relaxed, the person is unprepared, and dislocation occurs; if, on the other hand, the muscles are tense, and the patient is prepared for the strain, then the result will be either a sprain of the joint or a fracture of one of the adjoining bones. The wrist and ankle are rarely dislocated; ...