DISLOCATIONS.
dislocation is a perma nent, abnormal, total, or partial displace ment from each other of the articular portions of the bones entering into the formation of a joint. .
A sprain is a temporary, partial dis placement, reduced immediately and spontaneously.
In total, or complete, dislocations the articular surfaces are completely sepa rated, or touch each other only at their edges. In ball-and-socket joints the dislocation is said to be complete when the centre of the globular head is dis placed beyond the rim of the concave socket.
Lesser forms of displacement are termed partial, or incomplete, luxations, or sublnxations.
A diastasis is a subluxation in which the separation occurs in a plane perpen dicular to that of the articular surfaces, without lateral gliding of one upon the other. The most frequent examples of this condition are the so-called "sub luxation" of the head of the radius in children, and the tibio-fibular diastasis in Pott's fracture.
A dislocation is complicated by in juries to surrounding tissues of sufficient importance to affect materially the symp toms, prognosis, diagnosis, or treatment. It is rendered compound if the laceration of the soft parts establishes a communi cation between the cavity of the joint and tbe outside air.
Symmetrical dislocations on both sides of the body (viz., both shoulders, both hips) are termed double. If they occur in the one bone (jaw, vertebrffl), they are called bilateral.
Varieties.—Dislocations are classified, according to their etiology, as traumatic and spontaneous. Traumatic disloca tions are caused, not only by external vio lence, but also by muscular force. Such, for example, as the forward dislocation of the mandible. Spontaneous disloca tions are due to pathological processes in or about the joint which so weaken its normal supporting structure that luxa tion occurs gradually (or suddenly) and without recognizable trauma. Occurring in extra-uterine life these dislocations are termed pathological, while, if their ori gin is prenatal, they are congenital.
Nomenclature. — 'Usually the distal member of a joint is said to be dislo cated,—the most notable exception is the so-called dislocation of the outer end of the clavicle (acromio-clavicular joint); and the direction of the dislocation is that taken by the dislocated bone: thus a back-ward dislocation of the humerus means that the head of the humerus has been dislocated backward from the gle noid cavity, and lies behind it (unless it has been shifted by a secondary or con secutive displacement). Sometimes, how
ever, we speak of a dislocation as of the joint itself, dislocation of the elbow, of the knee; here, again, the direction of the dislocation being that taken by the distal segment. Thus, instead of saying a "backward dislocation of the humerus," we might say "a backward dislocation of the shoulder." Subvarieties are named, according to the new anatomical posi tion of the distal segment, as subcoracoid, dislocation of the humerus, iliac (or dor sal) dislocation of the thigh.
Finally, it is well to bear in mind the distinction between "typical" and "atyp ical" dislocations, typical dislocations being, those in which the attitude of the limb is characteristic, and atypical those in which, owing to the laceration of some opposing structure, whose integrity is usually preserved, the characteristic po sition is not present. An "atypical" backward dislocation of the hips is the so-called "everted dorsal," in which, ow ing to the rupture of the outer branch of the Y-ligament, the thigh is everted instead of assuming the usual attitude of inversion and adduction.
Symptoms.—Deformity is always pres ent. The displacement of the articular surface changes the normal contour: a change which can be accurately verified by ascertaining by palpation the abnor mal position of the various bony promi nences; moreover, the new position of the head of the bone makes a new and abnormal centre for the movements of the joint, and, in connection with the restricting influence of untorn ligaments or bony prominences, gives rise to a more or less characteristic attitude and restric tion of motion in certain directions.