Mechanism of axillary nerve palsy in the terrible triad of the shoulder: the role of excessive dislocation.

Background: The "terrible triad of the shoulder" (TTS) is a rare condition involving the simultaneous occurrence of shoulder dislocation, rotator cuff tear, and nerve palsy. The mechanism underlying its development remains unclear. This study aimed to examine the dislocation characteristics in patients with TTS. Methods: Eight patients with primary anterior shoulder dislocation, electromyography-confirmed axillary nerve palsy, and magnetic resonance imaging-verified rotator cuff tears were classified as the TTS group. Another 8 patients with primary anterior shoulder dislocation but without clinical evidence of brachial plexus palsy served as the control group. On anteroposterior radiographs taken during dislocation, the distance between the inferior glenoid margin and the medial anatomical neck of the humerus was measured as the dislocation distance. The superolateral tip of the greater tuberosity was assessed as being medial or inferior to the glenoid. The presence of Hill-Sachs lesions was evaluated on postreduction computed tomography scans. Results: The dislocation distance was significantly greater in the TTS group than in controls (49.3 ± 4.4 mm vs. 33.3 ± 3.6 mm, Conclusion: Excessive displacement in anterior shoulder dislocation with rotator cuff tear may be associated with axillary nerve traction. Imaging performed at the time of dislocation may help predict the occurrence of TTS.