Dr. Ly is a Resident at the Department of Radiology at Wilford Hall Medical Center in San Antonio, TX and Dr. Beall is an Assistant Professor at the Department of Radiology at the Uniformed Services University in Bethesda, MD.

Rotator-cuff disease is a common cause of shoulder pain in both young adults (athletes in particular) and the elderly. The cause can be acute or repetitive chronic trauma to the rotator-cuff tendons; in the older age group, degenerative changes are common and can contribute to rotator-cuff injury. Conventional magnetic resonance (MR) imaging and MR arthrography have become the standard for the noninvasive imaging assessment of shoulder pain. A high degree of sensitivity and specificity in the detection of rotator-cuff tears is achievable with conventional MR imaging. Moreover, MR arthrography can demonstrate an even higher degree of accuracy in the assessment of the rotator injuries and is helpful in the detection of partial-thickness tears, which become more conspicuous following intra-articular contrast injection. If rotator-cuff disease is not detected and treated early, significant loss of shoulder function and range of motion can occur. Therefore, recognizing cuff injury as well as its specific site of occurrence, size, and extent, are a few of the important factors for determining the best outcome for the patient. We present a review of the MR findings of rotator-cuff pathology through a series of images and illustrations.

Normal anatomy

The rotator cuff is comprised of four tendons, easily recalled with the mnemonic "SITS," which stands for supraspinatus, infraspinatus, teres minor, subscapularis. These four tendons interdigitate distally to form a tendinous hood at their insertions onto the tuberosities of the humerus. The supraspinatus tendon lies between the acromial undersurface and the top of the humeral head. The critical zone of avascularity is located 1 to 2 cm proximal to the tendinous insertion at the superior impression of the humeral head and is the most common site for tears. The supraspinatus, infraspinatus, and teres minor tendons attach to the greater tubercle from superiorly to inferiorly, respectively. The subscapularis inserts onto the lesser tubercle. The coracoacromial arch is bordered posteriorly by the humeral head, superiorly by the acromion and coracoacromial ligament, and anteriorly by the coracoid process and intervening coracoacromial ligament. Located within the coracoacromial arch are the subacromial/subdeltoid bursa, supraspinatus muscle and tendon, and the long head of the biceps tendon.

The muscles of the rotator cuff each have well-defined functions. The supraspinatus abducts the humerus. The infraspinatus and teres minor collaborate to externally rotate the humerus. The function of the subscapularis is to internally rotate the humerus. Figure 1 demonstrates the important anatomic relationships of the rotator-cuff tendons and muscles.

Technical factors

Surface coils are necessary for high-resolution images. There are two basic types of shoulder surface coils: 1) coils that conform to the shoulder; and 2) coils that are mounted on a platform. A combination of coils may also be used. For example, a flexible coil that conforms to the patient's shoulder may be used when the patient is in the abducted and externally rotated (ABER) position, and a platform coil may be used to perform the remainder of the exam. One of the main disadvantages of the flexible or curved coil is its susceptibility to artifact secondary to patient motion. The platform coil, on the other hand, provides a high signal-to-noise ratio but does not allow imaging in the ABER position. The imaging protocol involves scanning a supine patient with mild external rotation of the humerus or with the patient's thumbs pointing toward the ceiling. Imaging is obtained with thin 3- to 4-mm slices in three planes: coronal oblique, sagittal oblique, and axial. The actual imaging sequences can vary from institution to institution, but the majority of shoulder MR imaging usually invloves at least a fat-saturated fast-spin-echo (FSE) sequence. The coronal oblique plane of imaging should be perpendicular to the glenoid fossa or parallel to the supraspinatus tendon. The sagittal oblique images should be parallel to the glenoid fossa, from the scapular neck through the lateral aspect of the rotator cuff. The axial images are performed from just above the acromioclavicular joint through the inferior axillary recess.

Impingement syndrome

In 95% of cases of rotator-cuff pathology, injury is preceded by subacromial impingement syndrome, which is a purely clinical diagnosis defined as a painful compression, in an awake patient, of the supraspinatus tendon, subacromial/subdeltoid bursa, and long head of the biceps tendon between the coracoacromial arch and the humeral head. Pain is usually progressive and aggravated by raising the arm in a position of abduction and external or internal rotation. ¹ Impingement syndrome commonly occurs in patients under the age of 40 and is usually secondary to repetitive elevation and abduction of the arm during occupational or athletic activities; it also commonly occurs in the elderly as a result of subacromial osteophyte formation and other degenerative changes around the acromial-clavicular joint and coracoacromial arch. Predisposing factors include certain acromial shapes, slopes, and positions (Figures 2A through 2E, Table 1); presence of an os acromiale (Figure 2F); acromioclavicular joint degenerative and hypertrophic changes (Figure 2G); chronic glenohumeral joint instability resulting in degenerative changes (Figure 3A); a thickened coracoacromial ligament (Figure 3B); supraspinatus muscle hypertrophy (Figure 3C); posttraumatic deformities; calcific tendonitis (Figure 3D); a prominent greater tuberosity; and posterior superior glenoid impingement. The supraspinatus tendon is by far the most frequently injured rotator-cuff tendon, largely because of its more superior location relative to the other rotator-cuff tendons, resulting in its closer proximity to the acromion and distal clavicle, two common offending agents. The severity of impingement is directly related to rotator-cuff degeneration or tears. The diagnosis of "impingement syndrome" cannot be made with MR imaging, but is inferred from narrowing of the acromiohumeral distance or stenosis of the supraspinatous outlet.

MR imaging characteristics of rotator-cuff pathology

MR imaging, with its excellent soft-tissue contrast and multiplanar capability, is the imaging study of choice for the evaluation of the painful shoulder. Normally, the uninjured rotator-cuff tendon shows uniform low signal intensity. Any deviation from this MR appearance is suspicious for pathology. There are four major types of tendon pathology: 1) tendonapathy or tendinosis; 2) surface degeneration or fraying; 3) partial-thickness tear; and 4) full-thickness tear. Early collagen fiber breakdown within a tendon is termed tendonapathy or tendinosis and is an early sign of impingement. Use of the term tendonitis should be avoided, as there is no histologic evidence of significant inflammation. MR imaging characteristics of tendonapathy include intermediate signal intensity (higher signal than normal tendon, but lower than joint fluid) on T1-weighted (T1W) imaging, proton-density­weighted (PDW) imaging, and T2-weighted (T2W) imaging, without focal tendon disruption (Figures 4A and B). The signal intensity on T2W images is equal to or higher than that on T1W images, but should be less than joint fluid signal. Tendon enlargement is often an associated finding in tendonapathy. ² Histological examination of tendinopathy shows mucoid and fibrillary degeneration, with evidence of scarring.

Tendon surface degeneration or fraying is an irregularity of the tendon surface that is more common on the bursal side (Figure 4C) and is often associated with increased signal in the subacromial/ subdeltoid bursa. It is important, especially in athletes, to be able to distinguish these injuries from partial tears, as the latter can be surgically repaired for restoration of function. However, making this distinction can often be difficult on conventional MR imaging due to the magic-angle effect, focal fat, muscle fibers, or subclinical degeneration. ^3,4
The addition of frequency selective fat-suppression technique improves the detection of rotator-cuff tears. The use of gadolinium-DPTA in MR arthrography to distend the joint is useful in revealing small undersurface or articular surface abnormalities of the supraspinatus tendon. ⁵ To further improve detection of undersurface abnormalities, the ABER position can be used to increase the separation of the supraspinatus tendon from the humeral head and encourage spreading of any torn and frayed edges, thereby increasing the visibility of the tear. ⁶

The characteristics of partial-thickness tears on MR imaging are focally increased signal on T1W and T2W images (signal intensity similar to that of joint fluid on T2W images), with involvement of either the articular (Figure 5A) or bursal tendon surface or intrasubstance of the supraspinatus tendon (Figure 5B). ⁷ Partial-thickness tears can be graded according to the depth of the tear.

On MR imaging, a full-thickness tear of a tendon has the appearance of high signal involvement of the tendon, usually extending vertically or obliquely from the articular to the bursal surface (Figure 6A). Conventional MR imaging has a sensitivity of 92% to 96% for the detection of full-thickness tears. ⁸ A complete tear or rupture is complete disruption of the entire tendon, leaving a fluid-filled gap separating the torn tendinous ends, usually with medial retraction of the torn tendon as a secondary sign (Figure 6B). The Patte classification is frequently used to describe and communicate the degree of tendon retraction in complete tears (Figure 7, Table 2). ⁹ Over time, atrophy and fatty infiltration of the associated muscle develop and can be a useful secondary sign of chronic rotator-cuff tears (Figure 8). Subacromial/subdeltoid bursal fluid is a sensitive but relatively nonspecific finding in rotator-cuff tears. ¹⁰ Unlike injuries of the supraspinatus tendon, subscapularis tendon tears (Figure 9) are relatively uncommon and typically present in elderly patients with recurrent anterior shoulder dislocation. They often occur in conjunction with other rotator-cuff injuries. Isolated subscapularis tendon tears are less common but can result from trauma, eccentric overload, or rotator-cuff degenerative disease. Primary signs of a tear may be limited to the upper third of the tendon. ¹¹ Other reported causes of subscapularis injury include long head of the biceps tendon dislocation, massive rotator-cuff tears, and subcoracoid impingement (congenital or posttraumatic/postsurgical). Tears of the infraspinatus tendon (Figure 10) are relatively common; only tears of the supraspinatus tendon are more common. The pathogenesis and mechanism of tendon injury is similar to that of the supraspinatus tendon, and concomitant tears of these two tendons form the most common combination when tears of more than one tendon are present. Tears of the teres minor tendon are rare, particularly in isolation.

It is important to be cognizant of an artificial high signal appearance within the supraspinatus tendon known as the magic-angle phenomenon, which usually occurs at the critical zone of the supraspinatus tendon and can mimic tendon pathology. A useful discriminator between the magic-angle phenomenon and a true tendon tear is the appearance of high signal intensity on T2W imaging, indicative of a true tear. In contrast, the increased signal of the magic-angle phenomenon seen on T1W images remains isointense to the adjacent muscle or does not increase in signal intensity on T2W images.

Asymptomatic shoulder MR imaging

Surprising findings have occurred in asymptomatic patients who undergo shoulder MR imaging. Rotator-cuff tears have been reported in 34% of asymptomatic patients. ¹² It has been noted that the frequency of tears increases with age, with partial-thickness tears being more common than full-thickness tears. It is important to be aware that MR imaging findings may not correlate with the patient's symptoms.

The long head of the biceps tendon

The long head of the biceps tendon stabilizes the anterosuperior portion of the rotator cuff by its position in the bicipital groove anteriorly and its attachment to the glenoid labrum superiorly. Tears of the subscapularis tendon in particular have been associated with medial subluxation or dislocation of the biceps tendon or tendonopathy. ¹³ The biceps tendon is also frequently injured (Figure 11) in association with the tears of the superior and anterior portions of the rotator cuff. When tears of the supraspinatus and subscapularis tendons are detected, specific attention should be directed to the condition of the long head of the biceps tendon. ¹⁴

Conclusion

Rotator-cuff injuries are a common cause of shoulder pain. Conventional MR imaging provides high accuracy in the detection of rotator-cuff tears. MR arthrography demonstrates even higher accuracy, but is invasive to some degree and is used more often when there is evidence of glenohumeral instability. This pictorial review should help to familiarize radiologists and clinicians with characteristic MR findings of rotator-cuff pathology.