Advancing high-resolution musculoskeletal ultrasound: A histology- and anatomy-driven approach for enhanced shoulder imaging. Part I: Posterior and coronal shoulder

Ultrasound is a reliable imaging modality for diagnosing and assessing musculoskeletal disorders. Recent advancements in ultrasound technology have substantially improved image resolution, enabling visualization of anatomic structures on a near-microscopic level. However, there is as yet no atlas comparing ultrasound to anatomical and microscopic images that can be used for the proper allocation of ultrasound findings to anatomic structures. Existing guidelines for standardized shoulder ultrasound rely primarily on earlier machine models and settings that may not harness the full potential of these high-resolution imaging capabilities. Musculoskeletal ultrasound is a valuable diagnostic tool capable of observing soft tissue structures (Fig. 1) in addition to bone and cartilage⁽¹⁾. Frequencies most often utilized vary from 5 MHz to 24 MHz, depending on the tissue or joint being studied. Musculoskeletal structures are evaluated statically with the benefit of multiplanar views as well as dynamically in real-time.

Fig. 1.

Illustration of the rotator cuff of the shoulder. acr – acromion; cal – cora-coacromial ligament; cp – coracoid process; it – infraspinatus tendon; lhbt – long head biceps tendon; tmt – teres minor tendon, sscmt – sub-scapularis tendon, sumt – supraspinatus tendon, bu – bursa

This study aims to propose a contemporary approach to shoulder ultrasound. This includes a standard atlas of images that compares ultrasound standard images to anatomy, respectively histology and leverages state-of-the-art high-resolution imaging technology to enhance the interpretation of ultrasound findings.

To advance the standard of care in shoulder ultrasound, we implemented a series of systematic steps. Initially, we reviewed established ultrasound guidelines to identify recent developments in ultrasound anatomy, technology, and imaging settings, with a primary emphasis on enhancing the visualization of the detailed shoulder anatomy. In parallel, to establish a robust anatomical reference, anatomical and histological images were obtained from three body donors. Sections in different planes were prepared using standard methods and stained with hematoxylin and eosin (H&E) staining, ensuring a comprehensive understanding of the anatomical details. The preparation of the histological and anatomical images was performed in accordance with the Declaration of Helsinki.

To link anatomy and histology with ultrasound, we acquired ultrasound images from 10 volunteers and annotated the different anatomical and histological structures in the ultrasound images. To ensure reproducibility and validity, two sonographers performed this step. When structures were differently annotated, a decision was made based on consensus. The selected images and planes of views are based on official guidelines. The ultrasound device used is a LOGIQ E10 system from GE^® with R3 software. The probes used are ML6-15-D, ML4-20, L8-18i-D and L6-24-D, and frequencies ranged from 8 MHz to 24 MHz, depending on the structure.

A dynamic ultrasound examination from posterior (dorsal) to anterior (ventral) is the most common approach in clinical practice^(2,3). A chair or a rotating stool with or without a backrest is used to position the patient. The patient is seated upright in a relaxed position, with the patient’s forearm resting on their thigh in a supinated position at the beginning of the examination. Depending on personal preference, the sonographer can stand or sit behind, in front of, or beside the patient. Based on our own experience, we suggest examining the posterior structures first, then the anterior and lateral structures (Fig. 2, Tab. 1). During this procedure, the examiner is seated behind the patient, with the ultrasound machine on the examiner’s left side to adjust settings if necessary. Compared to having the patient in a supine position, sitting behind the patient and examining the posterior structures first allows for a more systematic, efficient, and ergonomic assessment with minimal patient repositioning, ensuring both thoroughness and patient comfort during the diagnostic process.

Fig. 2.

Illustration of a selection of probe positions, posterior/dorsal shoulder. a – posterior; b – anterior, c – lateral in the coronal plane

Ultrasound also allows the patient to view the examination via the device monitor or an optional second display, which may be helpful for explaining their pathology. This is, in our experience, well-accepted by patients and is a unique feature of ultrasound over other imaging modalities⁽⁴⁾.

Standard scans of the posterior shoulder (Fig. 3) (Tab. 2)

Fig. 3.

Probe positions, posterior/dorsal shoulder on the model

Anatomical structures of the posterior shoulder (Tab. 3)

The patient’s hand is relaxed and placed on the thigh, with the elbow bent to examine the posterior part of the shoulder. The glenoid and humeral head are seen on opposite sides of the screen when the probe is positioned transversely above the glenohumeral joint (Fig. 4). From here the examiner may perform dynamic maneuvers; for example, internal rotation, external rotation, and place the hand on the opposite shoulder so that the muscles and tendons are put under tension. Rotating the arm both internally and externally allows the examiner to see how the humeral head moves inside the glenoid fossa (Fig. 5)⁽⁵⁾. The insertion of the teres minor tendon and the infraspinatus tendon on the inferior and posterior aspects of the greater tuberosity are the two main points of static and dynamic examination of the humeral head⁽⁶⁾ (Fig. 6, Fig. 7).

Fig. 4.

Posterior transverse overview. A. Gray-scale ultrasonography posterior transverse panoramic image. B. Anatomical image. C. Histological image. bu – bursa; cu – cutis; dm – deltoid muscle; fa – fascia; gl – glenoid; hc – hyaline cartilage; hh – humeral head; im – infraspinatus muscle; it – infraspinatus tendon; jc – joint capsule/ligament; la – labrum; sc – scapula; sgn – spinoglenoid notch; su – subcutis

Fig. 5.

Posterior transverse. A. Gray-scale ultrasonography posterior transverse image in external rotation. B. Grayscale ultrasonography posterior transverse image in internal rotation. C. Anatomical image. D. Histological image. bu – bursa; cu – cutis; dm – deltoid muscle; e – enthesis; fa – fascia; gl – glenoid; hc – hyaline cartilage; hu – humeral head; im – infraspinatus muscle; it – infraspinatus tendon; jc – joint capsule/ligament; la – labrum; sc – scapula; sgn – spinoglenoid notch; su – subcutis

Fig. 6.

Posterior transverse. A, B. Gray-scale ultrasonography posterior transverse image in internal rotation. C. Anatomical image. D. Histological image. arrow – enlarged view of the cutout (box); bu – bursa; cu – cutis; dm – deltoid muscle; e1 – enthesis of the joint capsule/glenohumeral ligament; e2 – enthesis of the infraspinatus tendon; fa – fascia; gl – glenoid; hc – hyaline cartilage; hu – humeral head; im – infraspinatus muscle; it – infraspinatus tendon; jc – joint capsule/ligament; la – labrum; su – subcutis

Fig. 7.

Posterior longitudinal. A. Gray-scale ultrasonography posterior longitudinal image. B. Gray-scale ultrasonography posterior longitudinal in internal rotation. C. Gray-scale ultrasonography posterior transverse (left) and longitudinal (right). D. Gray-scale and Power-Doppler ultrasonography posterior longitudinal. E. Anatomical image. axn – axillary nerve; ca –glenohumeral joint cavity; cu – cutis; dm – deltoid muscle; fa – fascia; gl – glenoid; gt – posterior facet of the greater tubercle; im – infraspinatus muscle; it – infraspinatus tendon; la – labrum; pca – posterior circumflex artery; su – subcutis; spi – scapular spine; tma – teres major muscle; tmm – teres minor muscle; tmt – teres minor tendon; trm – triceps muscle; trt –triceps tendon; qs – quadrilateral space

While the teres minor tendon connects to both the surgical neck of the posterior humerus and the inferior facet of the greater tuberosity, the infraspinatus tendon is predominantly bound to the middle facet⁽⁷⁾. From the infraspinatus fossa, the infraspinatus muscle and the teres minor muscle may be visualized lower down. The probe is advanced laterally to demonstrate the myotendinous junction, and then the tendon until it reaches the insertion at the greater tuberosity. To better see the hyaline cartilage, bone, joint capsule, glenohumeral ligament, and separated enthesis of the tendons more clearly, it is useful to place the patient’s hand on the opposite shoulder. When the arm is kept in the maximum external rotation position, special attention should be given to the dorsal and inferior recess of the joint capsule, since intra-articular effusion is frequently observed in this position⁽⁵⁾. During the external rotation of the shoulder, the posterior capsule-synovial recess of the glenohumeral joint shows a “fingershaped retroflection” in between the infraspinatus muscle/tendon and the posterior labrum. This sonographic finding is very important because it can be useful to both assess the intra-articular effusion and confirm the stiffness of the joint^(8). It is also frequently seen that during vigorous internal rotation, the posterior/inferior (axillary) recess fills up considerably. From the biomechanical point of view, during an additional ABER (abduction + external rotation) maneuver, the posterior capsule “rolls up” over the posterior labrum/glenoid⁽⁹⁾. This dynamic maneuver can be used to assess the eventual presence of internal posterior impingement of the shoulder with pinching of the posterior glenohumeral joint recess (capsule and synovium) and the deep fibers of the infraspinatus between the humeral head and glenoid of the scapula. To see the inferior glenohumeral ligament, the joint capsule, and the axillary recess, move the probe caudally. Examining the transition from muscle to tendon is aided by longitudinal sonograms of the muscles in the infraspinatus fossa and more distally at the level of the teres minor muscle. When evaluating the infraspinatus in the long axis, it is possible to observe bursitis, glenohumeral synovitis, and tears of the infraspinatus tendon at the insertion level⁽¹⁰⁾. The axillary nerve⁽¹¹⁾ and posterior humeral circumflex artery in the quadrilateral space, as well as the triceps muscle in the inferior section of the glenoid, can be observed (Fig. 7, Fig. 8). The quadrilateral space of Velpeau area is bounded by the triceps brachii long head medially, the surgical neck of the humerus laterally, the teres major inferiorly, and the teres minor superiorly⁽¹²⁾.

Fig. 8.

Posterior longitudinal overview. A, B. Gray-scale ultrasonography posterior longitudinal panoramic image. C, D. Anatomical image. bu – bursa; cl – clavicle; cu – cutis; dm – deltoid muscle; fa – fascia; gl – glenoid; im – infraspinatus muscle; it – infraspinatus tendon; sc – scapula; sgn –spinoglenoid notch; spi – scapular spine; su – subcutis; sum – supraspinatus muscle; tmm –teres minor muscle; tmt – teres minor tendon; tr – trapezius muscle; trm – triceps muscle; trt – triceps tendon; qs – quadrilateral space

Standard scans of the frontal shoulder (Tab. 4) (Fig. 9)

Fig. 9.

Probe positions, frontal/coronal

Anatomical structures of the frontal shoulder (Tab 5.)

The probe is advanced from the posterior shoulder region to the frontal (coronal) level and positioned across the top of the shoulder in the coronal plane⁽¹³⁾. The acromioclavicular (AC) joint is typically simple to locate (Fig. 10). As an alternative approach, the probe is placed over the palpable clavicle; subsequently, the ultrasound transducer is shifted laterally in the direction of the acromion to the AC joint⁽¹⁴⁾. The superficial AC ligament, joint capsule, disc-like fibrocartilage, and surrounding bone structures are evaluated⁽¹⁵⁾. We suggest to continuously scan in both longitudinal and transverse planes. Observe the acromion and the transition to the scapular spine. The subacromial portion of the subdeltoid bursa, the greater tuberosity where the supraspinatus tendon inserts, the whole superior part, and the lateral humeral head are evaluated at the frontal level. In certain cases – and using deeper probe frequencies (<12 MHz) – the supraspinatus tendon, the subacromial-subdeltoid bursa, and the deep (inferior) AC ligament can be seen by scanning through the AC joint. When performing a dynamic movement (such as an abduction), the supraspinatus tendon may be seen moving through the AC joint window. The supraspinatus fossa is reached by moving the probe posteriorly and medially from the AC joint (Fig. 11). The supraspinatus fossa is concave⁽¹⁶⁾, wider at the medial than at the lateral end, and smaller than the infraspinatus fossa. The supraspinatus muscle originates from its medial two-thirds⁽¹⁷⁾. Here, you may also evaluate the subacromial portion of the subdeltoid bursa under the trapezius muscle, the supraspinatus muscle, the myotendinous junction, the superior glenoid, the biceps-labral complex^(18–21), the joint capsule, the suprascapular notch, and the superior labrum.

Fig. 10.

Coronal region. A, B. Gray-scale ultrasonography coronal transverse image. C. Gray-scale ultrasonography coronal longitudinal image. D. Anatomical image of the AC joint. E. Histological image of the superior part of the labrum and the glenohumeral joint. acj – acromioclavicular joint cavity; acr – acromion; cl – clavicle; cu – cutis; dm – deltoid muscle; hc – hyaline cartilage; hu – humeral head; iacl – inferior acromioclavicular ligament; jc – joint capsule; la – labrum; sacl – superficial acromioclavicular ligament; su – subcutis; sum – supraspinatus muscle; sumt – supraspinatus tendon; sun – suprascapular notch

Fig. 11.

Coronal views. A. Gray-scale ultrasonography coronal transverse panoramic image. B. Gray-scale ultrasonography coronal longitudinal image. C. Grayscale ultrasonography coronal transverse image. D. Power-Doppler ultrasonography coronal transverse image. E. Anatomical image. acr – acromion; a – artery; cl – clavicle; cu – cutis; dm – deltoid muscle; fa – fascia; hu – humeral head; sc – scapula; spi – scapular spine; ssn – suprascapular nerve; su – subcutis; sum – supraspinatus muscle; sumt – supraspinatus tendon; sun – suprascapular notch; v – vein

The shoulder is particularly accessible for ultrasonic imaging since the joint is covered by relatively few osseous structures. A thorough understanding of sonoanatomy is necessary to provide an accurate structural evaluation, which is the goal of high-resolution musculoskeletal ultrasonography of the shoulder. In optimal conditions, modern high-resolution musculoskeletal ultrasound can improve shoulder imaging by correlating structures to cross-sectional anatomy and detailed histology. This atlas provides an overview of the normal findings.