Posterior Traumatic Dislocation of the Shoulder

¹ GREENFIELD, MASSACHUSETTS

Posterior dislocation of the shoulder is a rare condition compared with anterior dislocation and is often missed on first examination. Diagnosis is made difficult by the fact that clinical signs are sometimes minimum or absent. Although it is a diagnostic challenge, it can frequently be recognized clinically if one keeps this condition in mind. The diagnosis is suggested by locking of the shoulder in adduction and internal rotation and is established by palpation of the head posteriorly. This is often easier with the arm in forward flexion. Many of the clinical signs suggested in the literature, such as flattening of the anterior shoulder contour, prominence of the coracoid or acromial process, and inability to rotate the humerus externally, do not identify a posterior dislocation, as they may be found in anterior dislocation as well.

The most important single reason for missing the diagnosis is that the routine anteroposterior roentgenograms often look normal. A clue sometimes found is the absence of the usual half-moon shadow of the humerus overlapping the glenoid fossa. When posterior dislocation is suspected clinically or when the antero-posterior roentgenogram lacks the normal semilunar shadow, an axillary roentgenogram (or an oblique roentgenogram if the arm cannot be abducted) is essential to clarify the diagnosis.

The typical pathological lesion may be detachment of the glenoid labrum, detachment or relaxation of the capsule forming a posterior recess, a grooved defect in the anterior part of the humeral head, or a combination of two or more of these defects.

Prompt recognition and reduction of acute dislocation are essential because prognosis deteriorates rapidly with delay. Closed reduction should be gentle, attempting to make the humeral head retrace the path of dislocation. Choice of surgical procedure for repair of recurrent or irreducible posterior dislocations should be determined by the specific lesion. The posterior approach by Kocher, with its modifications, affords a good exposure.

Periods of two and a half to three weeks of immobilization have proved adequate for uncomplicated cases. Since some lesions by their nature cannot heal spontaneously, consideration should be given to the need for early surgical repair rather than prolonged immobilization when instability persists after about five weeks.

Among the ten posterior dislocations presented, three were acute, six were recurrent (two bilateral), and one was permanent. One of the acute cases (Case 2) was an unusual posterior subglenoid dislocation. There were three cases of snapping shoulder which proved to be recurrent posterior dislocations (Cases 8, 9, and 10); three (Cases 5, 9, and 10) were due to congenital joint laxity.

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