Tendinosis histopathologically refers to tendon degeneration with collagen fiber disorientation, increased intrasubstance deposition of mucoid, and absence of inflammatory cells (thus the term tendinitis is inappropriate). The typical MR appearance of tendinosis is abnormal signal intensity associated with morphology changes (Figs. 2 and 3). The signal changes of tendinopathy typically are of intermediate signal on both short and long TE imaging. In particular with tendinosis there is no fluid-like signal intensity on long TE images. This allows one to differentiate tendinosis from frank tendon tearing. Because of the pitfalls described above that can lead to intermediate signal on short TE images in normal tendons, we believe that one should be very hesitant to call tendinopathy in tendons with normal morphology. The associated morphology changes typically consist of abnormal thickening of the involved tendon. Classically, rotator cuff tears occur in the insertional fibers of the cuff tendons in regions of preexisting tendinopathy [3].

Tendon tears are classified into full thickness or partial thickness tears. Full thickness tears are subclassified as complete or incomplete depending on if all or only some of the tendons of a muscle are involved. Partial thickness tears are sub-divided by the location of the tear into articular-sided, intratendinous, and bursal-sided tears. Although rotator cuff tears typically appear as areas of fluid signal intensity on T2-weighted images, in about 10% of tears, the region of tendon discontinuity is low in signal on T2-weighted images, possibly because of chronic scarring and fibrosis. These tears may be visualized at MR arthrography because intraarticular contrast fills the tear. On conventional MR imaging, secondary signs of cuff tear, such as tendon retraction (measured in the medial-lateral dimension), may be the only indication of a full-thickness tear.

Partial-thickness rotator cuff tears can be described according to the surface of the tendon involved as well as the percentage of tendon involved (Drawing 2).

Partial-thickness articular surface tears are characterized by a focal region of fiber discontinuity that is filled with fluid-like signal intensity on T2-weighted imaging. Fat-suppressed T2-weighted imaging can increase lesion conspicuity by better demonstrating the high T2 signal tendon defect). Articular surface tears are the most common type and are easily diagnosed with standard MR imaging when a joint effusion is present (Fig. 4). In the absence of a joint effusion, articular surface partial-thickness tears may be difficult to identify, particularly in the setting of granulation tissue or scarring. Delamination of the involved portion of the tendon may occur, most often involving the articular surface of the supraspinatus tendon (Drawing 3). Delaminated tears can lead to cyst formation within the associated or adjacent muscle, the so called sentinel cyst. Those cysts usually are restricted to the muscle belly and typically do not cause symptoms, in contrast to the paralabral ganglion cysts that may cause nerve compression.

Intrasubstance or concealed tears are characterized by intratendinous T2 fluid-like signal without extension to either the bursal or articular surface (Fig. 5). These lesions will not fill with gadolinium on MR arthrography because of lack of communication between the tear and the articular surface of the tendon. Intrasubstance tears will not be seen by the arthroscopist as they do not communicate with the tendon surface.

Partial-thickness bursal surface tears demonstrate abnormal increased T2 signal along the superior (bursal) surface of the tendon (Fig. 6). The articular surface remains intact. When there is fluid in the subacromial bursa, the tears are well visualized, particularly on T2-weighted images. However, these tears may not be visible on T1-weighted MR arthrographic images as the intra-articular gadolinium will not enter the gap in the tendon because of intact articular surface fibers. In addition, the presence of bursal fluid may not be appreciated on T1-weighted imaging. For these reasons, it is crucial to include at least one T2-weighted sequence on all MR arthrographic exams to assess for fluid-filled bursal surface tears. The extent of the partial tear can further be declared according to the depth; a commonly used grading system is shown in (Table 1) [4, 5].

Tears of the supraspinatus tendon most commonly arise at the anterior aspect of the tendon immediately adjacent to its attachment onto the greater tuberosity (Fig. 7). Supraspinatus tears can extend posteriorly into the infraspinatus tendon or anteroinferiorly through the rotator interval to involve the medial aspect of the coracohumeral ligament and superior subscapularis tendon fibers, a situation that is associated with more severe supraspinatus atrophy and poor prognosis [6]. A full-thickness supraspinatus tear allows communication between the articular and the bursal compartments.

Infraspinatus tendon tears are often associated with supraspinatus tendon tears and may be observed in younger athletes with overhead activities and posterosuperior impingement, in which there is often an articular side delamination of the cuff. The so-called ABER view (abduction and external rotation arm position) in conventional MRI or MR arthrography has been proposed to increase the sensitivity for detection of these tears. For younger patients and patients suspected of having posterior superior impingement and partial-thickness undersurface tears, the ABER position is valuable in demonstrating lesions of posterior superior impingement and undersurface tears of the rotator cuff as well as non-displaced tears of the anterior inferior labrum in patients with glenohumeral instability [7, 8] (Drawing 4). Teres Minor tendon tears are rare and present most commonly as partial tears accompanied by infraspinatus tears.