MR arthrography including abduction and external rotation images in the assessment of atraumatic multidirectional instability of the shoulder

Objective: To evaluate diagnostic signs and measurements in the assessment of capsular redundancy in atraumatic multidirectional instability (MDI) of the shoulder on MR arthrography (MR-A) including abduction/external rotation (ABER) images. Methods: Twenty-one MR-A including ABER position of 20 patients with clinically diagnosed MDI and 17 patients without instability were assessed by three radiologists. On ABER images, presence of a layer of contrast between the humeral head (HH) and the anteroinferior glenohumeral ligament (AIGHL) (crescent sign) and a triangular-shaped space between the HH, AIGHL and glenoid (triangle sign) were evaluated; centring of the HH was measured. Anterosuperior herniation of the rotator interval (RI) capsule and glenoid version were determined on standard imaging planes. Results: The crescent sign had a sensitivity of 57%/62%/ 48% (observers 1/2/3) and specificity of 100%/100%/94% in the diagnosis of MDI. The triangle sign had a sensitivity of 48%/57%/48% and specificity of 94%/94%/100%. The combination of both signs had a sensitivity of 86%/90%/ 81% and specificity of 94%/94%/94%. A positive triangle sign was significantly associated with decentring of the HH. Measurements of RI herniation, RI width and glenoid were not significantly different between both groups. Conclusions: Combined assessment of redundancy signs on ABER position MR-A allows for accurate differentiation between patients with atraumatic MDI and patients with clinically stable shoulders; measurements on standard imaging planes appear inappropriate. Key Points: • MR arthrography has the possibility to accurately identify patients with atraumatic MDI. • Imaging of the shoulder in abduction and external rotation provides additive information. • Capsular enlargement of the shoulder can be diagnosed on MR arthrography.