The statement that mobility is in creased in fracture and decreased in dis locations is misleading and inaccurate. In fractures mobility is not increased, but created where before it was not. In dislocations it is decreased in some direc tions, but it is not infrequently increased in others; and, indeed, with sufficient laceration of all the soft parts it may be increased in all directions. A disloca tion may be readily differentiated from a contusion or sprain by examination under ether.
Etiology. — PREDISPOSING CAUSES. Normal predisposing causes exist to a greater or less degree in all joints; other wise no joint not diseased could be dis located: a theory long since rejected. These causes mainly exist in the con formation of the bony surfaces which make up or surround the joint. In some positions there is little resistance to a dislocating force properly applied. Thus, the wide-open jaw may be dislocated for ward by a relatively-small force, there being but the slightest resistance of bone and ligament to overcome. Or, again, the normal angle at the joint, as at the elbow, predisposes to dislocation by ap plying a transmitted force (from the hand) in a direction oblique (upward and inward) to the long axis of the joint, and thus tends to force the articular surfaces over each other in an abnormal direc tion. "Moreover, certain outlying promi nences may aid dislocation by acting as fulcra to pry out the head of the bone, as does the olecranon in hyperextension of the elbow and the acromion in hyper extension of the shoulder. Some joints are also more frequently exposed to ex ternal violence than others. Patholog ical predisposing causes are fracture or disease of the bones, disease of the liga ments or atrophy of muscles that act as ligaments, and distension of the joint with fluid.
—External violence may cause disloca tion directly by acting upon the articula tion itself, as a dislocation of the humerus by a blow upon the shoulder, or indi rectly by force transmitted through the shaft of the bone, as in the same dislo cation caused by a fall upon the out stretched hand, or more complexly by leverage, as when a fall upon the shoulder dislocated the inner end of the clavicle upward by leverage exerted on the first rib as a fulcrum.
Muscular action may also be exerted either directly or indirectly. Thus, yawning is a common cause for disloca tion of the lower jaw. In fact, certain persons can voluntarily dislocate one or other of their joints. The most common example is the backward subluxation of the first phalanx of the thumb; but there are also a few subjects who can throw out their larger joints, as, for example, a man who is at present traveling about exhib iting his power of dislocating both hips and both shoulders.
Pathology of Recent Dislocations.— In joints relaxed by paralysis or effusion (and in the jaw) dislocation habitually takes place without laceration of the cap sule. In all other cases (excepting the voluntary dislocations before mentioned) the capsule is torn. In enarthrodial joints the rent is on the side toward which the round head of the distal bone is displaced. In other joints any or all of the ligaments may be torn. The firmer bands, instead of giving way themselves, may strip up the periostemn or tear away the bony prominences to which they are attached. Opposing muscles put upon the stretch may act in the same way. The bones may also be broken by impact on each other; thus fracture of the olec ranon occurs in anterior dislocation of the elbow, and a mutual bruising of the head of the humerus and shattering of the rim of the glenoid cavity in disloca tions of the shoulder.
Complications. — Fractures worthy of the name of complications may occur. Some, indeed, such as fracture of the anatomical neck of the humerus, may prove insurmountable obstacles to reduc tion. External wounds, especially if they compound the dislocation, may prove serious complications. Adjoining vessels may be ruptured and give rise to fatal hmorrhage or to occlusion and gan grene, or to traumatic aneurisms. The rupture of nerves, of which the most common is circumflex at the shoulder, may cause permanent paralysis and an sthesia. The viscera are rarely injured unless by some other associated trauma.
In old unreduced dislocations the lac erated connective tissue about the head of the bone becomes thickened and forms a pseudocapsule, while the periosteum on which the head of the bone now rests is stimulated and throws out a ridge of bone so as to form a new articular cavity, sometimes lined with fibrocartilage. The muscles and ligaments shrink or elongate to adapt themselves to their changed cir cumstances, and thus a comparatively useful new joint may be formed. In the meanwhile an opposite train of events takes place in the old joint-cavity. It is obliterated either by adhesion of the cap sule or by filling up with granulation tissue. Thus not only is the dislocated bone fixed in its new position, but also the old socket is obliterated and rendered unfit for its reception. It is important to note that the scar may include neigh boring vessels or nerves and by pressing on them give rise to neuralgia or cedema without any direct pressure by the bone itself, and, moreover, the tearing of this tissue in attempts at reduction may re sult in fatal injuries to vessel or nerve.