Musculoskeletal ultrasound of the shoulder

INTRODUCTION — The shoulder is susceptible to a variety of traumatic and nontraumatic injuries. Due to the relatively superficial location of the shoulder, many of these pathologic conditions can be diagnosed or assessed using ultrasound (US).

This topic will review sonographically relevant shoulder anatomy, describe a systematic approach to the sonographic shoulder examination, and discuss the sonographic appearance of common pathologic findings in the shoulder. Topics devoted to shoulder pain and specific shoulder pathologies are found separately. (See "Evaluation of the adult with shoulder complaints" and "Physical examination of the shoulder" and "Rotator cuff tendinopathy" and "Subacromial (shoulder) impingement syndrome" and "Frozen shoulder (adhesive capsulitis)" and "Multidirectional instability of the shoulder" and "Presentation and diagnosis of rotator cuff tears" and "Clinical manifestations and diagnosis of osteoarthritis", section on 'Shoulder'.)

USES, ADVANTAGES, AND LIMITATIONS OF SHOULDER ULTRASOUND — Musculoskeletal ultrasound (US) uses high frequency sound waves (1 to 20 megahertz, MHz) to produce high-resolution images of soft-tissue structures (eg, nerves, tendons, muscles, ligaments, bursae) and bony surfaces.

US has many advantages over other imaging modalities, including [1]:

●Portability

●Relatively low equipment costs

●Absence of ionizing radiation

●Dynamic imaging capabilities (ie, provides a "movie" rather than a "picture")

●Minimal artifact from metal objects

●Ability to perform immediate side-to-side comparisons

●Provides physiologic information (eg, blood flow assessment using Doppler US)

●Enables interaction between the examiner and patient (eg, shoulder can be examined while it is manipulated)

●No known contraindications

The patient cost of an US is less than other imaging modalities used to evaluate similar types of pathology [2]. The authors of a retrospective review of United States government Medicare data concerning the imaging of musculoskeletal disorders concluded that 30.6 percent of the pathology found on shoulder magnetic resonance imaging (MRI) studies could have been identified using US [3]. According to the study, if US were appropriately substituted for MRI for diagnostic imaging of specific shoulder conditions, the United States would save approximately USD $6.9 billion between 2006 and 2020. In addition, patients prefer US to MRI for a number of reasons [4].

However, like all imaging modalities, US has limitations. Sound waves do not penetrate bone, and therefore, US cannot image the interior of joints, identify intramedullary lesions, or evaluate pathology in tissues deep to bones or calcified tissues. US enables focused imaging of a specific region. Therefore, it is most appropriate to use US when the patient's symptoms are relatively localized. If a patient presents with diffuse pain, they are often better served by an imaging modality that provides a larger field of view, such as MRI or computed tomography (CT). Sound waves attenuate as they pass through tissues, and therefore, structures located deep inside the body are difficult to image with US. This is particularly true for high-frequency sound waves, which attenuate to a greater degree than low-frequency sound waves. Thus, when imaging structures in a deep location, a lower frequency transducer should be selected. Finally, the quality of the US image obtained depends upon the skill of the examiner and the quality of the US machine. Consequently, it is important for the sonologist to recognize the limits of their skills and equipment when performing an examination.

SHOULDER ANATOMY FOR ULTRASOUND EVALUATION

Scapula and related structures — The scapula is a relatively large, flat bone located on the posterior thorax (figure 1 and figure 2 and figure 3 and figure 4 and figure 5). The scapula has a prominent bony ridge on its posterior surface called the scapular spine. The supraspinous fossa is located cephalad to the scapular spine and is the site of the supraspinatus muscle origin. The infraspinous fossa is located below the scapular spine and is the location of the infraspinatus and teres minor muscle origins. A small bony ridge in the infraspinous fossa divides the infraspinatus and teres minor muscle origins. This bony ridge is readily identified with ultrasound (US) and can be used to assist the sonologist with identification of the infraspinatus and teres minor muscles (image 1).

Laterally, the scapular spine forms the acromion process, which articulates anteriorly with the clavicle to create the acromioclavicular joint (figure 6 and figure 7). As the supraspinatus muscle travels laterally, it passes beneath the acromion process. This is a potential site for shoulder impingement. The subacromial-subdeltoid bursa is located between the acromion and deltoid (superficially) and supraspinatus (deep). Bursae function to reduce friction and facilitate movement between adjacent soft tissue and bony structures, but they are susceptible to direct and indirect injury that can cause pain and dysfunction.

The scapula has a finger-like bony projection off of its antero-superolateral surface called the coracoid process. The pectoralis minor, coracobrachialis, and short head of the biceps brachii tendons arise from the coracoid process. The subcoracoid bursa lies caudal to the coracoid process between the aforementioned tendons and the subscapularis musculotendinous unit. Three ligaments also attach to the coracoid process: the coracohumeral, coracoclavicular, and coracoacromial. The coracoclavicular ligament has two parts: the medially located conoid and the laterally located trapezoid.

Laterally, the scapula has an oval fossa called the glenoid fossa. The glenoid fossa articulates with the humeral head to form the glenohumeral joint. With ultrasound, the glenohumeral joint is best viewed posteriorly. A small, triangular cartilage ring called the glenoid labrum surrounds the rim of the glenoid fossa. The glenoid labrum is susceptible to injury. Ultrasound can be used to evaluate the posterior portion of the glenoid labrum, but other areas of the labrum are not well seen due to their depth and orientation, and bony obstructions. Furthermore, although US demonstrates relatively high specificity for identifying posterior labral tears, it has poor sensitivity [5].

Glenohumeral joint and related structures — The glenohumeral joint is enclosed in a joint capsule. The glenohumeral ligaments are focal areas of thickening within the joint capsule and contribute to the static stability of the glenohumeral joint (figure 6 and figure 7). Several distinct joint recesses are present in the shoulder joint, two of which can readily be imaged with US: the posterior recess and the long head of the biceps brachii tendon sheath [6].

The suprascapular nerve and vasculature pass over the superior aspect of the scapula through the scapular notch (figure 8). They traverse through the suprascapular fossa, deep to the supraspinatus muscle, and enter the spinoglenoid notch. The spinoglenoid notch is located between the superoposterior glenoid rim (deep) and spine of the scapula (medial), and underneath the acromion process (superior). The suprascapular neurovascular structures can be identified with US at the scapular and spinoglenoid notches, both of which are common sites of entrapment.

The long head of the biceps brachii tendon (also known as the bicipital tendon) originates from the supraglenoid tubercle and courses anterolaterally across the humeral head where it enters the intertubercular groove (figure 1 and figure 6 and image 2). The portion of the long head of the biceps brachii tendon proximal to the intertubercular groove lies within the glenohumeral joint (ie, intra-articular) and is located between the supraspinatus (posterior) and subscapularis (anterior) tendons. This area is called the rotator interval. The coracohumeral ligament travels superficial to the long head of the biceps brachii tendon, and the superior glenohumeral ligament courses deep to it in the rotator interval. The reflection pulley is a sling of connective tissue in the rotator interval, located anterior to the long head of the biceps brachii tendon (image 3). The reflection pulley holds the long head of the biceps brachii tendon in place and assists with preventing anterior dislocation of the tendon.

As the long head of the biceps brachii tendon traverses laterally, it exits the glenohumeral joint and enters the intertubercular groove. The intertubercular groove is bordered by the lesser tuberosity (medially), greater tuberosity (laterally), and transverse humeral ligament (superficially). The transverse humeral ligament is formed from blended fibers of the coracohumeral ligament, supraspinatus tendon, and subscapularis tendon [7]. The transverse humeral ligament assists with stabilizing the long head of the biceps brachii in the bicipital groove. The long head of the biceps brachii tendon is enclosed in a tendon sheath from where it exits the glenohumeral joint until a location approximately 3 to 4 cm distal to the intertubercular groove [7]. Due to its communication with the glenohumeral joint, this tendon sheath is considered a glenohumeral joint recess. The ascending branch of the anterior humeral circumflex artery is located lateral to the long head of the biceps brachii tendon in the intertubercular groove. The location of this artery should be taken into consideration when planning and performing an injection into the long head of the biceps brachii tendon sheath.

The long head of the biceps brachii tendon exits the intertubercular groove caudally and courses distally across the proximal anterior arm. The musculotendinous junction of the long head of the biceps brachii tendon is located where it passes underneath the pectoralis major tendon in the anterior proximal arm.

Rotator cuff — Four muscles comprise the rotator cuff. From anterior to posterior, they are the subscapularis, supraspinatus, infraspinatus, and teres minor (figure 4 and figure 1 and figure 3 and figure 2). The subscapularis originates from the subscapularis fossa of the scapula and traverses superolaterally in the scapular plane to attach onto the lesser tuberosity. The broad subscapularis tendon has multiple tendon slips with interspersed muscle, giving it a heterogenous appearance on US (image 4). The inferior fibers of the subscapularis tendon are extra-articular, whereas the superior fibers are intra-articular.

The supraspinatus muscle lies in the supraspinous fossa of the scapula and has an anterior and posterior portion. The muscle courses laterally in the scapular plane, becomes tendon as it passes under the acromion, and attaches to the superior facet and anterior portion of the middle facet of the greater tuberosity. The superior and middle facets of the greater tuberosity are easily recognizable on ultrasound (image 5). The supraspinatus measures approximately 2 to 2.5 cm in the anterior to posterior (short axis) dimensions. However, only the anterior 1.5 cm of the tendon contains strictly supraspinatus fibers. The remaining 0.5 to 1 cm of posterior tendon contains overlapping supraspinatus and infraspinatus fibers [7].

The anterior and posterior portions of the supraspinatus muscle give rise to distinct portions of the supraspinatus tendon. The anterior portion of the supraspinatus muscle forms an oval-shaped group of tendon fibers within the superficial anterior portion of the supraspinatus tendon. The posterior portion of the supraspinatus muscle gives rise to flatter tendon fibers that are located in the posterior portion of the supraspinatus tendon. Some of the tendon fibers arising from the posterior supraspinatus muscle course underneath the anteriorly located oval portion of the supraspinatus tendon. The different orientations of the supraspinatus tendon fibers result in disconjugate stress patterns within the tendon, which may predispose to tendon injury in this location [7].

The infraspinatus muscle arises from the infraspinous fossa of the scapula and courses in a superolateral direction, becomes tendon over the posterior humeral head, and attaches to the middle facet of the greater tuberosity. As previously stated, the anterior 0.5 to 1 cm of the infraspinatus tendon overlaps with the supraspinatus tendon. The infraspinatus-supraspinatus overlap begins approximately 1.5 cm posterior to the anterior margin of the supraspinatus tendon. Independent infraspinatus fibers begin between 2 to 2.5 cm posterior to the anterior margin of the supraspinatus tendon and extend posteriorly, approximately 1 to 1.5 cm [7].

The teres minor muscle originates from a distinct region on the posteroinferior surface of the infraspinous fossa of the scapula, just caudal to the infraspinatus origin. As previously stated, a small bony ridge, with a caudomedial to cephalolateral orientation, separates the infraspinous fossa from the teres minor fossa (image 1). The teres minor travels in a cephalolateral direction, becomes tendon as it traverses across the posterior humeral head, and inserts onto the inferior facet of the greater tuberosity. The cross-sectional area of the teres minor muscle is typically approximately 50 percent less than that of the infraspinatus muscle [7].

ULTRASOUND EXAMINATION OF THE SHOULDER

Guidelines, structures to image, and positioning — The American Institute of Ultrasound in Medicine (AIUM) and European Society of Skeletal Radiology (ESSR) recommend performing a complete shoulder examination rather than a regional examination, as is the case with other joints, regardless of the pathology suspected [8].

A list of the structures that should be imaged during a complete sonographic shoulder examination is provided in the accompanying table (table 1). This list was developed based upon the AIUM Practice Guideline for the Performance of a Musculoskeletal Ultrasound Examination [8]. All relevant structures should be evaluated with short and long-axis views. A high frequency (eg, 12 to 5 MHz), linear array transducer should be used for the shoulder evaluation. The transducer should be manipulated to ensure the ultrasound beam is perpendicular to the structure of interest to minimize anisotropy, a common sonographic artifact. When appropriate, pathologic regions should be evaluated with color or power Doppler ultrasound (US) to detect hyperemia, and measurements in orthogonal planes of the pathology (ie, sound waves are perpendicular) should be taken.

During the examination, the patient can be positioned in a dependent (supine or side-lying) or seated position, depending upon the sonologist's preference. If the seated position is chosen, the sonologist usually stands behind the patient and places the US machine in front of the patient so that the sonologist and patient are both facing the US machine (picture 1). The dependent position is the author's preferred position for performing an US examination of the shoulder. Thus, the majority of the pictures in this topic demonstrate how to perform the US examination of the shoulder with the patient in the dependent position.

Long head of the biceps brachii tendon — The shoulder examination begins by imaging the long head of biceps brachii tendon. The patient should be instructed to supinate their forearm, which moves the tendon anteriorly (picture 2). The clinician then places the US transducer over the anterior proximal arm in a short axis relative to the biceps brachii muscle (picture 3). At this level, the transducer lies over the anterior aspect of the biceps brachii muscle. The clinician then glides the transducer proximally to image the musculotendinous junction of the long head of the biceps brachii muscle, which is located at the level of the pectoralis major tendon (image 6). Next, the clinician continues to glide the transducer proximally toward the intertubercular groove. The long head of the biceps brachii tendon sheath terminates between the musculotendinous junction (distally) and intertubercular groove (proximally), approximately 3 to 4 cm distal to the intertubercular groove. Frequently, a small amount of physiologic, hypoechoic-appearing fluid is located in the tendon sheath near its terminus. Fluid circumscribing the tendon suggests pathology and should prompt further investigation of its source.

The clinician continues to glide the US transducer proximally until the intertubercular groove is reached. The width and depth should be noted, as should the presence of cortical irregularities within the groove. Instability of the long head of the biceps brachii tendon may be associated with an intertubercular groove depth of less than 4 mm [7]. In the intertubercular groove, the long head of the biceps brachii tendon should have an oval shape and a hyperechoic, fibrillar echotexture (image 2). The transverse humeral ligament is located superficial to the long head of the biceps brachii tendon, runs from the lesser tuberosity to the greater tuberosity, and has a hyperechoic linear appearance [7]. To assess for tendon instability, the clinician maintains US visualization of the long head of the biceps brachii tendon in the intertubercular groove while the patient internally and externally rotates their shoulder.

Next, the clinician glides the US transducer further proximally and medially to image the intra-articular portion of the long head of the biceps brachii tendon. The transducer should be "wagged" or "toggled" to eliminate anisotropy. The tendon should continue to demonstrate an oval, hyperechoic, fibrillar appearance (image 7). After imaging the tendon in the short axis, the transducer should be rotated 90 degrees to evaluate it in the long axis. A heel-toe maneuver, pressing the distal end of the transducer into the soft tissues, will help maintain a perpendicular orientation of the transducer relative to the long head of the biceps brachii tendon and minimize anisotropy (picture 4 and image 8).

Subscapularis and acromioclavicular (AC) joint — The next structure to be evaluated during a standard shoulder US examination is the subscapularis tendon. To begin doing so, the US transducer should be placed over the intertubercular groove in the anterior shoulder in the same orientation that would be used to image the long head of the biceps brachii tendon in a short axis. The patient's shoulder should be externally rotated, and the clinician should glide the US transducer medially over the subscapularis insertion on the lesser tuberosity (picture 5). In this position, the subscapularis tendon can be imaged in the long axis (image 9). Next, the clinician rotates the US transducer so it has a slight cephalolateral to caudomedial orientation to ensure a proper long-axis orientation of the transducer relative to the tendon fibers. The clinician then glides the US transducer cephalad, caudad, medial, and lateral to image the entire extent of the subscapularis tendon. Particular attention should be paid to the cephalad fibers of the subscapularis tendon since this is the most frequent location of subscapularis pathology [7].

When the US transducer is glided medially, the musculotendinous junction of the subscapularis can be imaged and a dynamic evaluation for subcoracoid bursopathy and impingement can be performed (image 10 and movie 1). The dynamic evaluation involves maintaining the transducer in a long axis relative to the subscapularis musculotendinous unit, with the medial aspect of the probe located over the conjoint tendon of the coracobrachialis and short head of the biceps muscles. The patient’s shoulder is then passively or actively rotated both internally and externally to identify compression of the subscapularis muscle or tendon or the subcoracoid bursa. The US transducer should then be rotated 90 degrees to evaluate the subscapularis tendon in the short axis (picture 6 and image 4). The clinician glides the transducer cephalad, caudad, medial, and lateral to image the entire extent of the tendon. In the short axis, the subscapularis tendon has a heterogenous appearance due to the mixed echotexture of the multiple tendon slips (hyperechoic and fibrillar) interspersed with muscle fibers (hypoechoic).

Next, the clinician glides the US transducer cephalad to the top of the shoulder where the acromioclavicular joint is imaged in an anatomic coronal-oblique plane (picture 7). The medial end of the US transducer should be slightly anterior to the lateral end of the US transducer due to the orientation of the clavicle relative to the acromion. The acromion and clavicle will have the characteristic hyperechoic appearance of bone with posterior acoustic shadowing (image 11). Between the two bones, a hypoechoic, ground glass-appearing, intra-articular fibrocartilaginous disc may be seen. The superior acromioclavicular ligament and joint capsule will appear as hyperechoic linear structures connecting the acromion to the clavicle superficially. The clinician then glides the US transducer anteriorly and posteriorly to image the entirety of the joint. Acromioclavicular joint provocative maneuvers, such as axial traction of the arm or horizontal adduction of the shoulder, can be performed to assess the acromioclavicular joint dynamically.

Infraspinatus and teres minor — The patient is asked to roll onto the side that is not being examined and the clinician places the US transducer in a sagittal-oblique plane over the mid-scapular spine on the patient's posterior shoulder (picture 8). The cephalad portion of the US transducer should be slightly medial relative to the caudal portion of the US transducer such that the transducer is perpendicular to the plane of the scapular spine. The clinician then glides the US transducer caudally over the infraspinatus muscle. In the short axis, the infraspinatus muscle appears hypoechoic with hyperechoic foci. These foci represent the normal fibroadipose septa of the muscle (image 12). The infraspinatus tendon is located in the center of the muscle and has a hyperechoic "seagull-shaped" appearance. The clinician glides the transducer laterally in the plane of the scapular spine to reach the insertion of the infraspinatus tendon on the middle facet of the greater tuberosity on the humeral head (picture 9 and movie 2). In this location, the infraspinatus will have a hyperechoic and fibrillar appearance (image 13). The transducer should then be rotated 90 degrees, and a long-axis examination of the entire infraspinatus muscle and tendon should be performed in a similar manner (image 14).

Next, the clinician rotates the transducer 90 degrees over the mid-portion of the infraspinatus muscle and glides it caudally to image the teres minor muscle in the short axis (picture 10). The muscle has a similar appearance to the infraspinatus muscle in the short axis but should be approximately 50 percent smaller (image 15) [7]. As previously mentioned, a small bony ridge separates the infraspinatus from the teres minor and can be used as a sonographic landmark to differentiate between these two muscles (image 1). The clinician glides the transducer laterally in the plane of the scapular spine to reach the insertion of the teres minor tendon on the inferior facet of the greater tuberosity on the humeral head (picture 11). In this location, the teres minor has a hyperechoic and fibrillar appearance (image 16). In the author's experience, cortical irregularities are common in the region of the inferior facet of the greater tuberosity and are usually not associated with pathology. Therefore, their presence in this location should be interpreted with caution. The transducer should then be rotated 90 degrees, and a long-axis examination of the entire teres minor muscle and tendon performed in a similar manner (image 17).

Posterior glenohumeral joint — Next, the US transducer should be placed in a long-axis over the mid-portion of the infraspinatus muscle and glided medially and laterally until the posterior glenohumeral joint is identified (picture 12). The humeral head has a round, hyperechoic appearance with posterior acoustic shadowing (image 18). The articular cartilage of the humeral head appears as a hypoechoic/anechoic region between the bony surface of the humeral head (deep) and the overlying infraspinatus muscle and tendon. The fibrocartilaginous, arrowhead-shaped glenoid labrum has a granular, hyperechoic appearance. During dynamic scanning, the labrum can be seen moving and bending in response to pressures from adjacent tissues (movie 3). With shoulder external rotation, glenohumeral joint fluid pools in the posterior glenohumeral joint recess, thus enhancing the ability to identify a joint effusion in this location. Medial to the fibrocartilaginous labrum is the bony glenoid, which has the characteristic hyperechoic, smooth appearance of bone with posterior acoustic shadowing. The sonologist should glide the US transducer cephalad and caudad to ensure the entire extent of the posterior glenohumeral joint is evaluated.

Spinoglenoid and suprascapular notches — Following evaluation of the posterior glenohumeral joint, and starting with the US transducer in the same orientation, the clinician should glide the US transducer slightly medially to image the spinoglenoid notch (picture 13 and image 19). The suprascapular neurovascular structures pass through the spinoglenoid notch on their way to the infraspinous fossa. This is a potential site for nerve injury, often due to traction when the arm is in an abducted and externally rotated position, or from compression from a spinoglenoid notch cyst that arises from a tear of the glenoid labrum [9]. With the shoulder externally rotated, the suprascapular vein dilates and can mimic the appearance of a spinoglenoid notch cyst [7]. Thus, this region should be examined with the shoulder in a neutral or internally rotated position. Color or power Doppler US imaging can be used to assist with the identification of the suprascapular vasculature in the spinoglenoid notch, but they are frequently difficult to identify even with Doppler US assistance due to their small size and deep location. The roof of the spinoglenoid notch is formed by the inferior transverse scapular ligament, which appears hyperechoic and fibrillar on US.

The US transducer can then be glided cephalad and slightly medially over the scapular spine to the supraspinous fossa, where the suprascapular notch can be identified (picture 14 and image 20). The suprascapular nerve enters the supraspinous fossa through the suprascapular notch. The roof of the suprascapular notch is formed by the superior transverse scapular ligament, which has a hyperechoic and fibrillar appearance. The suprascapular vasculature crosses over the top of the superior transverse scapular ligament rather than passing through the suprascapular notch. Color or power Doppler US imaging can be used to assist with the identification of the suprascapular vasculature in this region. But, similar to the spinoglenoid notch region, it is often difficult to identify the vasculature even with the aid of Doppler US. The suprascapular notch is another potential site of suprascapular nerve compression [9].

Supraspinatus — Next, ask the patient to place their hand on their posterior hip thereby extending the shoulder (picture 15). The elbow should be kept adducted. This position is called the modified Crass position and is one of the preferred methods for evaluating the supraspinatus tendon. The patient can also be placed in the Crass position, which places more tension on the supraspinatus tendon, but may make it difficult to image the anterior margin of the supraspinatus tendon, which is the most common site of pathology (picture 16) [7]. The clinician then glides the transducer over the intertubercular groove with the long head of the biceps brachii tendon in the short axis. The transducer should be glided slightly posterior so that most of the ultrasound image consists of the greater tuberosity, but the long head of the biceps brachii tendon is still seen at the anterior border of the screen (picture 17). The clinician then glides the transducer cephalad over the top of the greater tuberosity (picture 18). As the transducer crosses over the greater tuberosity, the supraspinatus tendon insertion will come into view (image 21 and movie 4). The tendon should have a hyperechoic, fibrillar appearance. The surface of the superior facet of the greater tuberosity of the humeral head should be smooth and devoid of cortical irregularities.

After evaluating the supraspinatus tendon insertion on the superior facet of the greater tuberosity, the clinician glides the transducer medially until the image of the supraspinatus is obstructed by the acromion process. The long head of the biceps brachii tendon should be kept in sight at all times to orient the sonologist to the location of the anterior margin of the supraspinatus (image 22). After imaging the anterior portion of the supraspinatus tendon and muscle in the short axis, return the transducer to the superior facet of the greater tuberosity, then glide it posteriorly over the middle facet of the greater tuberosity, which is the location of the supraspinatus-infraspinatus tendon overlap (image 5). The clinician then glides the transducer medially such that this portion of the supraspinatus tendon can be evaluated in the short axis. The transducer is then returned to its starting position over the long head of the biceps brachii tendon in the intertubercular groove, and rotated 90 degrees such that the long head of the biceps brachii tendon is seen in the long axis (picture 19). The clinician then glides the transducer posteriorly over the supraspinatus insertion on the superior facet of the greater tuberosity (image 23). The transducer should be glided posteriorly, until the entire supraspinatus tendon insertion has been evaluated in the long axis, and then glided medially to the edge of the acromion. Finally, the clinician glides the transducer anteriorly and posteriorly to image the entire mid-portion of the supraspinatus tendon.

Subacromial-subdeltoid bursa — While imaging the supraspinatus tendon in the long axis, the overlying subacromial-subdeltoid bursa can be seen as a thin, hypoechoic line surrounded by hyperechoic peribursal fat (image 24). The subacromial-subdeltoid bursa extends beyond the supraspinatus footprint and down the lateral aspect of the greater tuberosity approximately 3 to 4 cm [7]. This region should be evaluated during the US examination since subacromial-subdeltoid fluid frequently pools in this area, resulting in a "teardrop" sign (image 25) [7].

Subacromial impingement — A dynamic evaluation looking for signs of subacromial impingement can be performed toward the completion of the examination. To perform this evaluation, the patient is placed in an upright position with the US transducer over their anterolateral shoulder, oriented in a long axis over the supraspinatus tendon (picture 20). The medial aspect of the US transducer should be over the lateral edge of the acromion process. The patient is asked to abduct their shoulder while the sonologist maintains the US transducer over the supraspinatus and watches as the supraspinatus tendon and subacromial-subdeltoid bursa pass under the acromion process (movie 5). Impingement is suggested by pain, bunching of bursal or tendon tissue, milking of subacromial-subdeltoid bursal fluid laterally, and inability to pass the greater tuberosity under the acromion [10].

SONOGRAPHIC APPEARANCE OF SHOULDER PATHOLOGY

Rotator cuff pathology — Suspected rotator cuff pathology is the most common indication for performing a shoulder ultrasound (US) examination [11,12]. Rotator cuff pathology includes tendinopathy (including calcific tendinopathy), partial-thickness tears, and full-thickness tears. (See "Rotator cuff tendinopathy" and "Presentation and diagnosis of rotator cuff tears".)

The sensitivity and specificity of US for rotator cuff tears are comparable to standard magnetic resonance imaging (MRI) [13,14]. While magnetic resonance arthrography (MRA) is slightly more sensitive than MRI and US for partial thickness rotator cuff tears, US has advantages that make it an ideal imaging modality for evaluating the rotator cuff [13,15]. (See 'Uses, advantages, and limitations of shoulder ultrasound' above.)

Tendinopathy — Tendinopathy of the rotator cuff most commonly involves the supraspinatus, but other tendons can be affected [11]. Supraspinatus tendinopathy has a similar sonographic appearance to tendinopathy in other locations, including thickening (>5 to 6 mm), hypoechogenicity, and heterogeneity (image 26) [7,16,17]. Thickening of the rotator cuff can result in impingement (subacromial or subcoracoid). Therefore, a dynamic assessment for impingement should be performed when tendinopathy is identified (movie 5). Pathologic rotator cuff tendons remain susceptible to anisotropy, which can lead to the misdiagnosis of a rotator cuff tear. To avoid this pitfall, the sonologist needs to ensure that the transducer is positioned so the sound waves are perpendicular to the tendon, and the tendon is imaged in two orthogonal planes. Two additional features that distinguish tendinopathy are the noncompressible status of the tendon and the absence of secondary signs of a rotator cuff tear [11]. (See "Rotator cuff tendinopathy" and "Subacromial (shoulder) impingement syndrome".)

In some individuals, the area of rotator cuff tendinopathy develops calcium deposits. This condition is referred to as calcific rotator cuff tendinopathy and it occurs most frequently in the supraspinatus tendon, although up to 15 percent of cases involve the infraspinatus, and 5 percent the subscapularis [18]. Calcific rotator cuff tendinopathy is rarely associated with a rotator cuff tear. Risk factors include female gender, middle age, and history of underlying endocrine disorder (eg, diabetes mellitus) [18]. Calcific tendinopathy has four phases, beginning with the pre-calcific phase in which fibrocartilaginous metaplasia occurs within the rotator cuff tendon fibers. This progresses to the calcific phase. Unless there is mechanical impingement, the calcific phase is not painful. During the resorptive phase, vasculature invades the calcified area and a dramatic inflammatory response ensues. This phase often is accompanied by severe pain. Finally, after the calcification has been resorbed, there is a healing phase at which time the pain resolves [18].

Calcific tendinopathy appears as hyperechoic foci within the tendon (image 27). During the calcific phase, the calcification has significant posterior acoustic shadowing [19]. The posterior acoustic shadowing becomes less prominent as the calcification progresses into the resorptive phase [19]. Occasionally, hyperemia can be seen within the calcification or surrounding tendon tissue during the resorptive phase [19]. During the resorptive phase, the calcification may be amenable to treatment via US-guided lavage and aspiration of the calcific material [20].

Tendon tears — There are three types of partial-thickness rotator cuff tears: articular surface, bursal surface, and intra-substance tears. Partial-thickness rotator cuff tears appear as focal areas of hypo- or anechogenicity with fiber discontinuity and (typically) volume loss [21]. Occasionally, the torn tendon fibers at the edge of the tear appear hyperechoic due to the surrounding hypoechoic fluid [21]. When performing a sonographic examination, any rotator cuff tear should be imaged in orthogonal planes to confirm its presence. The sonologist should include all of the following information in their report: tear location, tendon(s) involved, tear dimensions (anterior-posterior and medial-lateral), percentage of the tendon involved (with partial-thickness tears), and degree of rotator cuff atrophy. Muscle atrophy manifests as decrease in muscle size, increased echogenicity, and loss of the normal muscle architecture (movie 6) [22]. The overlying trapezius or deltoid muscles can be used as a reference for normal muscle echogenicity and architectural pattern [22]. (See "Presentation and diagnosis of rotator cuff tears".)

The most common partial-thickness rotator cuff tear is an articular surface tear of the anterior supraspinatus tendon near the anatomic neck of the humerus (image 28) [23]. If the partial-thickness articular surface tear is located at the insertion on the greater tuberosity, it is often referred to as a "rim-rent tear.” Partial-thickness articular surface tears can occur in other regions, including the supraspinatus-infraspinatus overlap (often seen in the overhead athlete with internal impingement), or the cephalad portion of the subscapularis [21]. Partial-thickness articular surface tears are the most difficult type of rotator cuff tear to identify using either US or MRI [13]. With articular surface partial-thickness rotator cuff tears, volume loss may or may not be present due to the continuity of the overlying tendon fibers. However, two other associated findings suggest the presence of an articular surface rotator cuff tear. The first is the cartilage interface sign, a hyperechoic line at the interface between the hypo- or anechoic hyaline cartilage and the hypo- or anechoic area of tendon fiber discontinuity (image 29). The second finding is cortical irregularity at the insertion site of the supraspinatus on the greater tuberosity (image 28). The presence of cortical irregularities suggests that the tear is chronic.

Bursal surface partial-thickness rotator cuff tears tend to be more painful than other partial-thickness rotator cuff tears due to the adjacent subacromial-subdeltoid bursa, which is highly innervated [11,21]. In addition to focal hypo- or anechogenicity and fiber discontinuity, bursal surface partial-thickness rotator cuff tears are associated with loss of the normal convexity of the superficial rotator cuff surface (image 30 and image 31) [11,21]. A reactive bursopathy commonly accompanies bursal surface partial-thickness rotator cuff tears [11,21]. Bursal hypertrophy can be isoechoic with tendon fibers and thus may appear to fill in the area of the tear, making it difficult to identify. However, with shoulder movement the rotator cuff will move dyskinetically relative to the overlying bursa (ie, the bursal tissue remains relatively static while the underlying rotator cuff fibers move during shoulder abduction). In addition, the bursal tissue typically has a granular, rather than fibrillar, appearance. The subacromial-subdeltoid bursa extends 3 to 4 cm beyond the edge of the greater tuberosity, whereas the supraspinatus inserts on the superior facet and cephalad portion of the middle facet of the greater tuberosity [11,21]. Using these clues, the sonologist can identify bursal surface partial-thickness rotator tears.

The third type of partial-thickness rotator cuff tear is an intra-substance tear. These tears do not communicate with either the bursal or articular surface of the rotator cuff, and so are not apparent during arthroscopy. Therefore, when an intra-substance tear is identified, it is important to document its location relative to known anatomic landmarks (eg, distance from the anterior margin of the supraspinatus or superior margin of the subscapularis) in order to convey the information to the referring clinician. Intra-substance partial-thickness tears are less common than bursal or articular surface partial-thickness rotator cuff tears. They frequently occur in association with high-grade tendinopathy and have the appearance of focal areas of anechogenicity and fiber discontinuity, and are identifiable in orthogonal planes (image 32) [24]. Occasionally, fluid can accumulate in intra-substance tears, leading to tendon delamination and formation of intratendinous or intramuscular cysts [24].

A full-thickness rotator cuff tear is defined as a tear that communicates from the bursal to the articular surface of the tendon [11,25-28]. Sonographically, full-thickness rotator cuff tears appear hypo- or anechoic with fiber discontinuity from the bursal to the articular surface (image 33). The tear can be focal (ie, involving part of the anterior-posterior tendon width) or complete (ie, involving the entire anterior-posterior tendon width). Rotator cuff tears can result from trauma or degeneration. Similar to partial-thickness tears, the most common location of full-thickness rotator cuff tears is the anterior margin of the supraspinatus [11]. Thus, it is important to evaluate carefully the anterior margin of the supraspinatus during an US examination of the shoulder.

Full-thickness focal tears of the anterior margin of the supraspinatus tendon tend to propagate posteriorly, while full-thickness focal tears of the superior margin of the subscapularis tendon (another common place for rotator cuff tears) tend to propagate inferiorly. Supraspinatus tendon tears greater than 2.5 cm in their anterior-posterior dimensions involve the entire supraspinatus tendon and extend into the infraspinatus tendon. Full-thickness complete tears of the infraspinatus and teres minor are rare in the absence of trauma [11]. Large full-thickness rotator cuff tears often are accompanied by significant tendon retraction, which uncovers the surface of the humeral head and greater tuberosity. This is commonly referred to as the "naked tuberosity sign" [25,26,29].

There are several indirect sonographic signs of rotator cuff tears, including the following [30]:

●Tendon thinning (image 31)

●Cortical irregularities (image 34)

●Cartilage interface sign (hyperechoic line at interface between hypo- or anechoic hyaline cartilage and hypo- or anechoic area of tendon fiber discontinuity (image 29))

●Loss of the convex contour of the superficial surface of the tendon (image 31)

●Fluid within the glenohumeral joint and subacromial-subdeltoid bursa (image 35)

The presence of fluid in a glenohumeral joint recess (eg, the bicipital tendon sheath) and the subacromial-subdeltoid bursa has relatively high specificity for full thickness rotator cuff tears according to some observational studies (image 35) [14]. On US, intra-articular fluid is most commonly detected in the bicipital tendon sheath or posterior glenohumeral joint. Therefore, when fluid is seen in one of these joint recesses and the subacromial-subdeltoid bursa, the clinician should look carefully for a full thickness tear. The Geyser sign, discussed below, is an example of such fluid (image 36). (See 'Acromioclavicular joint pathology' below.)

Bursal pathology — Due to its intimate association with the rotator cuff muscles, the subacromial-subdeltoid bursa is frequently affected by rotator cuff pathology [31]. A pathologic subacromial-subdeltoid bursa appears thickened (>2 mm), hypoechoic, and may be associated with hyperemia (image 37). If fluid accumulates within the subacromial-subdeltoid bursa, it frequently is found in its dependent portion, which is located approximately 3 to 4 cm distal to the rotator cuff insertion on the greater tuberosity. This is referred to as the "teardrop" sign (image 25) [31]. Ultrasound cannot differentiate between septic and aseptic bursopathy. Therefore, if septic bursopathy is suspected, the fluid should be aspirated and sent for appropriate laboratory testing to assess for infection. (See "Septic bursitis".)

Dynamic imaging for subacromial impingement is part of the standard ultrasound examination of the shoulder, and it is particularly important when subacromial-subdeltoid bursopathy is suspected. If subacromial impingement and subacromial-subdeltoid bursopathy are present, such testing may reveal fluid that can be milked laterally from underneath the acromion, or bursal tissue may bunch up laterally as the patient abducts their shoulder and the greater tuberosity approaches the acromion (movie 7).

Subcoracoid bursopathy usually occurs in the setting of subscapularis pathology or subcoracoid impingement. Subcoracoid impingement involves compression of the soft tissues between the humerus and the coracoid process [32]. Most often this occurs with horizontal adduction and internal rotation of the shoulder, and results in anterior shoulder pain. When measured using US, a coracohumeral interval of less than 8 mm is suggestive of coracohumeral narrowing, which may predispose to the development of this condition [33]. Subcoracoid bursopathy appears as hypoechoic thickening of the subcoracoid bursa with or without accompanying hyperemia and fluid accumulation (image 38). In a patient with subcoracoid bursopathy, dynamic US examination of the subcoracoid bursa as the shoulder is internally and externally rotated may demonstrate milking of fluid laterally within the bursa or bunching of bursal tissue against the coracoid process or overlying conjoint tendon.

Long head of the biceps brachii tendon disorders — The long head of the biceps brachii tendon, also known as the bicipital tendon, is susceptible to several different types of pathology including tenosynovitis, tendinopathy, partial-thickness tears, full-thickness tears, and instability. Studies of the sensitivity and specificity of US to detect bicipital tendon pathology generally compare ultrasound findings with direct surgical observation [17,34]. According to several small observational studies of this type, US appears to be highly sensitive and specific for most of the aforementioned pathologies, but it is not a reliable tool for detecting partial-thickness intra-substance bicipital tendon tears [17,34]. (See "Biceps tendinopathy and tendon rupture".)

Bicipital tendon pathology is most frequently located at the level of the intertubercular groove and is best imaged in the short axis [34]. Bicipital tenosynovitis is characterized by hypoechoic thickening of the bicipital tendon sheath with or without associated hyperemia and excessive simple (hypoechoic or anechoic) or complex (heterogenous, isoechoic or hyperechoic) fluid within the bicipital tendon sheath (movie 8) [34]. Bicipital tenosynovitis can occur in isolation or can be associated with bicipital tendinopathy or tears [7].

The presence of fluid within the tendon sheath surrounding the bicipital tendon is considered abnormal and can represent pathology of the bicipital tendon, bicipital tendon sheath, or glenohumeral joint (image 39) [7,30,34]. If the bicipital tendon and tendon sheath appear normal, the most likely source of the fluid is the glenohumeral joint, as the bicipital tendon sheath is found in a recess of the glenohumeral joint (image 40) [7,16]. However, if the bicipital tendon or tendon sheath is pathologic, then fluid is likely secondary to a local pathologic process. It is important to be aware that the subacromial-subdeltoid bursa extends distally over the bicipital groove, and fluid within the subacromial-bursa can be mistaken for fluid within the bicipital tendon sheath (movie 9).

Bicipital tendinopathy is characterized by tendon enlargement and heterogenous hypoechogenicity with or without associated hyperemia on Doppler US (image 41) [34]. High grade tendinopathy is frequently associated with partial-thickness intra-substance tears, although these can be difficult to identify with US [34]. Predisposing factors include bony pathology within the intertubercular groove (eg, intra-groove cortical irregularities, enthesophytes, or groove narrowing) [16,34]. Although bicipital tendinopathy most frequently occurs in the intertubercular groove, it can also involve the intra-articular portion of the tendon. Therefore, it is important to scan the entire long head of the biceps brachii tendon during the shoulder US examination to ensure that pathology is not missed.

Partial-thickness tears of the bicipital tendon appear as focal regions of anechogenicity, fiber discontinuity, and, in the case of high grade partial-thickness tears, volume loss [7]. Partial-thickness intra-substance tearing can also occur in the setting of tendinopathy, in which case the tendon will appear enlarged, heterogenous, and hypoechoic, with focal areas that are anechoic and represent the partial-thickness intra-substance tears (image 26 and image 28) [34].

Full-thickness bicipital tendon tears are characterized by an "empty groove sign" in which the bicipital tendon is absent from the intertubercular groove (image 42) [35]. A common pitfall is mistaking granulation and scar tissue within the intertubercular groove for an intact bicipital tendon. Another reason for an empty groove is dislocation of the bicipital tendon medially. Both possibilities should be kept in mind when evaluating a patient for a potential full-thickness bicipital tendon tear. To help avoid mistaken diagnoses, the clinician should begin the US examination distally over the biceps brachii muscle and trace the tendon proximally. In the case of a chronic full-thickness bicipital tendon tear, the muscle of the long head of the biceps brachii will be hyperechoic and atrophic (movie 10). Furthermore, as the clinician glides the US transducer proximally while maintaining a short-axis view of the bicipital tendon, in the setting of a full-thickness tear, the bicipital tendon is typically not visible proximal to the level of the pectoralis major tendon. If the bicipital tendon has dislocated, its medial location will be noted as the clinician glides the transducer proximally (image 43).

Instability of the long head of the biceps brachii tendon can be either partial (subluxation) or full (dislocation) (image 44). Instability of this tendon is frequently associated with an injury to the reflection pulley, transverse humeral ligament, or a tear of the cephalad border of the subscapularis tendon [35]. These areas should be carefully evaluated in the setting of long head of the biceps brachii tendon instability. (See 'Ultrasound examination of the shoulder' above.)

Acromioclavicular joint pathology — Ultrasound can assist with the assessment of patients with acromioclavicular joint osteoarthritis, sprains, periarticular cysts, and distal clavicular osteolysis [36]. Although most of these conditions are readily diagnosed with standard radiographs, US has the advantage of being able to assess periarticular ganglion cysts and acromioclavicular joint stability [37]. (See "Acromioclavicular joint injuries ("separated" shoulder)" and "Acromioclavicular joint disorders".)

The sonographic appearance of acromioclavicular joint osteoarthritis includes hyperechoic, hypertrophic (eg, osteophytes) bony changes on the acromial and clavicular sides of the joint (image 45). The joint capsule and superior acromioclavicular ligament are often thickened and hypoechoic with or without hyperemia and calcifications. In contrast to acromioclavicular joint osteoarthritis, distal clavicular osteolysis presents with cortical irregularities on the clavicular side of the acromioclavicular joint, tends to affect a younger population, and is more common in people who perform significant strength-training exercises involving the chest (eg, bench press) or shoulder muscles (eg, military press) [38].

Periarticular cysts can occur in association with osteoarthritis or in the setting of a large, full-thickness rotator cuff tear [39,40]. In the latter case, cephalad migration of the humeral head erodes the inferior acromioclavicular ligament and joint capsule, causing direct communication between the glenohumeral and acromioclavicular joints [39,40]. Fluid from a glenohumeral joint effusion can pass into the acromioclavicular joint and herniate through the superior acromioclavicular joint capsule and ligament, causing a periarticular cyst (image 36) [39,40]. This is referred to as the "geyser sign" and can present as a pseudotumor [39,40].

The sonographic appearance of acromioclavicular joint sprains includes hypoechoic thickening or disruption of the superior acromioclavicular ligament and capsule, periarticular hematoma formation, and potential widening of the acromioclavicular joint or depression of the acromion relative to the clavicle (image 46) [41]. Acutely, hematoma is relatively hyperechoic with a complex appearance representing the cellular and serum components of hemorrhage [30]. Subacutely, the hematoma will develop a more hypoechoic appearance [30]. Chronically, the hematoma will become smaller and more echogenic as tissue forms in place of the hematoma [30].

Ultrasound can be used to dynamically assess the stability of the acromioclavicular joint following injury [41]. Common maneuvers to assess acromioclavicular joint stability include horizontal adduction or axial traction of the arm. Instability is identified when the acromioclavicular joint widens significantly or acromion and clavicle move in a dysconjugate manner (movie 11).

Glenohumeral joint pathology — As with other joints, intra-articular pathology is best evaluated with imaging modalities other than US, such as radiographs, CT, or MRI. However, US can identify several types of glenohumeral joint pathology including osteoarthritis, posterior glenoid labral tears, and glenohumeral joint effusions [42,43]. (See "Clinical manifestations and diagnosis of osteoarthritis", section on 'Shoulder'.)

Glenohumeral joint osteoarthritis has a similar sonographic appearance to osteoarthritis in other joints, including synovial/capsular hypertrophy (hypoechoic thickening of the periarticular soft tissues) with or without associated hyperemia on Doppler US imaging, cortical irregularity and osteophytes (hyperechoic bony changes on the joint periphery), and possibly fluid or loose bodies within glenohumeral joint recesses (image 47) [43]. Osseous loose bodies have a hyperechoic appearance with associated posterior acoustic shadowing and are intra-articular [6].

Glenohumeral joint effusions are easily identified with US in the posterior joint recess and bicipital tendon sheath (image 40) [6,42]. Other glenohumeral joint recesses are not as readily assessed with US (eg, axillary recess, subscapular recess). The fluid of glenohumeral joint effusions can appear simple and hypo- or anechoic, or complex and hyperechoic [5,6,42]. US cannot differentiate between septic and aseptic effusions. Therefore, joint fluid should be aspirated and sent for appropriate analysis whenever the possibility of a septic joint is entertained.

On US, posterior glenoid labral tears appear as a hypoechoic cleft within the hyperechoic labrum (image 48). Although US is highly specific for labral tears, its sensitivity is low compared with other imaging techniques such as MRI arthrography [5,6]. Therefore, US is not the diagnostic study of choice when evaluating patients with a suspected labral tear. (See "Superior labrum anterior to posterior (SLAP) tears".)

Miscellaneous disorders — Occasionally, tears in the posterior glenoid labrum lead to the development of a cyst that protrudes into the spinoglenoid notch and (potentially) up to the suprascapular notch. Spinoglenoid notch cysts can reliably be detected with US and appear as a hypoechoic cystic mass in the spinoglenoid or suprascapular notch, superficial to the suprascapular neurovascular structures (image 49) [44]. The cyst can be a single cystic structure or multi-septated. The stalk that communicates between the glenoid labral tear and the cyst may be identified with US. When the shoulder is externally rotated, the suprascapular vein dilates, mimicking the appearance of a spinoglenoid notch cyst. Therefore, it is important to ensure that the patient is examined with their shoulder in a neutral or internally rotated position when evaluating for a spinoglenoid notch cyst.

Spinoglenoid notch cysts can occasionally compress the suprascapular nerve, leading to denervation of the infraspinatus muscle. The sonographic appearance of a denervated muscle includes atrophy, hyperechogenicity, and loss of the normal muscle echotexture (movie 8). It is often described as having a granular or ground glass appearance. Normally, the infraspinatus should be twice the size of the teres minor. A decrease in this 2:1 ratio suggests the presence of infraspinatus atrophy [30]. If the cyst extends to the suprascapular notch, then the suprascapular nerve branch to the supraspinatus may be affected, resulting in similar denervation changes in the supraspinatus.

The quadrilateral space is another potential site of neurovascular entrapment at the posterior shoulder. The quadrilateral space is bordered by the teres minor (cephalad), teres major (caudad), humeral diaphysis (lateral), and long head of the triceps (medial). The posterior humeral circumflex artery and axillary nerve pass through the quadrilateral space. Occlusion and stenosis of the posterior humeral circumflex artery in the quadrilateral space can be detected with US [45]. Neurovascular compression is frequently exacerbated with shoulder abduction and external rotation [45]. With sufficient compression of the axillary nerve, denervation of the teres minor will occur [46]. When denervated, the teres minor appears atrophic, hyperechoic, and has a granular/ground glass echotexture. These findings in isolation are suggestive of quadrilateral space syndrome [46].

The pectoralis major tendon crosses over the bicipital tendon in the anterior shoulder and attaches to the lateral aspect of the distal intertubercular groove. The pectoralis major is susceptible to tendinopathy, muscular strains, and partial and full-thickness tendon tears [47-49]. Muscular strains demonstrate muscle fiber disruption and (acutely) a fluid-filled hematoma with mixed echogenicity (image 50). The hematoma undergoes the sonographic changes discussed previously. (See 'Acromioclavicular joint pathology' above.)

Pectoralis strains occur most often at the musculotendinous junction [48]. On US, pectoralis major tendinopathy appears as a thickened, heterogenous, and hypoechoic tendon, and possibly hyperemia on Doppler US and intratendinous calcifications. Partial-thickness tears show tendon thinning and discontinuity of a portion of the tendon fibers. Full-thickness tears demonstrate complete discontinuity of the tendon fibers, medial retraction of the musculotendinous unit, and a hematoma interposed between the torn ends of the tendon.

ADDITIONAL ULTRASOUND RESOURCES — Instructional videos demonstrating proper performance of the ultrasound examination of the shoulder and related pathology can be found at the website of the American Medical Society for Sports Medicine: Shoulder ultrasound examination. Registration must be completed to access these videos, but no fee is required.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Musculoskeletal ultrasound".)

SUMMARY AND RECOMMENDATIONS

●Use and limitations of ultrasound shoulder examination – The shoulder is susceptible to a variety of traumatic and nontraumatic injuries. Due to the relatively superficial location of the shoulder, many of these pathologic conditions can be diagnosed or assessed using ultrasound (US). The advantages of US include portability, absence of radiation, and dynamic imaging capability. US cannot image the interior of joints, identify intramedullary lesions, or evaluate pathology in tissues deep to bones or calcified tissues. (See 'Uses, advantages, and limitations of shoulder ultrasound' above.)

●Anatomy – An understanding of shoulder anatomy is essential for interpreting US images. The anatomy most important for shoulder US is reviewed in the text; more detailed descriptions of shoulder anatomy and biomechanics are provided separately (figure 6 and figure 5 and figure 4 and figure 1 and figure 2 and figure 3). (See 'Shoulder anatomy for ultrasound evaluation' above and "Evaluation of the adult with shoulder complaints", section on 'Anatomy and biomechanics'.)

●Transducer selection and examination performance – A complete rather than a regional shoulder examination is preferred when using US. A list of the structures that should be imaged during a complete examination is provided in the accompanying table (table 1). All relevant structures should be evaluated with short and long-axis views. A high frequency (eg, 12 to 5 MHz), linear array transducer should be used for the shoulder evaluation. Performance of the US examination for each major structure, along with examples of normal and abnormal US findings, is described in the text. (See 'Ultrasound examination of the shoulder' above and 'Sonographic appearance of shoulder pathology' above.)