Imaging description

As opposed to the relative ease of recognizing classic anterior subglenoid dislocation of the shoulder, the findings of posterior shoulder dislocation on the anterior-posterior (AP) view of the shoulder are subtle and require a high degree of suspicion to detect [1–3]. Nearly one-quarter of posterior shoulder dislocations are missed on initial radiographic assessment [4]. The normal anatomic appearance of the shoulder is reviewed in Figure 83.1. Signs to look for on the AP view include the “lightbulb” appearance of the humeral head, due to fixed internal rotation of the humerus (Figure 83.2), and a “vacant” glenoid fossa due to lateral displacement of the humeral head, creating the “rim sign” (Figure 83.3). The “rim sign” can be due to hemarthrosis or septic arthritis. There may be absence of normal half-moon overlap between the humeral head and glenoid. The “trough sign” is caused by impaction of the humeral head on the glenoid, and reflects the parallel lines created by the medial humeral head cortex and the reverse Hill–Sachs fracture fragment (Figure 83.4). More recently, the “Mouzopoulos” sign on AP radiographs has been described [2]. In posterior shoulder dislocation, projection of the greater and lesser tuberosities on the AP view of the internally rotated humeral head creates a capital “M” appearance (Figure 83.5). A false-positive Mouzopoulos sign may be seen when there is marked internal rotation of the humerus in the absence of dislocation [2].