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.
Dislocations, like fractures, may be _complicated_ by injuries to large blood vessels or nerve-trunks, by injuries to internal organs, or by a wound of the soft tissues which does not communicate with the joint. Further, a fracture may coexist with a dislocation--a most important complication.
_Clinical Features._--The most characteristic signs of dislocation are _preternatural rigidity_, or want of movement where movement should naturally take place; _mobility in abnormal directions_; and _deformity_, the part being "out of drawing" as compared with the uninjured side (Fig. 18). The bony landmarks lose their normal relationship to one another; and the deformity is characteristic, and is common to all examples of the same dislocation.