The superior part of the rotator cuff consists of the anterior (SSa) and posterior (SSp) parts of the supraspinatus tendon, the infraspinatus (IS) tendon plus the articular capsule. An overlap of the distal SSp tendon by the anterior part of the IS one has been anatomically demonstrated; the insertion area of the IS is more anterior than currently believed. The aim of our study was to assess this complex architecture through standard MRI scans.

Twenty-five healthy volunteers underwent a shoulder MRI. Three planes T2 fat saturation sequences were read in consensus by two radiologists. The SSa, the SSp, the IS tendons and the articular capsule were assessed for visibility. The patterns of demarcation of each structure from adjacent ones were assessed. The width and the thickness of each tendinous band were measured on sagittal images.

The SSa, the SSp and the IS tendons were distinguishable in all patients. The anterior part of the IS tendon overlapped the SSp tendon to reach a quite anterior insertion into the greater tuberosity of the humerus. The SSa, the SSp and the IS tendons were 6.5-3.4, 15.1-2.8 and 26.8-2.2 mm wide and thick, respectively.

MR images of the normal superior rotator cuff are consistent with latest anatomical descriptions. The distal superposition of the IS over the SSp tendon should be considered regarding the linear increased signal areas and the commonly named "partial thickness ruptures" of the superior rotator cuff as well as the fatty infiltration of the IS muscle.