EDITOR'S NOTE

The most common cause of shoulder pain is rotator cuff disease secondary to impingement syndrome. With its great sensitivity to soft-tissue abnormalities, MRI is ideally suited for the evaluation of impingement syndrome. It can detect the full gamut of rotator cuff disease easily­­from bursitis through tendonitis to frank tears of the rotator cuff tendons. This issue of Applied Imaging will discuss these findings, as well as the diagnosis of shoulder dislocation.­­William G. Bradley, Jr., MD, PhD, FACR

MRI of the Shoulder

The traditional imaging evaluation of the shoulder has been performed with plain films. While plain films can demonstrate fractures, advanced osteoarthritis, and radiodense or radiolucent bone-marrow abnormalities, they are insensitive to soft-tissue abnormalities involving the rotator cuff tendons, the biceps tendon, or the fibrocartilaginous glenoid labrum. For evaluation of these structures, magnetic resonance imaging (MRI) is required. ¹ This issue of Applied Imaging describes the major indications for MRI in the evaluation of the shoulder.

Rotator Cuff Disease

The rotator cuff is composed of the supraspinatus, infraspinatus, teres minor, and subscapularis tendons that attach on the humeral head. MR imaging of the shoulder for suspected rotator cuff disease is best performed in the coronal oblique plane (parallel to the supraspinatus muscle), in the sagittal oblique plane (perpendicular to the angled coronal), and in the axial plane. ¹ Typically, 3- to 4-mm thick proton-density and T ₂ -weighted (T2WI) conventional or fast spin echo images are acquired with an additional fat-saturated T2WI fast spin echo or short TI- inversion recovery (STIR) sequence (both of which null fat signal and bring out subtle fluid collections) in the angled coronal plane. ¹ Neither intravenous nor intra-articular contrast are generally used for shoulder pain without a history of dislocation.

The concept of rotator cuff impingement dates back to Neer, ² who first called attention to an abnormally shaped acromion as the initial cause of impingement (Figure 1). An abnormally hooked (Type III) or laterally down-sloping acromion impinges upon the supraspinatus tendon during shoulder elevation, leading initially to bursitis, then to tendonitis, and finally to frank tearing and disruption. The primary treatment for impingement syndrome, therefore, is an anterior acromioplasty. ³

The earliest MR finding in impingement syndrome is thickening of the fat pad of the subacromial-subdeltoid bursa ¹ (Figure 2). While the normal bursal fat pad is 1-mm thick, it becomes thicker with bursitis. On fat suppressed T2WI (either fat saturated T ₂ -weighted fast spin-echo at high field or STIR at any field strength), fluid may be seen in the bursa. With isolated bursitis, the supraspinatus tendon is of normal thickness and intensity.

The normal supraspinatus muscle is recognized as a horizontal, cigar-shaped structure on the coronal oblique view (Figure 3). The supraspinatus tendon normally appears as a dark structure (Figure 4) passing under the acromion, inserting on the greater tuberosity of the humerus. ⁴ The musculotendinous junction (Figure 3), i.e., the junction between the supraspinatus muscle and the darker tendon, should be at approximately 12 o'clock on the humeral head "clock" in the angled coronal plane. With the shoulder in a neutral position, i.e., neither internally nor externally rotated, the supraspinatus tendon curves anteriorly to attach on the greater tuberosity of the humerus; therefore, it is best visualized on more anterior sections than its muscle belly in the angled coronal plane. The actual attachment of the supraspinatus tendon on the humeral head is the least well vascularized portion of the tendon ("critical zone") and is generally the earliest site of tearing (Figure 3). ⁵

The diagnosis of tendonitis is made on the basis of tendon thickening and increased signal on T2WI (Figure 5). ^1,6 Increased signal without tendon thickening is indicative of myxoid degeneration or "tendinosis" (Figure 6). ⁷ Signal as bright as fluid on a T2WI is diagnostic of a tear (Figure 7). These can be partial thickness tears in either a superior-inferior (SI) direction or an anterior-posterior (AP) direction. Partial thickness tears in the SI direction are subcategorized according to involvement of the bursal surface or the articular surface. Partial thickness tears in the AP direction are categorized as to whether they are anterior or posterior. In general, partial thickness tears do not result in retraction of the supraspinatus tendon.

Complete tears of the supraspinatus tendon are recognized on the basis of fluid-intensity signal in the expected position of the normally dark tendon (Figures 3 and 7). Complete tears lead to retraction of the muscle belly and the medial displacement of the musculotendinous junction (Figure 3). The length of retraction should be the same length as the fluid collection along the former course of the supraspinatus tendon. MR is also excellent at diagnosing supraspinatus muscle atrophy (Figure 3), which, if sufficiently severe, may be a contraindication to rotator cuff repair.

Larger tears of the supraspinatus tendon are generally associated with tears (or at least tendonitis) of the infraspinatus tendon as well. Since both tendons angle anteriorly toward their attachments on the humeral head, the tendon identified on the most posterior coronal oblique sections through the supraspinatus muscle belly is actually the infraspinatus. Further posteriorly, the muscle fibers of the infraspinatus run obliquely rather than horizontally (like the supraspinatus). The teres minor has the same oblique course as the infraspinatus only more inferiorly, i.e., below the coracoid process.

The subscapularis tendon and muscle are best evaluated in the axial plane. The subscapularis tendon attaches near the bicipital groove. Fibers of the subscapularis tendon may blend with the fibers of the transverse ligament, which covers the bicipital groove. Thus, tears of the subscapularis tendon (Figure 8) are often associated with dislocation of the biceps tendon (Figure 9) medial to the bicipital groove. ⁸

Rotator cuff degeneration is often associated with bicipital tendonitis (Figure 10). This appears as increased size of the bicipital tendon with or without increased signal intensity. Fluid around the bicipital tendon without enlargement is inconsequential, i.e., it does not constitute tenosynovitis (as the bicipital tendon sheath communicates freely with the glenohumeral joint). Since the biceps tendon must pass through the rotator cuff "interval" to get to the superior glenoid labrum, fluid surrounding the tendon could potentially be mistaken as a tear of either the supraspinatus or the subscapularis tendons, particularly on sagittal views (Figure 11). ¹

Occasionally synovial cysts or masses in the suprascapular (spinoglenoid) notch can lead to pain, simulating a rotator cuff tear (Figure 12). The suprascapular nerve supplies sensory to the supraspinatus tendon and motor to the infraspinatus muscle. Therefore, compression of this nerve by a cyst or mass can lead to fatty atrophy of the infraspinatus muscle, as well as symptoms of a rotator cuff tear. ⁹

While most cases of supraspinatus impingement are thought to be due to an abnormal shape of the acromion or an under-surface spur, degenerative changes in the acromioclavicular joint can also cause impingement (best visualized in the oblique coronal plane). ²

Instability

The shoulder is a ball-and-socket joint; however, the bony glenoid "socket" per se is very shallow. The fibrocartilage of the glenoid labrum (Figure 8) increases its surface area by 60%, increasing the stability of the joint; however, it is still the joint most prone to dislocation. Shoulder dislocation can be classified as "anterior," "posterior," or "bidirectional," referring to the position of humeral head relative to the glenoid. Anterior shoulder dislocations are most common with the humeral head dislocating anterior and inferior to the coracoid process. With sufficient force, the bony labrum may be fractured (Bankart lesion) and the posterolateral superior aspect of the humeral head may be impacted (Hill-Sachs fracture) (Figure 2).

Frank fractures are generally evident on plain films, however, they can be equally well visualized by MRI. MRI also has the advantage that it can diagnose milder forms of trauma where the humeral head is bruised but its cortex is not frankly disrupted. Such "bone bruises" are well seen on MR techniques, which tend to highlight fluid while suppressing the normal fatty signal in the humeral epiphysis, e.g., fat-saturated T2WI fast spin echo techniques at high field ¹⁰ or STIR techniques at any field.

Milder trauma sufficient to cause shoulder dislocation without fracture of the bony glenoid may strip the cartilaginous glenoid labrum (Figure 13). On axial images, the low-intensity triangle of the glenoid labral fibrocartilage is absent or displaced. It should be noted that the middle glenohumeral ligament runs adjacent to the anterior glenoid labrum in a superior to inferior course and can easily be mistaken for a labral tear. ¹ When in doubt, injection of a dilute solution of gadolinium (1:100 in normal saline) into the shoulder joint under fluoroscopy is an extremely sensitive­­if somewhat more invasive­­ technique to demonstrate labral tears. ¹¹ With intra-articular gadolinium, imaging is generally performed with thin (i.e., 3 mm) T1WI in the axial, angled coronal, and angled sagittal planes (with fat saturation at high field).

Posterior shoulder dislocations are much less common than anterior dislocations and, generally, are seen in the setting of seizures. In this situation, the geometry is reversed: the posterior glenoid labrum is torn or fractured and the superior/anterior humeral head is bruised or fractured. These are called "reverse Bankart" and "reverse Hill-Sachs" lesions.

Conclusion

Over the past decade, MRI has evolved into the imaging modality of choice for the evaluation of the painful or unstable shoulder. MRI can demonstrate the full spectrum of rotator cuff disease easily, as well as evaluating the bony abnormalities leading to impingement syndrome in the first place. MRI with intra-articular gadolinium is useful for demonstrating tears of the glenoid labrum. Finally, as in other parts of the body, MRI is significantly more sensitive and specific for the detection of bony abnormalities (infarcts or metastatic disease) than plain films.

Clinical Quiz: True or False

1. MRI is the procedure of choice to evaluate any patient with shoulder pain.

2. MRI is the procedure of choice to diagnose rotator cuff disease.

3. MRI frequently affects management of rotator cuff tears.

4. MRI can diagnose glenoid labral tears.

5. CT is better than MRI to detect subtle fractures of the humeral head.

Answers

1. False. Certain patients (e.g., those with pacemakers or ferromagnetic intracranial aneurysm clips) can't have MRI. Also, for fractures, plain films are often sufficient­­and less expensive.

2. True. MRI can diagnose bursitis, tendonitis, and frank tears with disruption of one or more rotator cuff tendons.

3. True. The amount of retraction may determine whether a surgeon approaches a shoulder arthroscopically or through an arthrotomy. Also, patients with marked atrophy of the supraspinatus muscle may not be candidates for surgery.

4. True. Large tears can be diagnosed on a routine MRI study, however, smaller, nondisplaced tears require the injection of a dilute solution of gadolinium into the joint space for optimal visualization.

5. False. Even without frank cortical disruption, MRI can diagnose the bone marrow edema associated with trabecular fractures or bone bruises much more readily than CT.