Multidirectional instability of the shoulder: rotator interval dimension and capsular laxity evaluation using MR arthrography

To compare the rotator interval and capsular dimension as measured on MR arthrography between patients with clinically diagnosed multidirectional instability (MDI) and control subjects with no instability. We retrospectively reviewed a consecutive series of 658 shoulders that had undergone MR arthrography between 2006 and 2010. Of these, 97 shoulders were included in the present study. These shoulders were divided into two groups according to the clinically established diagnoses. The MDI group comprised 47 shoulders with atraumatic multidirectional shoulder instability, and the control group comprised 50 shoulders with no instability. Two independent observers measured the width and depth of the rotator interval, and the capsular dimensions at the anterior, anteroinferior, inferior, posteroinferior, and posterior regions in the two groups using MR arthrography. The rotator interval width and depth were significantly greater in the MDI group (width, observer 1, 17.7 mm, observer 2, 17.9 mm; depth, observer 1, 8.9 mm, observer 2, 8.8 mm) than in the control group (width, observer 1, 14.3 mm, observer 2, 14.5 mm; depth, observer 1, 5.9 mm, observer 2, 6.2 mm) (p < .001). The capsular dimensions at the inferior and posteroinferior regions were significantly larger in the MDI group (inferior, observer 1, 27.9 mm, observer 2, 27.8 mm; posteroinferior, observer 1, 27.0 mm, observer 2, 27.1 mm) than in the control group (inferior, observer 1, 25.7 mm, observer 2, 25.3 mm; posteroinferior, observer 1, 23.3 mm, observer 2, 23.6 mm) (p < .05). A width greater than 15.2 mm or a depth greater than 6.4 mm of the rotator interval, suggesting MDI, had sensitivities of 81 and 92 % for observer 1, and 79 and 94 % for observer 2, and specificities of 66 and 72 % for observer 1, and 62 and 66 % for observer 2, respectively. Measurements of the rotator interval and the size of the distended inferior and posteroinferior joint capsule on MR arthrography are greater in shoulders with clinical MDI than in stable shoulders.