Dislocation (Lat. dis, apart, and locus, place), in surgery, that displacement in the osseous system which results from the direct application of force or other long continued cause. All the joints are liable to dislocation, but it most commonly occurs to those which possess the greatest mobility; hence the shoulder joint is the most frequent seat of this accident. The head of the humerus or bone of the upper arm, forming a ball-and-socket joint in connection with the scapula or shoulder blade, is regulated in its motions by very strong muscles, and is but slightly impeded in its free motions by the very shallow socket in which it rests. While this arrangement bestows great freedom of action upon this joint, it renders it liable to dislocation in almost every direction. The most common is that which occurs when the arm is elevated above the head, by means of which the head of the humerus is thrown into the armpit. Next in frequency is the dislocation of the hip joint, which is generally produced by a sudden blow upon the knee when the thigh is flexed toward the abdomen, whereby the head of the thigh bone is drawn backward by the action of the gluteal muscles upon the dorsum of the ileum or pelvis. The jaw bone is often thrown out of place in laughing, and much more frequently in yawning.

This accident sometimes occurs while speaking under undue excitement. It may be easily remedied by placing the thumbs on the back teeth so as to press them downward while the chin is raised by the fingers slowly upward. Care should be taken, however, to remove the thumbs quickly on the restoration of the joint, or they may be painfully compressed between the teeth. - The chief difficulty in restoring a dislocation consists in the opposition offered by the muscles, rendered acutely irritable by the unnatural position of the head of the luxated bone. In some instances this is overcome by reducing the heart's action by general bleeding. The warm bath and emetics are likewise used to relax the muscles, and with the same view tobacco moistened with water is sometimes laid upon the abdomen until it induces sickness and a disposition to syncope. But the safest and most efficient means of securing complete relaxation of the muscles is probably the etherization of the patient; and in this way a dislocation may sometimes be reduced with the exertion of a comparatively slight degree of force.

The surgeon in reducing a luxated joint endeavors, by a steady application of force exerted in the direction of the joint, either to fatigue the muscles, or seize some moment when they are relaxed to slip the bone into its socket. Various degrees of force and different appliances are used to effect this object. In the case of the shoulder joint the surgeon frequently forms a lever of the arm, with the heel of his boot placed in the armpit for a fulcrum, and by pressing the arm inward over this toward the body, overcomes the resistance of the muscles, and restores the joint. In the case of the hip joint, the force required is necessarily more considerable, and pulleys are often resorted to, by which means not only a greater but a steadier traction is exerted. A recent dislocation is much more easily reduced than one of long standing; indeed, no time should be permitted to elapse between the accident and an attempt at its reduction, for every hour adds to the uncertainty of success. - In geology, the term dislocation is applied to the change in the position of rocks caused by their being torn from their original place, either by upheaval or subsidence.