The use of ultrasound in the assessment of the glenoid labrum of the glenohumeral joint. Part I: Ultrasound anatomy and examination technique

Abstract

The glenohumeral joint is a spherical articulation with a remarkable range of motion in several planes and decreased stability. The maintenance of joint stability is influenced by the functioning of specific muscle groups in the shoulder region, a complex system of ligaments reinforcing the joint capsule, and the labrum which augments the glenoid fossa. Lesions of the aforementioned structures require accurate diagnosis prior to a decision for operative treatment. Ultrasound is one of the imaging methods that has been widely used in the assessment of various shoulder pathologies. In the author opinion, this imaging modality may also be applied for the evaluation of labral tears. Being attached along the glenoid rim, the labrum forms a collar deepening the glenoid fossa thus increasing area of its contact with the head of the humerus. To better describe the location of lesions, the glenoid labrum is usually divided into certain zones. Most of them may be visualized sonographically. The US examination of the posterior labrum can be performed during evaluation of the infraspinatus and teres minor muscles. The anterior labrum along with capsulolabral complex is seen at the glenoid edge under the subscapularis tendon. Sonographic examination of the inferior labrum is best performed using axillar approach. The superior labrum is only partially available for US examination. A crucial part of the sonographic assessment of the labrum is the dynamic examination during rotation of the upper extremity. The paper presents normal sonographic anatomy of the glenoid labrum and technique of the examination.

Introduction

The glenohumeral joint (in short referred to as the shoulder joint) is a ball-and-socket joint between the head of the humerus (the ball) and the glenoid fossa of the scapula (the socket). The joint surface area of the humeral head is approximately fourfold greater than that of the glenoid cavity; this is often compared to a golf ball resting upon the tee. Thanks to such morphology, a large range of motion of the shoulder joint is possible in several planes, augmented by the sliding of the scapula with respect to the thoracic wall, as well as rotation within the acromioclavicular and sternoclavicular joints. This tremendous range of movement also makes the shoulder less stable and far more prone for dislocation and injury than other joints. However, during movements in the shoulder under normal circumstances, there exists a dynamic balance between the forces pushing the humeral head onto the glenoid fossa versus those forces pulling it away. The stability of the joint depends upon both active and passive mechanisms. Active stabilization is related to muscle function, most importantly the rotator muscles and the tendon of the long head of the biceps brachii muscle. The tendons of the rotator muscles form a characteristic “cuff” surrounding the joint, along with the rotator cable of the joint capsule. Passive stabilization comes from the shape, size and positioning of the glenoid, as well as the presence of a capsular-ligamentous complex, and the glenoid labrum. The former plays a significant role. Being attached along the glenoid rim, the labrum forms a collar deepening the glenoid fossa and increasing the area of its contact with the head of the humerus (fig. 1). This allows for the maintenance of negative intra-articular pressure, which is essential for joint stability^{(1, 2)}. Histologically, the glenoid labrum is composed of fibrocartilaginous tissue with a varied course of collagen fibers, mainly in a circular arrangement, reinforced in the external zone by fibers from the glenohumeral ligaments, joint capsule and surrounding tendons. Numerous collagen fibers run through the fibrocartilaginous matrix, containing small lacunae with chondrocytes. There may also appear a few elastin fibers within the glenoid labrum. The labrum is vascularized by branches of the supra-scapular, the circum-scapular, and the posterior circumflex arteries, which reach it from the capsule and the periosteum near the bony attachment. These vessels mainly supply the peripheral area of the glenoid labrum, while the vascularization of its middle part is rather poor. Similarly to menisci the vascularization of the glenoid labrum decreases with age^{(3, 4)}. In its inferior half, both in the anterior and posterior parts, the labrum forms a continuation of the joint cartilage of the glenoid, with a transitional fibrocartilaginous zone. This explains the significantly limited mobility of this part of labrum. The outline of the labrum in this part is usually slightly rounded, although there exist variable anatomic variants in the shape of the labrum⁽⁵⁾. Therefore, even irregular contours of the labrum should not be primarily considered pathological. On the other hand, the superior half of the glenoid labrum has a slightly less dense fibrous structure, a looser bony attachment, and is thus more mobile. In the transverse section, the labrum has a shape resembling the meniscus of the knee joint. In its superior half, the glenoid labrum becomes firmly attached to the tendon of the long head of the biceps (which also attaches to the supraglenoid tubercle of the scapula), forming a common biceps-labral complex with the interweaving of collagen fibers of both structures⁽³⁾. At the level of fusion of both structures, between the labrum and the glenoid rim, there exists a small synovial recess (sublabral recess)^{(1, 5)}. In the anterosuperior area, there may be a complete discontinuity between the labrum and the glenoid (sublabral foramen), so that the joint communicates with the subscapularis recess⁽¹⁾. There may also be a lack of anterosuperior labrum combined with a cordlike thickening of the medial glenohumeral ligament (Buford complex)⁽⁶⁾.

Fig. 1.

A scheme of the capsular-ligamentous-labral complex with the rotator cuff of the shoulder. Gl – the glenoid, L – the labrum, LHB – the tendon of the long head of the biceps brachii muscle; the superior – SGHL, middle – MGHL, and inferior – IGHL glenohumeral ligaments, CHL – the coracohumeral ligament, CAL – the coracoacromial ligament, Sub – the subscapularis muscle, Sup – the supraspinatus muscle, Is – the infraspinatus muscle, TM – the teres minor muscle (courtesy of: Michael Stadnick, Radsource)

The peripheral part of the glenoid labrum connects to the joint capsule and the glenohumeral ligaments, to form an anatomical-functional unit, called the labral capsular ligamentous complex⁽¹⁾. The glenoid attachment of the anterior joint capsule exhibits certain anatomical variants. In more than 50 percent of cases, the anterior capsule attaches directly to the labrum and the adherent part of the glenoid. In the remaining cases, the insertion of the anterior capsule is located beyond the labrum, on the scapular neck (usually within 1 cm from the edge of the glenoid, rarely more medially). The posterior capsule is usually attached directly to the glenoid labrum^{(2, 5, 7, 8)}. The glenohumeral ligaments also join the glenoid labrum. The superior glenohumeral ligament (SGHL) usually inserts to the superior part of the labrum and partially to the supraglenoid tubercle of the scapula. The middle glenohumeral ligament (MGHL) attaches at the level of the SGHL insertion or right below it, either to the glenoid, the labrum, or has a common origin with the SGHL. Sometimes the MGHL may be a very prominent structure (as in the Buford complex), but there are also cases in which this ligament is completely missing⁽⁵⁾. The inferior glenohumeral ligament (IGHL) consists of two bands – the anterior and posterior, has distinct attachments to the labrum and the glenoid, and significantly reinforces the joint capsule⁽¹⁾.

To better describe the location of lesions, the glenoid labrum is usually being divided into certain zones. There are two main alternative mapping methods used for this purpose. One is based upon a clock face with 12 zones of the labrum, whereas the simpler and more practical second method, divides the labrum into 6 zones⁽⁹⁾ (fig. 2). Both scales are quite similar to each other and both distinguish the following labral zones (going from superior to inferior anteriorly, clockwise for the right side, and counterclockwise for the left):

• the superior labrum (between 11 and 1 o'clock);

• the anterosuperior labrum (between 1 and 3 o'clock for the right shoulder, while between 11 and 9 o'clock for the left one);

• the anteroinferior labrum (between 3 and 5 o'clock for the right shoulder, while between 9 and 7 o'clock for the left one);

• the inferior labrum (between 5 and 7 o'clock);

• the posteroinferior labrum (between 7 and 9 o'clock for the right shoulder, while between 5 and 3 o'clock for the left one);

• the posterosuperior labrum (between 9 and 11 o'clock for the right shoulder, while between 3 and 1 o'clock for the left one).

Fig. 2.

The topographical division of the glenoid labrum into 6 areas⁽⁸⁾

Sometimes the labrum's superior half is further divided into superoanterior and superoposterior parts, while the inferior half into inferoanterior and inferoposterior parts. This subdivision has more relevance for the superior labrum which is more frequently injured (SLAP type injuries), whereas injuries to the inferior labrum alone are extremely rare. An even simpler topographical description divides the labrum into 4 zones (quadrants): the anterosuperior, anteroinferior, posteroinferior and posterosuperior ones⁽¹⁰⁾. This classification may be used in diagnosing anterior and posterior instability of the shoulder, but is insufficient when describing injuries of the superior labrum.

The main imaging modality used to assess the glenoid labrum along with glenohumeral ligaments is magnetic resonance (MRI), especially with intra-articular administration of paramagnetic contrast medium (MR arthrography)^{(2, 5–7, 11)}. Sometimes computed tomography arthrography (CT arthrography) might be an option⁽⁶⁾. Based upon published data, as well as the author's personal experience of several years, ultrasound (US) may also be valuable in the evaluation of the glenoid labrum^{(10, 12–15)}. For many years, ultrasound has been widely used for diagnosing various pathologies of the shoulder, and its role is still increasing with further technological progress of this imaging tool^(16–18). The US study has an indisputable role, particularly in diagnosing rotator cuff pathologies. The addition of an assessment of the glenoid labrum to the routine US imaging of the shoulder would make this examination even more complete, and influence further diagnostic-therapeutic proceedings.

Ultrasound anatomy and examination technique of the glenoid labrum

For the US examination of the glenoid labrum, a linear probe with a frequency of 6–12 MHz (the broadband probes are the best) is used, since it provides an appropriate resolution and penetration to the depth of a few centimeters. In obese patients or those with increased muscle mass, a probe of 5–6 MHz frequency (for example the convex type) may be used. Unfortunately, the latter does not provide enough resolution for the evaluation of small post-traumatic changes of the labrum. The glenoid labrum, being a fibrous structure, exhibits the anisotropy phenomenon, as the joint capsule does. Both the anterior and posterior parts of the labrum are located at a depth usually not exceeding 4.0–5.0 cm, sometimes less (between 2.5 and 3.5 cm in the author's experience). This depth varies, depending on the thickness of the subcutaneous fat tissue and the muscle “cuff”. Contrary to the suggestions of some authors⁽¹⁸⁾, it seems not to be true that the anterior part of the labrum is significantly more deeply located than its posterior part, which would make it less accessible for the US study. The difference in depth does not exceed a few millimeters, and has in author's opinion little practical significance.

The posterior part of the glenoid labrum

The US examination of the posterior part of the labrum is usually carried out with the evaluation of the infraspinatus and teres minor muscles. The patient sits comfortably on a rotating chair, with his/her back to the examiner. The main imaging plane is that perpendicular to the long axis of the forearm (axial cross-sections) (fig. 3). Both the posterior part of the humeral head and the glenoid rim should be first visualized. The labrum is usually seen as a triangular echogenic structure, located right alongside the edge of the glenoid. It may be slightly rounded from the joint side, or may be flat, which is another anatomic variant. The joint capsule, which lies deep to the tendons of the posterior rotator cuff muscles, attaches to the labrum directly (fig. 4).

Fig. 3.

Positioning of the US probe for the evaluation of the posterior part of the labrum

Fig. 4.

The posterior labrum (*), the joint capsule (arrows); H – the head of the humerus, Gle – the glenoid, ISP – the infraspinatus muscle

The labrum should be scanned as much as possible, from the level right below the scapular spine to the lower edge of the glenoid. The labrum is inaccessible to US imaging at the level of the scapular spine. The dynamic assessment is an important aspect of the US examination, and consists of alternating internal and external rotation of the upper extremity in the adduction position. In internal rotation, the tightening of the joint capsule may be observed, with slight pulling on the glenoid labrum. In external rotation, the joint capsule gets lax, while the labrum becomes slightly flattened and minimally bends in or out. If there is an effusion in the joint, a more or less filled posterior recess of the joint capsule is usually visible during external rotation. It extends medially from the glenoid rim and labrum along the scapular neck (fig. 5). Incidentally, it is possible to visualize even small amounts of fluid in this location in the course of shoulder diseases (a second such place is the tendon sheath of the long head of the biceps).

Fig. 5.

The bulging joint capsule (arrows) with anechoic fluid filling the posterior recess of the joint (+), above the external outline of the glenoid (Gle) and the labrum (*); H – the head of the humerus, ISP – the infraspinatus muscle, D – the deltoid muscle

The anterior part of the glenoid labrum

The US examination of the anterior part of the labrum is somewhat more difficult and requires some practice. It is best to examine the patient in the supine position, as this ensures better stability of the probe, particularly during the dynamic assessment. The main imaging plane is that perpendicular or slightly oblique to the long axis of the arm (axial cross-sections) (fig. 6). Both the anterior part of the humeral head and the glenoid should be visualized. The structure of the anterior capsular-labral complex is more complicated than the posterior equivalent, thus their US image differs. The labrum is visible at the edge of the glenoid as an echogenic structure adjacent to the joint capsule, which is thicker than in the posterior part because of the presence of the anterior band of the IGHL (fig. 7). The echogenicity of the joint capsule is similar to that of the labrum, thus it is difficult to differentiate these structures, although sometimes during the dynamic examination the joint capsule may slide against the labrum. Sometimes at the base of the labrum, there may be recognizable a thin linear area (width <2 mm) of lower echogenicity, which represents the transitional layer of fibrocartilage between the joint cartilage and the labrum. The assessment of the anterior labrum should be started from the highest part visible in the study (just below the level of the coracoid process of the scapula), and continued to the inferior edge of the glenoid. In the anterosuperior part, the labrum's shape and echogenicity are quite similar to the menisci of the knee. The labrum is inaccessible to US imaging at the level of the coracoid process of the scapula, while the lower portion of the anterosuperior (approximately from 2/10 o'clock) and the entire anteroinferior parts of the labrum are usually well visible. As mentioned previously, the dynamic assessment is an important element of the US examination, and is performed during alternate external and internal rotation of the upper extremity in the adduction position as well as in 90° abduction (fig. 8). In external rotation the joint capsule gets taut and the labrum slightly pulled, whereas in internal rotation the capsule becomes loosened. The labrum changes its shape insignificantly in the dynamic examination. In normal conditions during internal rotation of the upper limb, a minimal external deviation of the labrum may be noticed, under the pressure of the humeral head, with accompanying creasing of the capsularligamentous structures (fig. 9). The aforementioned “pressure” effect of the humeral head on the labrum with loosening of the joint capsule is used by the author to determine labral stability after trauma. This approach differs from those presented in the scientific literature, where such assessment has been performed usually during external rotation^{(10, 13)}.

Fig. 6.

Positioning of the US probe for the evaluation of the anterior labrum

Fig. 7.

The capsular-labral complex in the anterior part of the glenohumeral joint (arrows): the glenoid labrum (asterisk), the anterior band of the IGHL (+), H – the head of the humerus, Gle – the glenoid, SSC – the subscapularis muscle

Fig. 8.

The dynamic US examination of the anterior labrum, with the limb in adduction and abduction of 90°, in external rotation (A, C) and internal rotation (B, D). respectively

Fig. 9.

The dynamic USG examination of the anterior labrum. In external rotation (A), the tension of the joint capsule is visible (arrows) with the labrum being pulled (*). Whereas in internal rotation (B), the joint capsule gets loosened (arrows) while the labrum gets rounded and slightly deviates (*); H – the head of the humerus, Gle – the glenoid, SSC – the subscapularis muscle

The inferior part of the glenoid labrum

The US examination of the inferior part of the glenoid labrum is performed through the axillary approach with lying patient having the upper limb in abduction. The dynamic component of the study takes place during internal and external rotation (fig. 10). The glenoid labrum is found most superficially from the axillary fossa. Despite this fact, the dynamic assessment of the inferior labrum may be challenging due to the rather thick capsular part of the capsular-labral complex, and difficulties in obtaining an optimal imaging cross-section (fig. 11). However, isolated injuries to the inferior part of the labrum are very rare. Sometimes it is possible to visualize posttraumatic changes of the IGHL (ex. HAGL type injuries) using axillar approach, as well as the presence of loose bodies in the axillary recess of the glenohumeral joint.

Fig. 10.

The US examination of the labrum from the axillary fossa approach with the limb in 90° abduction and being externally rotated (A) or internally rotated (B)

Fig. 11.

The inferior labrum (*), the capsular-ligamentous complex (+ + +), H – the head of the humerus, Gle – the glenoid, SSC – the subscapularis muscle

The superior part of the glenoid labrum

The US study of the superior part of the labrum is somewhat problematic due to limited access to the parts of the labrum covered by bony structures (the acromion and the clavicle) as well as difficulties in good stabilizing the probe during the dynamic examination. The examination is performed with the patient sitting. The anterior part of the superior labrum (between 12 and 1 o'clock for the right shoulder, and 11 and 12 o'clock for the left one) is usually accessible for the assessment. The probe is applied where a minor depression of tissues is palpated, anterior to the distal part of the clavicle and medial to the anterior part of the acromion (fig. 12 A). This position of the probe should allow for visualization of the humeral head and the upper part of glenoid with adjacent triangular echogenic structure representing the labral-bicipital complex, consisting of the labrum on the bottom and the overlying tendon of the long head of the biceps. Above this complex the anterior part of the tendon/cord tendon-belly of the supraspinatous muscle is present (fig. 13). In the author's experience, the labrum is best visualized with the upper limb internally rotated and adducted (ex. the Middle- ton/Crass position used to assess the tendon of the supraspinatus muscle) (fig. 12 B). After the labrum is identified, the dynamic examination is performed, relying on alternating rotations of the limb in adduction and abduction, while trying to maintain the US probe stable. The structures are also evaluated during passive pulling of the upper limb downward.

Fig. 12.

Positioning of the US probe for the evaluation of the superior labrum from the anterior aspect (A); the optimal position of the upper limb for visualizing the superoanterior part of the labrum (B)

Fig. 13.

The labral-bicipital complex in the superior part of the glenohumeral joint (arrows): the labrum (+), the tendon of the long head of the biceps (*), H – the head of the humerus, Gle – the glenoid, Cla – the acoustic shadow of the clavicle, SSP – the tendon/belly of the supraspinatus muscle, D – the deltoid muscle

In a favorable configuration of the acromion and in slender subjects, the posterior part of the superior labrum may also be imaged at 11 o'clock (for the right shoulder) and 1 o'clock (for the left). The probe is positioned right behind the clavicle, medial to the acromion of the scapula (fig. 14 A). The outlines of the superior part of the glenoid and the labrum should be visualized, as well as a small part of the humeral head that is not covered by the acoustic shadow of the acromion. Above these described structures, the supraspinatus muscle is found (fig. 15). The dynamic examination relies on external and internal rotation of the limb in 90° abduction (fig. 14 B). The US examination in this region is also used for diagnosing an internal posterosuperior glenoid impingement of the shoulder.

Fig. 14.

Positioning of the US probe for the evaluation of the superior labrum from the posterior aspect (A); the positioning of the upper limb for the dynamic examination (B)

Fig. 15.

The superior labrum from the posterior aspect: the labrum (*), Gle – the glenoid, H – the head of the humerus, Ac – the acoustic shadow of the acromion, SSP – the supraspinatus muscle

Summation

The diagnostics of injuries to the glenoid labrum and the capsular-ligamentous complex of the glenohumeral joint, aside from the clinical assessment, is mainly based upon the MRI, especially after intraarticular contrast administration (MR arthrography). As this article has shown, the glenoid labrum is also visible in the US study. Therefore, this imaging tool may be valuable in the diagnostics of labral pathologies, which will be presented in part II of the article.