Dr. Jacobson is a Clinical Associate Professor of Radiology and the Director of the Musculoskeletal Radiology Division at the University of Michigan, Ann Arbor, MI.

Evaluation of the shoulder continues to be the most common sonographic examination of the musculoskeletal system at most institutions. Several advantages of sonography over alternative imaging methods include availability, portability, and low cost. Most patients prefer sonography over magnetic resonance (MR) imaging, as it requires less time and it is more comfortable. This is especially true in the older patient, who is more likely to undergo evaluation for rotator cuff abnormality. Although learning this imaging method initially takes time, thorough knowledge of anatomy combined with standardized examination technique shortens the learning process. Once skilled in this technique, a complete shoulder sonographic examination can be easily completed in <5 minutes. Technologists can also be successfully trained in shoulder sonography. With the use of technologists acquiring images and documenting pathology, the sonographic images can be reviewed, similar to other sonographic studies, in a matter of minutes.

From the perspective of a radiologist who is trained in both sonography and MR imaging of the shoulder, shoulder sonography is often preferred primarily because of the improved resolution of sonography over MR imaging. For example, a tendon abnormality as small as several millimeters can be seen well, and the critical distinction between articular surface and isolated greater tuberosity extension can be made easily. This is important, as the tendon abnormality in the latter situation cannot be visualized at arthroscopy. From an orthopedic surgeon's perspective, accuracy of rotator cuff sonography can meet, and even exceed, that of MR imaging. ¹ It is important, however, that the strengths and limitations of shoulder sonography are understood. For example, sonographic evaluation of the rotator cuff is very effective. ² For evaluation of the glenoid labrum and ligamentous structures, MR arthrography is indicated. The proper utilization of sonography relies on patient selection. At our institution, a patient under the age of 35 years will typically have MR arthrography to evaluate cartilaginous structures, while a patient over the age of 40 years will more likely have sonography to evaluate the rotator cuff.

Technical considerations

Although most current ultrasound machines can perform musculoskeletal sonography, proper transducer selection is critical to success. For sonography of the shoulder, a linear transducer is preferred. More importantly, a frequency of >=10 MHz is recommended. There is a trade-off, as high frequency transducers offer the highest resolution, but at the expense of depth penetration. At our institution, a 12-MHz linear transducer with a 5-cm footprint is used for the rotator cuff. However, a 10-MHz or 7-MHz transducer may be used in patients with a large body habitus. All images used in this article were obtained on a dedicated musculoskeletal ultrasound unit (Model HDI 5000, Advanced Technology Laboratories, Bothell, WA).

To ensure complete evaluation of the shoulder, specific patient positions and transducer placements are used to optimize visualization of each structure. ³ To evaluate the supraspinatus, it is important that the patient internally rotates and hyperextends or adducts the arm so that the tendon is pulled down from beneath the acromion for visualization. ⁴ This is accomplished by asking the patient to place the dorsum of the hand in the small of the back, or alternatively the palm of the hand at the ipsilateral hip. This latter position allows visualization of the intra-articular portion of the long head of the biceps brachii tendon. This is an important landmark when assessing for and describing the location of supraspinatus tendon abnormalities. Identification of the biceps tendon also ensures that the full anterior extent of the supraspinatus tendon has been evaluated, as this area is commonly torn. The subscapularis is optimally visualized with the arm in external rotation to eliminate anisotropy. The infraspinatus and teres minor are identified from a posterior approach, while the long head of the biceps brachii tendon is identified in the bicipital groove from an anterior approach with the hand supinated in front of the patient. ³

Normal sonographic anatomy

Normal tendons appear hyperechoic with a fibrillar echotexture when imaged perpendicular to the ultrasound beam (Figure 1). ⁵ It is important to recognize that tendon fibers oblique to the ultrasound beam will appear artifactually hypoechoic, which is called anisotropy. ⁶ This should not be confused with a tendon tear, which may also be hypoechoic. Only that segment of tendon that is perpendicular to the ultrasound beam should be evaluated for tendon abnormality.

Normal muscle appears relatively hypoechoic at sonography, with interspersed hyperechoic fibroadipose septa (Figure 1). Bone cortex is highly reflective and will appear hyperechoic with posterior acoustic shadowing or reverberation artifact if a flat segment of cortex is imaged perpendicular to the ultrasound beam. Hyaline cartilage appears hypo-echoic, while fibrocartilage, such as the glenoid labrum, appears hyperechoic. The subacromial-subdeltoid bursa is located between the supraspinatus tendon and deltoid muscle and is outlined by hyperechoic peribursal fat and bursal walls. This bursa continues beyond the greater tuberosity, which allows differentiation from the adjacent rotator cuff inserting on the tuberosity.

Rotator cuff

The strength of shoulder sonography lies in its ability to accurately evaluate the rotator cuff. ^2,7 It has been shown that sonography can perform at least equal to MR imaging in this application. ¹ The following discussion will review the sonographic appearances of various rotator cuff pathologies, and will approach this from a differential diagnosis perspective. Abnormalities of the supraspinatus tendon are emphasized, as this tendon is most commonly involved.

Focal hypoechoic or
anechoic tendon abnormality

When a focal hypoechoic or anechoic abnormality of a tendon is identified, one must first differentiate tendinosis from a tendon tear. Tendinosis, or tendinopathy, is tendon degeneration, and this term is used instead of tendonitis, as active, inflammatory cells are absent. ⁸ In this differentiation, tendon tears are more likely well-defined (Figure 2), while tendinosis is more likely ill-defined (Figure 3). ⁹ In addition, tendon tears are more likely quite hypoechoic or even anechoic (Figure 2), while tendinosis is more likely minimally hypoechoic (Figure 3). Both processes, however, may coexist in the continuum of a diseased tendon.

Cortical irregularity

One indirect sign that is very helpful in differentiating tendon tear from tendinosis is cortical irregularity of the greater tuberosity at the supraspinatus tendon insertion (Figure 4). When a focal and well-defined hypoechoic or anechoic supraspinatus tendon abnormality is immediately adjacent to cortical ir-regularity of the greater tuberosity, this likely represents a tendon tear. ¹⁰ Greater tuberosity cortical irregularity is seen with full-thickness and partial-thickness supraspinatus tendon tears. One should note, however, that cortical irregularity of the greater tuberosity at the infraspinatus tendon insertion, and cortical irregularity of the lesser tuberosity at the subscapularis tendon insertion, does not carry the same significance and is commonly seen even with a normal rotator cuff.

Intrasubstance abnormality

In the presence of cortical irregularity, a focal and well-defined hypoechoic or anechoic supraspinatus tendon abnormality is considered a tear--unless it contacts only the greater tuberosity surface. In this case, it may still represent tendon fiber disruption but does not extend to the articular or bursal surfaces and cannot be visualized at arthroscopy or bursoscopy. ¹¹ The term intrasubstance abnormality is used in this situation, as it could either represent intrasubstance tear or severe tendinosis (Figure 5). The longitudinal view of the supraspinatus is most important in determining if the tendon abnormality extends to the articular or greater tuberosity surface.

Articular side abnormality

If the focal and well-defined hypo-echoic or anechoic supraspinatus tendon abnormality contacts only the articular surface, this indicates an articular side partial-thickness tear (Figure 4). Many of these tears will demonstrate a mixed echogenic appearance, with the hyper-echoic torn tendon fibers identified within the hypoechoic or anechoic tendon gap. ¹² If it is unclear if a tendon abnormality extends to the articular surface, the presence of a hyperechoic reflective interface between the tendon abnormality and hyaline articular cartilage of the humerus (the cartilage interface sign) is helpful, as this indicates articular extension (Figure 2). ¹³

Tendon thinning

If the normally convex superior surface of the supraspinatus tendon is flattened or concave from tendon thinning or absence, this indicates tendon fiber loss and may be seen with bursal side partial-thickness tendon tear (Figure 6) or full-thickness tendon tear (Figures 7 and 8). ^2,7 Extension of the tendon abnormality from the bursal to articular surface indicates full-thickness tear, while isolated bursal surface extension indicates bursal side partial-thickness tendon tear. This finding can also be applied to the other tendons of the rotator cuff. Generally speaking, as the tendon tear becomes larger, the degree of superficial tendon concavity or dipping the deltoid muscle into the tendon gap becomes more pronounced. With an acute tear, this tendon gap more likely fills with anechoic fluid (Figure 2); with chronic tears, fluid is lacking and often only tendon thinning or tapering is seen. ¹⁴

Tendon nonvisualization

When there is complete nonvisualization of a tendon, this indicates a massive full-thickness tear (Figure 8). In this situation, the deltoid muscle is typically lying on the proximal humerus in place of the torn rotator cuff. ^2,7 The torn and retracted tendon stump is often located beneath the acromion and not identified with sonography. It is important to determine the extent of a full-thickness tear. On transverse images, the intra-articular portion of the long head of the biceps brachii tendon is used as a reference point for this purpose (Figure 7B). A tendon tear that extends more than 2 cm to 2.5 cm posterior to the biceps tendon indicates infraspinatus tendon involvement. The presence of infraspinatus tendon atrophy should also be noted, as this is a negative prognostic indicator.

Diffuse hypoechoic
tendon abnormality

If a rotator cuff tendon is heterogeneous in echotexture and is diffusely hypoechoic without tendon thinning, this is characteristic of diffuse tendinosis (Figure 9). ¹ The involved tendon may be swollen with this process, which is a helpful sign. Cortical irregularity of the greater tuberosity at the supraspinatus tendon is typically absent. The patient may experience significant pain with this condition during the sonographic examination.

Focal hyperechoic tendon abnormality

If a focal hyperechoic tendon abnormality at the articular surface is surrounded by a hypoechoic area, this indicates an articular side partial-thickness tear. ¹² If the hyperechoic abnormality is larger and displays posterior acoustic shadowing, this is characteristic of calcium hydroxyapatite deposition, as seen in calcific tendonitis (Figure 10). ¹⁵ Although most of these deposits demonstrate shadowing, 7% display no posterior acoustic shadowing. ¹⁵ If this situation is suspected, anisotropy is helpful in detection. By imaging the involved tendon oblique to the ultrasound beam, the tendon becomes hypoechoic from anisotropy while the calcium deposit becomes more conspicuous, as it remains hyperechoic. If the calcium deposit incites inflammation, then the term calcific tendonitis is appropriate. Increased flow may be seen on color and power Doppler imaging. Sonography has been proven effective in aspiration and lavage of tendon calcium deposits. ¹⁶

Other shoulder pathology

Joint effusion

An increase in the volume of joint fluid can be visualized in the various joint recesses of the shoulder, including the long head of the biceps brachii tendon sheath (Figure 11), the subscapularis recess, and the posterior joint recess. Joint effusion visualization around the biceps tendon is common. This should be differentiated from a true biceps tenosynovitis. A focal collection of fluid associated with pain and hyperemia is characteristic of this latter condition. The presence of an isolated joint effusion is not specific for one type of pathology, and has low sensitivity and specificity for the diagnosis of rotator cuff tear. ¹⁷ Joint fluid may range from anechoic simple fluid to hyperechoic complicated fluid.

Subacromial-subdeltoid bursal fluid

Fluid within the subacromial-subdeltoid bursa typically appears anechoic or hypoechoic, surrounded by hyperechoic peribursal fat and bursal walls (Figure 12). Bursal wall distention <2 mm is considered normal. ¹⁸ Abnormal quantities of bursal fluid may be seen with impingement, as well as full-thickness and bursal side partial-thickness tendon tears. The presence of subacromial-subdeltoid bursal fluid and joint effusion has a high positive predictive value for the diagnosis of rotator cuff tear. ¹⁷ Bursal fluid may range from anechoic reactive fluid to hyperechoic complicated fluid. It is important not to mistake hyperechoic synovium within the subacromial-subdeltoid bursa as rotator cuff. Extension of this tissue beyond the greater tuberosity indicates bursal thickening as the etiology (Figure 5).

Biceps tendon subluxation and tear

The long head of the biceps brachii tendon is easily assessed with sonography. Its characteristic location within the bicipital groove of the proximal humerus aids in its identification. Absence of tendon within the bicipital groove may indicate either full-thickness biceps tendon tear (Figure 13) or biceps tendon subluxation or dislocation (Figure 14). ¹⁹ In the setting of a complete tendon tear, the distal retracted tendon stump can be visualized more distally, which confirms this diagnosis (Figure 13B). The biceps tendon may subluxate over the lesser tuberosity (Figure 14A), or dislocate into a subscapularis tendon tear and into the glenohumeral joint (Figure 14B). Abnormal medial position of the biceps tendon may indicate coracohumeral and superior glenohumeral ligament abnormality. With an empty bicipital groove, it is important to scan distally for a retracted tendon, and medially within the joint to differentiate tear from dislocation.

Glenoid labrum tear and cyst

MR arthrography is considered the imaging study of choice in evaluation of the glenoid labrum. However, the labrum can be identified with sonography, especially the posterior aspect. ²⁰ Evaluation of the posterior labrum can occur simultaneously during evaluation of the infraspinatus, although a 7-MHz transducer may be needed to optimally delineate the hyperechoic fibrocartilage labrum. When a well-defined hypoechoic or anechoic cleft is seen, a labral tear can be suggested. The presence of a hypoechoic or anechoic glenoid labral cyst adjacent to the labrum is more evidence that the labrum is torn (Figure 15). ²¹ Because symptoms related to a glenoid labral cyst may simulate that of rotator cuff pathology, it is important to consider this diagnosis, especially in the setting of infraspinatus and possible supra-spinatus muscle atrophy. Sonography has been used to guide cyst aspiration about the shoulder. ²² Because of the limitation of sonography in evaluation of the labrum, especially at the biceps anchor, any suspected labral abnormality is typically followed by MR arthrography for confirmation.

Greater tuberosity fracture

Nondisplaced greater tuberosity fractures of the proximal humerus are often overlooked on radiographs due to in-
adequate patient positioning, suboptimal radiographic technique, and subtlety of the fracture line. In the setting of a reported negative radiograph, a patient with continued shoulder pain after trauma may be evaluated with sonography to exclude
rotator cuff tear. At sonography, a greater tuberosity fracture will appear as dis-ruption of the echogenic cortex with step-off deformity (Figure 16). ²³ It is this step-off deformity that helps differentiate the true fracture from the greater tuberosity cortical irregularity that is seen with rotator cuff tears. With a greater tuberosity fracture, point tenderness is present with transducer pressure and the rotator cuff is often normal.

Conclusion

Sonography has been proven effective in the evaluation of the rotator cuff. Thorough knowledge of anatomy, a standardized technique, and familiarity with pathology as seen on sonography are important for success. The advantages of sonography include availability, low cost, patient comfort, and superior image resolution. AR