IRREDUCIBLE POSTERIOR DISLOCATION OF THE SHOULDER

Jeffrey C. Allard M.D.

Josiah Bancroft M.D.

From Department Of Radiology, Musculoskeletal Section,

University of Miami School of Medicine and Jackson

Memorial Hospital, P.O. Box 016960 ® 109), Miami,

Florida 33101, 305-549-7844.

Correspondence to JCA.

ABSTRACT

Failure to reduce an acute posterior dislocation of the shoulder is rare, and is usually due to the interposition of a structure into the joint. In this paper we report the MR and CT findings of a failed reduction due to interposition of a dislocated biceps tendon between the humeral head and anterior glenoid fossa. This was associated with an avulsed subscapularis tendon with its attachment to the lesser tuberosity and a nondisplaced fracture of the humeral neck, findings which were only evident on MR.

Index Terms: Magnetic resonance/ computed tomography/shoulder, dislocation/shoulder, rotator cuff tear/shoulder, fracture

Irreducible shoulder dislocations are uncommon. They have been most frequently reported with anteriordislocations. Interposed fracture fragments or soft tissue structures have been described, usually in association with tears of the rotator cuff. Especially important is the role of the long biceps tendon and its subluxation in many of these cases. We present an unusual case of irreducible posterior subluxation which was preoperatively evaluated with CT and MR imaging.

CASE REPORT

A 33 year old man with seizure disorder presented with complete posterior shoulder dislocations bilaterally after a grand mal episode. The right shoulder was easily reduced. The left shoulder was manipulated with partial reduction and post-manipulation radiographs demonstrated a small bony fragment anterior to the joint space, the origin of which was unclear.CT verified these findings but neither clarified the origin of the fragment, nor showed interposition of the fragment between glenoid and humeral head (Fig. 1a). MR did show that the subscapularis muscle was retracted with its tendinous termination in relation to the bony fragment, indicating that a lesser tuberosity avulsion existed (Figs 1b, 1c). In addition the biceps tendon had subluxed anteriorly out of its groove and was interposed between the humeral head and glenoid, probably preventing relocation. Supraspinatus muscle tear and fractured humeral neck were associated findings (Figs 1d, 1e).

Open reduction by a deltopectoral approach was then carried out. Division of the biceps tendon and resuturing was required. The subscapularis tendon remained attached to the avulsed lesser tuberosity and this was secured with suture wires. Postoperative-results were good with return of mobility over a 2-month period of time.

DISCUSSION

Acute dislocations of the shoulder are almost universally treated by closed reduction. Irreducible acute shoulder dislocations are rare, unless there is associated fracture (1). All reported cases to date have been irreducible anterior dislocations (2-5). When roentgenograms reveal no fracture of the head of the humerus or glenoid cavity, it must be assumed that soft tissue lying between the humeral head and glenoid fossa is blocking the reduction (1). The three soft tissue lesions that prevent reduction include the following:

(a) the torn rotator cuff in front of the glenoid fossa;

(b) the avulsed inferior portion of the capsule drawn into the joint between the humeral head and glenoid fossa;

In the reported cases of irreducible anterior dislocations, the tendon of the long head of the biceps was the most common factor preventing reduction (2-6). It may be dislocated or not in these cases, but often requires surgical division for relocation of the shoulder (6). In order for the biceps tendon to be dislocated posteriorly, either a displaced fracture of the greater tuberosity or a massive rotator cuff tear is necessary (1_5). Anterior and medial subluxation of the biceps tendon from its groove is common, but not a documented cause of irreducible shoulder (7,8). This condition has been noted in 6.5% of cadavers, indicating that it is related to a degenerative more often than an acute traumatic etiology. The subscapularis tendon may be intact in many of these cases, although its rupture is a prerequisite for entry of the biceps tendon into the joint space (8).

The subscapularis tendon in some manner is always involved in posterior dislocations; it may be stretched across the anterior glenoid or it may tear or be avulsed from the lesser tuberosity (1). Fracture of the lesser tuberosity is present in 25% of posterior dislocations (9). This has not been noted to prevent relocation of the shoulder previously.

The use of CT and MR in the preoperative evaluation of an irreducible shoulder dislocation can aid the surgeon in his operative approach. In our case CT showed the bony fragment but failed to show the magnitude of the associated rotator cuff tear seen with MR. The CT also failed to demonstrate the humeral neck fracture seen with MR. Humeral fractures can result from both the acute dislocating trauma or attempts at closed reduction, especially when excessive force is used (6).

The comparison of CT and MR images in our case suggest that, when a dislocated shoulder can not be reduced, MR offers the most effective method to evaluate the presence of a mechanical explanation for the persistent dislocation. As tendon and bone both are low signal on MR, correlation with CT can aid in differentiating these structures.

REFERENCES

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9. Cisternino SJ, Rogers LF, Stuffleban BC et al. The trough line: a radiographic sign in posterior dislocation. AJR 1978;130:951-5.