CASE 114

By: Dr. Nizar Al-Nakshabandi MD, FRCPC

HISTORY: 25-year-old male presented with shoulder pain following electrocution.

What are your findings?

What is the differential diagnosis?

What are the causes?

FINDINGS:

AP radiograph [A] demonstrates an internally rotated humeral head giving a rounded appearance [the lightbulb sign]. In addition, there is evidence of loss of normal half-moon overlap sign. Maloney’s  arch is disrupted. The glenohumeral joint space is widened by more than 6 mm consistent with the rim sign and the compression defect in the anterior medial humeral head [the trough sign].

Trans-scapular view [B] demonstrate posterior shoulder dislocation.

Axial fat sat post intra-articular gadolinium administration demonstrate the trough sign with evidence of loose intra-articular bodies from the compression fracture. The shoulder is now congruent post reduction. No evidence of labral tear present. The bony glenoid is also intact with no evidence of a reverse Bankart lesion.

DIAGNOSIS:

Posterior shoulder dislocation with evidence of trough sign from impacted fracture of the anterior medial humeral head.

PEARLS AND DISCUSSION:

Posterior shoulder dislocation accounts for less than 4% of all shoulder dislocations.

The mechanism of action is by forced internal rotation while abducting the arm. Such mechanism occurs commonly with convulsions/ECT and electrocution. Hence, it may happen bilaterally.

Always look for associated fractures particularly the dense vertical line in the medial humeral head [trough sign] and the reverse Hill-Sachs lesion. Associated injuries such as reverse Bankart lesions, proximal humeral fractures, HAGL lesions, and ALPSA lesions can be seen on MR arthrogram. You may be asked how to perform an arthrogram so know the technique well.

Most posterior dislocation reduces spontaneously. Caution about closed reduction especially in the elderly and frail should be taken. Prolonged posterior dislocation can lead to glenohumeral labral capsule tear which ultimately leads to shoulder instability.

FURTHER READING:

1.      Robinson CM, Aderinto J. Posterior shoulder dislocations and fracture-dislocations. J Bone Joint Surg Am. 2005;87 (3): 639-50.

2.      Weissleder R, Wittenberg J, Harisinghani MG. Primer of diagnostic imaging. Mosby Inc. (2003) ISBN:0323023282.