CHAPTER XXXV.
DISLOCATIONS.
A sprain has already been described as a momentary change of emplacement or disturbance of the normal relations between joint surfaces, which, so far as displacement is concerned, is but a momentary affair and is promptly overcome. The term dislocation implies something more permanent as well as complete in both respects. It indicates an absolute and direct separation of articular surfaces of much more than momentary duration and requiring skilled assistance for its reduction. It pertains to articular surfaces which are enclosed within a capsule. The term luxation is synonymous with dislocation. When the condition is evidently partial or incomplete it is often referred to as subluxation. As compared with fracture dislocations are about one-tenth as frequent.
Dislocations are described as compound when through a co-existing wound air may enter the cavity of the joint, and as complicated when accompanied by other lacerations or injuries. When unaccompanied by these conditions they are described as simple.
To dislocations which result from external violence or from sudden muscular action is given the term traumatic. Pathological dislocations are those which are brought about by slow morbid processes, muscle spasm being the most prominent factor in their production. A third variety of dislocations, the so-called congenital, do not belong strictly in this class; by common consent the term is applied to congenital abnormalities where, from errors in development, normal emplacements and relations are altered.
The distal bone is the one described as that which is dislocated; thus we speak of dislocations of the forearm upon the arm, of the leg upon the thigh, etc.
Subluxations or incomplete dislocations are frequently accompanied by fracture of a bony prominence, e. g., the rim of the acetabulum, the coronoid process of the ulna, etc. The direction in which the distal member of the joint has been displaced is indicated by one of the common terms, as forward, inward. A consecutive or secondary dislocation implies a shifting of position from that at first occupied by the displaced bone end. These injuries may occur at any age, although usually during the more active period of life, from childhood to middle age, when mankind are more subject to injuries.
Certain conditions predispose to dislocations. Abnormalities or previous injury or disease of joint structures figure especially in this respect. A joint already relaxed by hydrarthrosis will exercise a relatively small restraining influence and a subluxation, at least, may easily occur.
The immediate cause is violence, either from without or within, generally the former. This may be direct, as from a blow, or transmitted, as when the shoulder is displaced by a fall upon the open hand. It occasionally happens that the component bones of a dislocated joint were in a position of extreme flexion or extension at the time of injury. The factors of leverage and spiral tension or wrenching are also important ones. Luxation from muscular activity is occasionally met with; most frequently when the lower jaw is dislocated by the act of yawning or violent laughter. The shoulder has been displaced in a violent effort at throwing or pitching a ball, or in wild gesticulation.
A few individuals have been in the habit of exhibiting themselves whose normal ligament and joint arrangements are so lax that they can voluntarily displace one or more of them and as easily replace them. These may be spoken of as instances of voluntary dislocation.
A joint once displaced may never fully recover its normal degree of tension, and will yield more readily to subsequent similar injuries. In this way there may occur so-called recurrent or habitual dislocations. Expressions of this kind are seen most often in the lower jaw and in the patella.