TRAUMATIC DISLOCATIONS A dislocation or luxation is a persistent displacement of the opposing ends of the bones forming a joint. We are here concerned only with such dislocations as immediately follow upon injury. Those that are congenital or that result from disease will be studied later. _Causes._--The majority of dislocations are the result of _indirect_ violence, the more movable bone acting as a lever, on a fulcrum furnished by the natural check to movement in the form of ligament, bone, or muscle. It is in this way that most dislocations of the shoulder, hip, and elbow are produced. At the moment the violence is applied, the muscles are relaxed or otherwise taken at a disadvantage, so that the joint is for the time being deprived of their support. The joint is moved beyond its physiological range, and the end of one of the bones being brought to bear upon the capsule, tears it, and passes through the rent thus made. The muscles then contract reflexly, and pull the head of the bone into an unnatural position outside the capsule. The position assumed will depend upon such factors as the direction of the force, the structure of the joint, the position of the limb at the time of the accident, and the relative strength of the different groups of muscles acting upon the bone which is displaced. Violence applied _directly_ to the joint is a much less frequent cause of dislocation. In this way, however, the knee-joint may be dislocated, one bone being driven past the other--for example, by a kick from a horse; or the acromio-clavicular joint by a blow on the shoulder. _Muscular contraction_ is not often the sole cause of dislocation, although, as has been mentioned, it plays an important rôle in the production of the majority of these injuries. The shoulder, mandible, and patella are, however, not infrequently displaced by muscular action alone. Acrobats sometimes acquire the power of dislocating certain joints by voluntary contraction of their muscles. _Age and Sex._--Dislocations occur most frequently in adult males, doubtless on account of the nature of their occupations and recreations. In children the epiphyses are separated, and in old people the bones are broken by such forms of violence as cause dislocation in the middle-aged. Muscular debility and undue laxness of ligaments resulting from disease or previous dislocation are also predisposing factors. _Clinical Varieties._--The separation between the bones may be _complete_ or _partial_. When partial, portions of the articular surfaces remain in apposition, and the injury is known as a _sub-luxation_. Like fractures, dislocations may be _simple_ or _compound_, the latter being specially dangerous on account of the risk of infection. When seen within a few days of its occurrence, a dislocation is looked upon as _recent_; but when several weeks or months have elapsed, it is spoken of as an _old-standing_ dislocation. The latter will be described later.