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. 2021 Nov 1;240(4):761–771. doi: 10.1111/joa.13582

Anatomical, functional and biomechanical review of the glenoid labrum

Yousef A Almajed 1,2,✉, Andrew C Hall 3, Thomas H Gillingwater 1,3, Abduelmenem Alashkham 1
PMCID: PMC8930820  PMID: 34725812

Abstract

The glenohumeral joint is the most mobile joint in the human skeleton, supported by both active and passive stabilisers. As one of the passive stabilisers, the glenoid labrum has increasingly been recognised to play an important role in stability of the glenohumeral joint, acting to maintain intraarticular pressure, centralise the humeral head and contribute to concavity‐compression stability. Several studies have investigated the macro‐ and micro‐anatomical features of the labrum as well as its biomechanical function. However, in order to better understand the role of the labrum and its mechanics, a comprehensive anatomical, functional and biomechanical review of these studies is needed. Therefore, this article reviews the current literature detailing anatomical descriptions of the glenoid labrum, with an emphasis on its function(s) and biomechanics, as well as its interaction with neighbouring structures. The intimate relationship between the labrum and the surrounding structures was found to be important in glenohumeral stability, which owes further investigation into the microanatomy of labrum to better understand this relationship.

Keywords: anatomy, biomechanics, glenohumeral joint, glenohumeral ligaments, glenoid labrum

As one of the passive stabilisers, the glenoid labrum has increasingly been recognised to play an important role in stability of the glenohumeral joint, acting to maintain intraarticular pressure, centralise the humeral head and contribute to concavity‐compression stability. Several studies have investigated the macro‐ and micro‐anatomical features of the labrum as well as its biomechanical function. However, in order to better understand the role of the labrum and its mechanics, a comprehensive anatomical, functional and biomechanical review of these studies is needed.

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1. INTRODUCTION

The glenohumeral joint is a synovial, ball‐and‐socket joint, which is formed by the humeral head laterally and the glenoid fossa of the scapula medially. By possessing an extensive range of movements, the glenohumeral joint sacrifices stability for greater mobility, especially when compared with other large synovial joints, such as the hip, elbow and knee joints (Barnes et al., 2018; Jobe et al., 2018; Lippitt & Matsen, 1993). However, the glenohumeral joint is provided with some semblance of stability by a combination of ligaments, a fibrous joint capsule and surrounding skeletal muscles (Felli et al., 2012; Moore et al., 2015; Warner et al., 1999). The glenoid fossa is an inverted comma shape, which articulates with the humeral head and is comparatively smaller and flatter than the humeral head (McPherson et al., 1997; Moore et al., 2015). Importantly, the glenoid fossa is deepened by the presence of a fibrocartilaginous labrum—the glenoid labrum—which is attached around the entire perimeter of the glenoid fossa and has three sides: one facing the humeral head, another the glenoid bone and another the fibrous joint capsule (Clavert, 2015). The exact in vivo functions of the glenoid labrum are yet to be fully established, as it has been postulated to enhance shoulder stability by: (a) increasing the depth of the glenoid concavity; (b) increasing the articular surface area of the joint; (c) centralising the humeral head and (d) acting as a joint ‘seal’ to maintain intra‐articular pressure (Clavert, 2015; Habermeyer et al., 1992; Hill et al., 2008; Kumar & Balasubramaniam, 1985; Moore et al., 2015; Smith & Funk, 2010; Standring, 2016). Several studies have been undertaken to determine either the macro or micro‐anatomy of the labrum and/or its biomechanical function (in health or following injury), in an attempt to construct a detailed knowledge of its structure and function. Nevertheless, no definitive, comprehensive overview bridging macroanatomy, microanatomy, biomechanics and responses to injury have been forthcoming. Therefore, this article reports the current literature concerning the glenoid labrum and its attached structures, aiming to provide a more cohesive understanding of the role of the glenoid labrum in joint stability during normal function and following injury.

2. GLENOID FOSSA/CAVITY OF THE SCAPULA

The glenoid aspect of the scapula is commonly described as being elliptical, or pear‐shaped, due to the presence of the glenoid notch in the anterior margin of the glenoid cavity (Checroun et al., 2002). Furthermore, the glenoid fossa is longer in the superior‐inferior plane, with a height normally ranging from 31.2 mm to 50.1 mm compared with an anteroposterior width ranging from 22.6 mm to 41.5 mm (Checroun et al., 2002; Misir et al., 2019). The surface area of the glenoid ranges from 570.6 mm2 to 1316.3 mm2, which covers only around 25% to 30% of the humeral head during joint motion at the glenohumeral joint (Checroun et al., 2002; Misir et al., 2019). Zumstein et al. (2014) reported that the radius of curvature of the osseous glenoid is 51.5 mm in the maximum anteroposterior plane, whereas the superior‐inferior plane is only 33.7 mm, resulting in superior‐inferior plane that has a more defined curvature than the anteroposterior plane.

3. LABRAL GROSS ANATOMY AND BIOMECHANICS

The glenoid labrum can be considered as a ring‐like structure, as it encompasses the entire circumference of the glenoid fossa. Embryologically, the glenoid labrum forms around the 8th week of development, with the long head of biceps brachii and the glenohumeral joint capsule having a clearly defined insertion on the labrum by the 9th week (Hita‐Contreras et al., 2018). The glenoid fossa has a shallow depth in order to accommodate the more curved humeral head (Howell & Galinat, 1989; McPherson et al., 1997). Howell and Galinat (1989) reported that the superior to inferior glenoid depth was 41% of the overall depth of the radius of the humeral head, and was much shallower in the anterior to posterior regions. However, the depth and surface area of the rather shallow glenoid articular surface is increased through the presence of the labrum (Cooper et al., 1992). This, in turn, increases the area of the glenoid articular surface (Hata et al., 1992; Lippitt & Matsen, 1993), deepening the glenoid fossa by approximately 50% (Howell & Galinat, 1989). This points to an important role for the labrum in increasing the concavity of the fossa, as well as the congruity of the glenoid socket to the larger humeral head.

The labrum can be divided into distinct anatomical sub‐regions for ease of description and localisation, using one of two common approaches. In the first approach, an analogue (e.g., 12 h) clock face‐based approach is employed, where a clock face is superimposed on a lateral view of the glenoid fossa, with the 12 o'clock position directed superiorly and the 3 o'clock position directed anteriorly. The second approach uses anatomical directions that are based on either four regions (comprising superior, anterior, inferior and posterior) or eight regions (where anterosuperior, anteroinferior, posteroinferior and posterosuperior are added) (Chang et al., 2012; Cooper et al., 1992) (Figure 1).

FIGURE 1.

FIGURE 1

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Anatomical divisions of the glenoid fossa. A lateral view of a right glenohumeral joint (with the humerus removed) illustrating the anatomical and the ‘o'clock’ based localisation methods commonly used to subdivide the labrum. The 12 ‘o'clock’ position is used to identify the position of the superior labrum. Illustration adapted from Dekker et al. (2020)

The gross morphology of the glenoid labrum is known to exhibit biological variations around its circumference. For example, Cooper et al. (1992) reported that the superior region of the labrum has a meniscal‐like appearance, in that it is loosely attached to the glenoid rim and has a more triangular cross‐sectional shape. These authors also reported that the anterosuperior region of the labrum is similar to the superior labrum in shape; however, a similar attachment to the glenoid was found in only five specimens (45% of the total sample), being detached in four specimens (36%) and firmly attached in two others (18%). The posterosuperior labrum, on the other hand, was found to present with either a meniscal shape, being loosely attached to the glenoid bone, or a more rounded shape that is firmly attached. Two arthroscopic studies evaluated the anatomical appearance of the labrum and reported that the superior labrum may present with a triangular, flat, meniscoid or rounded shape (Clavert et al., 2005; Davidson & Rivenburgh, 2004). Triangular and meniscoid‐shaped morphology was commonly associated with a physiologically detached or loosely attached labrum. The inferior labrum is more anatomically consistent, with a rounded shape, slight lateral extension and a firm attachment to the glenoid bone (Cooper et al., 1992).

In an investigation to measure the thickness (articular side) and height (capsular side) of the glenoid labrum (Figure 2), Alashkham et al. (2019) reported that the superior region of the labrum was the tallest and thickest (mean: 5.96 mm and 6.02 mm, respectively), with the anterior region being the shortest and thinnest (mean: 3.63 mm and 3.94 mm, respectively). The larger superior region was attributed to the contribution provided by the tendon of the long head of biceps brachii. Likewise, the depth of the glenoid fossa, from the centre point of the glenoid cavity in relation to the edge of the labrum, was measured by Hata et al. (1992), who found that the inferior region of the labrum was deeper than the superior labrum (3.8 mm vs. 3.0 mm, respectively), meaning that it extended laterally more extensively than the superior region. However, no significant difference was noted between anterior and posterior regions with regards to their depth (Hata et al., 1992). The lateral extension seen at the inferior labrum can most likely be attributed to its ability to facilitate the centralisation of the humeral head within the glenoid socket by limiting inferior shift of the humeral head.

FIGURE 2.

FIGURE 2

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Schematic overview of the positioning, thickness and height of the labrum. A cross section of the triangular labrum (L) attaching on both the glenoid bone (B) and the articular cartilage (AC) illustrating the positioning, thickness and height of the glenoid labrum, as described in Alashkham et al. (2019)

Despite our growing understanding of regional labral variability, it is still unclear how labral morphology directly impacts on its biomechanical functions. The increase in the depth of the glenoid socket might have an impact on concavity‐compression stability and assist in maintaining the negative intra‐articular pressure. Concavity‐compression stability refers to a biomechanical model whereby a deeper concavity leads to a higher resistance to sliding of an object pressed into the concavity (Lippitt & Matsen, 1993). Halder et al. (2001) investigated the role of the labrum in concavity‐compression stability at the glenohumeral joint by assessing the contribution of the labrum to the concavity of the glenoid socket, combined with the compressive force generated by the rotator cuff muscles over the humeral head and into the glenoid concavity. This study found that the glenoid labrum contributes around 10% in concavity‐compression stability to the glenohumeral joint, thereby assisting other factors that contribute to joint stability, including muscular compression of the humeral head into the glenoid fossa (Halder et al., 2001; Lippitt & Matsen, 1993). Importantly, humeral stability was found to be decreased, particularly in the inferior direction, by around 20% after labral resection (Halder et al., 2001). Anatomically, this can be attributed to the inferior region’s lateral extension, as well as its larger fibrocartilaginous area, causing a higher opposing translational force of the humeral head (Hata et al., 1992; Yoshida et al., 2015). As for intra‐articular pressure, Habermeyer et al. (1992) found that in shoulders with anterior labral tears the negative intra‐articular pressure was not maintained, in contrast with shoulders with an intact labrum. This study reported that during surgery on patients without labral tears (n = 15), the intra‐articular pressure was maintained in neutral traction even when additional traction was applied to the arm (mean: −32 mmHg and −133 mmHg, respectively). However, in unstable shoulders (n = 17) that exhibited anterior labral tears, the intra‐articular pressure was not maintained under either neutral and traction‐applied conditions (mean: 0 mmHg and −2 mmHg, respectively).

By increasing the concavity of the glenoid fossa, the glenoid labrum is also thought to play an important role in centralising the humeral head, decreasing the contact area on the chondral plate. A biomechanical study, using a flexible tactile force sensor, investigated the effect of excising the anteroinferior labrum on the humeral head contact area, as well as joint pressure, on eight fresh‐frozen cadavers (Greis et al., 2002). It was found that the humeral contact area decreased by an average of 9% (range: 7% to 15%); the authors suggested that this change was due to the loss of surface area and an anteroinferior shift of the humeral head. The reduced contact area was accompanied by an increase in pressure on the glenoid socket by an average of 12% (range: 8% to 20%) (Greis et al., 2002). An anteroinferior shift of the humeral head was also evident in another study that incised the same labral region, leading to decentralisation of the humeral head by an average of 0.74 mm (Fehringer et al., 2003). Glenoid labrum lesions were associated with humeral head instability. A study which investigated simulated Superior Labrum Anterior to Posterior (SLAP) lesions found that there was a significant difference between intact and simulated SLAP lesions in terms of anterior and inferior humeral head translation. The humeral head translation was particularly evident during 60° abduction combined with either neutral rotation or 90° external rotation, and during 30° glenohumeral abduction with neutral rotation (Mihata et al., 2008). Therefore, the detachment of the superior glenoid labrum decentralises the humeral head, which could affect other anatomically related structures’ function.

4. ASSOCIATED STRUCTURES RELATED TO THE GLENOID LABRUM

While the evidence summarised above highlights a clear and direct role for the glenoid labrum in the structure and function of the shoulder region, it is also important to consider other anatomical relations of the labrum when assessing its anatomy and biomechanical roles and properties. Indeed, the majority of other anatomical structures contributing to the glenohumeral joint are known to have a direct relationship with the glenoid labrum. These structures are likely to exert significant biomechanical influences on the labrum and glenohumeral joint, particularly during movement. This section therefore explores the anatomical attachments and relationships of these associated structures (Figure 3).

FIGURE 3.

FIGURE 3

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Schematic overview of the glenoid fossa and related structures. Lateral view of a right glenoid fossa of the scapula showing the articular cartilage in the middle of the glenoid fossa surrounded by the glenoid labrum on its rim. The subscapular bursa is located anterior to the tendon of the long head of biceps brachii (LHB), in close proximity to the superior glenohumeral ligament (SGHL) and middle glenohumeral ligament (MGHL). The inferior glenohumeral ligament can be found inferiorly, highlighting its different segments: the anterior band (A‐IGHL), axillary pouch, and the posterior band (P‐IGHL). Illustration adapted from Dekker et al. (2020)

4.1. Long head of biceps brachii

The tendon of the long head of biceps brachii attaches to the superior part of the glenoid labrum and the supraglenoid tubercle (Bain et al., 2013; Vangsness et al., 1994) (Figure 3). However, during the earliest stages of development (9 to 12 weeks of gestation), the long head of biceps was reported to arise from only the glenoid labrum in 42% (n = 36) of shoulders, while the remainder had both a bony and a labral attachment (De La Cuadra‐Blanco et al., 2017). In adults, about 40% to 60% of bicipital attachment fibres were inserted into the superior labrum in the Vangsness et al. (1994) study of 100 shoulders. They also reported considerable variability in the projections of bicipital tendon fibres, which were divided into four categories: Type I, which all demonstrated preferential posterior orientation (found in 22% of shoulders); Type II, possessing mostly posterior attachments (33%); Type III, where fibres were divided equally anteriorly and posteriorly (37%); and Type IV, where fibres demonstrated preferential anterior orientation (8%). Similar findings have been recently reported in a study conducted on the superior labrum by Bain and colleagues (Bain et al., 2013).

The observed variances in the attachment of the long head of biceps brachii are likely to be of particular interest when considering glenohumeral and labral biomechanics, suggesting that further research is warranted. For example, since the long head of biceps brachii has a considerable labral attachment, it seems highly likely that forces being exerted on the tendon will be transmitted to the labrum. Indeed, Pagnani et al. (1996) postulated that tensile forces can be transmitted from the long head of biceps to the labrum, contributing directly to the overall stability of the glenohumeral joint. Moreover, Kanatli et al. (2011) found that patients with intra‐articular variations in the long head of biceps brachii had a higher prevalence of labral pathologies. In keeping with these findings, analyses using a single finite element model demonstrated that increasing load on the long head of biceps tendon leads to increased strain on the superior regions of the glenoid labrum (Hwang et al., 2014). From a functional perspective, Kuhn et al. (2003) demonstrated that load on the biceps tendon is transmitted to labrum during throwing motions. Thus, anatomical and functional relationships between the long head of biceps brachii and the glenoid labrum need to be taken into consideration when assessing labral anatomy and function at the glenohumeral joint.

4.2. Glenohumeral ligaments

While not being comparable anatomically or functionally to analogous ligaments surrounding the hip joint, for example, the glenohumeral ligaments surrounding the glenohumeral joint also contribute to the overall joint stability. There are four distinct glenohumeral ligaments surrounding the glenohumeral joint: a superior glenohumeral ligament, a middle glenohumeral ligament, an inferior glenohumeral ligament and a spiral glenohumeral ligament (Alashkham et al., 2018; Chahla et al., 2019; Ide et al., 2004; Steinbeck et al., 1998). The glenohumeral ligaments are primarily located in the superior, anterior and inferior aspects of the joint. Unlike ligaments in other joint types that have an isometric articulation (such as those regularly found supporting synovial hinge joints), the glenohumeral ligaments are lax in most joint positions and augment the humeral head in joint motion particularly in extreme motions (Barnes et al., 2018). This section will explore these ligaments and their relationship to the glenoid labrum.

4.2.1. Superior glenohumeral ligament

The superior glenohumeral ligament forms a part of the superior labral complex, along with the superior region of the glenoid labrum and the biceps tendon (Kuhn et al., 2003) (Figure 3). A systematic review carried out by Chahla et al. (2019) found that the glenoid attachment of the superior glenohumeral ligament was primarily to the supraglenoid tubercle, although the medial attachment ranged from the supraglenoid tubercle to the labrum, in close vicinity to the long head of biceps. In contrast, two other studies reported that the superior glenohumeral ligament can directly originate from the glenoid labrum (Ide et al., 2004; Steinbeck et al., 1998). Ide et al. (2004) found that the superior glenohumeral ligament arose, together with the middle glenohumeral ligament, from the labrum in 43% of shoulders (n = 23), with 17% (n = 18) of shoulders presenting with the same anatomical arrangement in the study by Steinbeck et al. (1998). A more recent larger sample study conducted by Alashkham et al. (2018) on 140 cadaveric shoulders reported that the superior glenohumeral ligament had a direct attachment to the anterosuperior region of the glenoid labrum in all samples examined. Subsequent work by Dekker et al. (2020) refined our understanding of the labral attachment in 10 cadavers and reported that the superior glenohumeral ligament inserted between the 12:15 and the 1:10 ‘o'clock’ positions. Thus, it appears that a direct physical interaction between the superior glenohumeral ligament and glenoid labrum is a common feature of glenohumeral joint anatomy in many individuals.

From the biomechanical perspective, the superior glenohumeral ligament was found to be taut, in cadaveric and a computer model of the glenohumeral joint, when the humerus was positioned between 0° and 30° of abduction, with a further increase in tension noted with additional external rotation (Debski et al., 1999; Felli et al., 2012; Mihata et al., 2008). Mihata et al. (2008) speculated that this finding might be responsible for increased humeral translation during joint movement between these angles in the case of SLAP lesions. Interestingly, the superior glenohumeral ligament can also be injured with excessive anterior‐superior subluxation (Savoie et al., 2001). A superior labrum anterior cuff (SLAC) lesion is a specific lesion that involves damage to the superior region of the labrum and superior glenohumeral ligament, alongside an anterior supraspinatus cuff tear (Savoie et al., 2001). Thus, interactions between the glenoid labrum and superior glenohumeral ligament are also important for understanding shoulder pathology.

4.2.2. Middle glenohumeral ligament

The middle glenohumeral ligament has more variable anatomy than the other glenohumeral ligaments, with the most commonly reported glenoid attachment being into the superior and anterosuperior regions of the glenoid labrum, positioned between 12:50 and 3:10 ‘o'clock’ (Alashkham et al., 2018; Chahla et al., 2019; Dekker et al., 2020) (Figure 3). Steinbeck et al. (1998) reported that the middle glenohumeral ligament was only identifiable in 84% (n = 88) of individuals. Likewise, Ide et al. (2004) were only able to identify the middle glenohumeral ligament in 63% (n = 53) of their studied samples, with 43% arising alongside the superior glenohumeral ligament from the labrum. The middle glenohumeral ligament can present as a cord‐like structure, but can also present as a Buford complex: a cord‐like middle glenohumeral ligament that is continuous with the superior labrum with absence of the anterosuperior labrum, as first described by Williams et al. (1994). Ide et al. (2004) reported the presence of a Buford complex in 1.5% of their sample and found a cord‐like middle glenohumeral ligament in 9%. Thus, any direct contribution of the middle glenohumeral ligament to the form and function of the glenoid labrum is likely to be highly variable between individuals.

When present as a stand‐alone structure (e.g., not a Buford complex), the middle glenohumeral ligament was found to be under tension in cadaveric specimens during abduction (up to 45°), particularly when accompanied by external rotation (Barnes et al., 2018; Felli et al., 2012). SLAP lesions have been associated with middle glenohumeral ligament variations, in which the SLAP lesion extends anteriorly into the area of the middle glenohumeral ligament (Beltran et al., 2002). Bents and Skeete (2005) studied the correlation between the presence of a Buford complex and SLAP lesions and found that in 235 shoulder arthroscopies only six cases had a Buford complex. Five of those six cases had a SLAP lesion, whereas in the remaining 229 cases, only 40 had this type of lesion. Thus, the incidence of SLAP lesions increases in the presence of a Buford complex, where there is an absence of the anterosuperior labrum (Bents & Skeete, 2005). This suggests that variation of the labrum affects the stability of the glenohumeral joint, being more prone to superior labral detachment.

4.2.3. Inferior glenohumeral ligament

The origin of the inferior glenohumeral ligament extends from the anteroinferior to the posteroinferior aspects of the glenoid (Chahla et al., 2019; Passanante et al., 2017) (Figure 3). The inferior glenohumeral ligament is comprised of three main structures: an anterior and a posterior band, as well as an axillary pouch, which extends from the anterior to the posterior bands (Alashkham et al., 2018; Chahla et al., 2019). Chahla et al. (2019) reported that the inferior glenohumeral ligament arises either from the glenoid labrum, the glenoid neck or both. Its presence is more consistent than the middle glenohumeral ligament, with Steinbeck et al. (1998) identifying a distinct inferior glenohumeral ligament in 97 out of 104 specimens. Furthermore, they reported that the ligament originated from between the 2 o'clock and 9 o'clock positions. In another study, Alashkham et al. (2018) investigated the inferior glenohumeral ligament in 140 embalmed cadavers, adding additional anatomical descriptions of the individual anterior and posterior bands. They reported that the anterior band was present in all of the shoulders examined, arising from the labrum between the 3 and 5 ‘o'clock’ positions. In contrast, the posterior band was present in only 79% of shoulders, arising from the labrum between the 7 and 9 ‘o'clock’ positions. These results were confirmed in a subsequent study carried out by Dekker et al. (2020), suggesting that the inferior glenohumeral ligament expands to a cover a wide area, thereby supporting the humeral head during motion.

From a biomechanical perspective, the inferior glenohumeral ligament has been described to act as a ‘hammock‐like’ brace, supporting the humeral head by cradling it during movement at the glenohumeral joint (Passanante et al., 2017; Warner et al., 1992). Warner et al. (1992) reported that at 90° abduction the inferior glenohumeral ligament prevented inferior translation of the humeral head. The anterior and posterior bands provide the greatest support for the humeral head during internal and external rotation, respectively (Warner et al., 1992), while the anterior band has been found to play an integral role in anterior stability (Debski et al., 1999; Speer et al., 1994; Warner & Beim, 1997). A Bankart lesion is one of the major causes of recurrent anterior instability that affects the anterior glenoid labrum, particularly with regards to the attachment of the inferior glenohumeral ligament, causing increased humeral head translation (Speer et al., 1994; Warner & Beim, 1997). Similarly, posterior band injury has also been associated with a reverse Bankart lesion on the posterior aspect of the glenoid (Bokor & Fritsch, 2010; Pokabla et al., 2010). Thus, the relationships between the inferior glenohumeral ligament and glenoid labrum are likely to be as important as those between the superior glenohumeral ligament and glenoid labrum for understanding glenohumeral joint stability and function, in health and following trauma.

5. LABRAL MICROANATOMY AND BIOMECHANICS

While the gross anatomy and anatomical relationships of the glenoid labrum undoubtedly play key roles in regulating and defining its form and function, recent research has begun to highlight the importance of understanding the microanatomy of the labrum. For example, a better understanding of how labral fibres are aligned to follow lines of force (an important characteristic of fibrocartilaginous tissues) will be key to generating a holistic overview of the labrum and its functions. Huber and Putz (1997) investigated gross fibre alignment of the glenoid labrum using India ink, thereby revealing that it is composed of fibres with connections to both glenohumeral ligaments and the biceps tendon. Fibre bundle dimensions were found to be larger in the superior and inferior quadrants, getting progressively smaller towards the anterior and posterior regions (Huber & Putz, 1997). Similarly, Nishida et al. (1996) used scanning electron microscopy (SEM) to demonstrate that fibrils from the biceps tendon, and capsuloligamentous structures, were interconnected with the labral fibrils running parallel to the glenoid rim. This, along with the increased incidence of SLAP lesion in cases when Buford complex is exhibited, shows the intimate microanatomical relationship between the labrum and surrounding attached structures that mechanically stabilises the glenohumeral joint working as biomechanical stabilising complex.

Three distinct layers can be identified in the glenoid labrum. Hill et al. (2008) described these as: (a) a superficial multidirectional mesh layer; (b) a circumferential loosely packed middle layer; and (c) a large, dense circumferential core layer. Transmission electron microscopy (TEM) identified large and small diameter collagen fibres in circular fibrils, suggesting different collagen types. The loosely packed layer seems to support the deeper core layer fibre bundles. Furthermore, the dense circumferential core layer function was related to hoop stress which accommodate the humeral head compression over the glenoid fossa thereby protecting the chondral plate (Hill et al., 2008). However, this function still needs to be investigated further.

As the dense circumferential core layer constitutes the largest portion of the mid‐labrum, its tensile and compressive properties were investigated by Smith et al. (2008, 2009). The mean compressive modulus was found to be 69.7 MPa, with the superior and anterior regions being significantly stiffer than the inferior and posterior regions (Smith et al., 2009). The mean elastic moduli, on the other hand, was 22.8 MPa with the anterosuperior region being the least at 15.4 MPa and the anteroinferior having the highest elastic modulus at 30.3 MPa (Smith et al., 2008). While these results are confined to the core of the labrum, it is interesting to note that the anterosuperior region had the least tensile modulus, as this region has a close relationship with the middle glenohumeral ligament, which can imply its reinforcement to the labrum; however, this low tensile moduli could mean that the fibres of the middle glenohumeral ligament might not infiltrate the deep core layer of the labrum. The stiffer nature of the anterior region could be an indicator as to its role in accommodating the compressive force of the capsuloligamentous structures in this region.

Investigations of the fibrocartilaginous area of the glenoid labrum point to a possible role in mechanical stress dissipation, in combination with related capsuloligamentous structures. The fibrous glenoid labrum attaches into the glenoid bone and the chondral plate via a fibrocartilaginous transition zone (Bain et al., 2013; Cooper et al., 1992; Huber & Putz, 1997; Nishida et al., 1996; Ockert et al., 2012). This transition zone resembles that of the fibrocartilaginous enthesis found in tendons and ligaments (Bain et al., 2013). Furthermore, the fibrocartilaginous enthesis consists of layers of dense fibrous tissue, uncalcified fibrocartilage, calcified fibrocartilage and then cortical bone. The uncalcified fibrocartilage in the enthesis, with its low compressibility, has been suggested to play a role in dispersing the stress exerted on angled fibres (Benjamin et al., 2002; Benjamin & McGonagle, 2009; Benjamin & Ralphs, 1998). Therefore, it can be proposed that the fibrocartilaginous elements of the glenoid labrum play a role in relieving the stress of taut capsuloligamentous structures.

To better understand the potential contribution of fibrocartilage stress dissipation in the labrum, it is important to examine the relationship between fibrocartilaginous area of the labrum and related capsuloligamentous structures. Yoshida et al. (2015) undertook such experiments using Safranin‐O to stain proteoglycans. The fibrocartilage area was found to be the largest in the inferior regions of the labrum, and the least in the anterosuperior region (Yoshida et al., 2015), suggesting that the inferior region may play a role in dissipating stress produced by the inferior glenohumeral ligament and overlaying capsule during joint movement. Furthermore, it has been suggested that the anterosuperior region of the labrum, having the least fibrocartilage, has minimal medial to lateral capsuloligamentous pull over the labrum with the presence of the opening to the subscapular bursa (Jobe et al., 2018; Moore et al., 2015). These potential roles and regional differences have yet to be biomechanically quantified in order to directly assess the amount of stress that capsuloligamentous structures can transmit to the glenoid labrum with regards to its fibrocartilage content (Figure 4).

FIGURE 4.

FIGURE 4

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Schematic illustration of biomechanical forces impacting on the glenoid labrum. A cross section of the glenoid bone (B) showing the capsule (C), labrum (L), and articular cartilage (AC), as well as the fibrocartilaginous transition zone, illustrating the role that the glenoid labrum can play in stress dissipation on the angled fibres of the capsule and labrum. The transfer of forces (arrows) is illustrated in the right panel, when force is applied to the capsule. The fibrocartilage area of the labrum limits flexion of the labrum towards the articular cavity, mediated by the low compressibility of the fibrocartilage zone

6. LABRAL INJURY AND MANAGEMENT

Repetitive movement combined with an excessive stress or acute traumatic event, such as a fall on an outstretched arm, on the glenohumeral joint can result in microtrauma, capsuloligamentous laxity and/or a labral tear (Elattrache et al., 2020; Snyder et al., 1990). Labral lesions can occur anywhere around the circumference of the labrum, with the most common site being affected being the superior region during a SLAP lesion (Clavert, 2015). Snyder et al. (1990) classified four categories of SLAP lesions, ranging from a simple fraying at the attachment site, through to marked detachment and tear reaching the biceps tendon (Table 1). Further categories and subcategories of labral tears have been recognised, which included larger labral tears or different tear presentations (Table 1) (Maffet et al., 1995; Morgan et al., 1998; Powell et al., 2004). SLAP tears have been attributed to different predisposing factors as well as reasons for SLAP repair failure, such as Buford complex, older age and excessive range of movement in the biceps brachii (e.g. in athletes) causing stress on the superior labrum (Bents & Skeete, 2005; Frank et al., 2013; Kanatli et al., 2010; Mihata et al., 2008; Wilk et al., 2012).

TABLE 1.

Classification and description of labral tears

Studies Type Description
SLAP
Snyder et al. (1990) I Fraying of the superior labral attachment with intact biceps and labral attachment
II Non‐variational detachment of the superior labrum along with the long head biceps tendon
Morgan et al. (1998) IIa A type II detachment extending anteriorly
IIb A type II detachment extending posteriorly
IIc A type II detachment having a combined extension from anterior to posterior
Snyder et al. (1990) III A detached proximal superior labrum displaced into the articular surface. Also referred to as ‘bucket handle’
IV A ‘bucket handle’ tear of the whole labrum with partial displacement of the biceps tendon into the articular surface
Maffet et al. (1995) V An anterior‐inferior lesion (Bankart) that extends superiorly reaching the biceps tendon
VI An unstable labral attachment with biceps tendon anchor release
VII Superior labral tear that extends to the middle glenohumeral ligament
Powell et al. (2004) VIII Superior labral lesion extending posteriorly and reaching the 6 ‘o'clock’ position
IX A ‘pan‐labral’ tear involving the whole circumference
X A reverse Bankart tear reaching the superior labrum
Bankart (1923) Bankart Detachment of the anterior labrum along with the joint capsule
Neviaser (1993) ALPSA An anterior‐inferior lesion (Bankart) that involves avulsion of the scapular periosteum
Simons et al. (1998) POLPSA A posterior‐inferior lesion (reverse Bankart) that involves avulsion of the scapular periosteum
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Abbreviations: ALPSA, anterior labro‐ligamentous periosteal sleeve avulsion; POLPSA, posterior labro‐capsular periosteal sleeve avulsion; SLAP, superior labrum anterior to posterior.

Other types of labral lesions have been described in the anteroinferior and posteroinferior regions. The most notable lesions in these sites are a Bankart lesion in the anteroinferior region and a reverse Bankart lesion in the posteroinferior region (Bankart, 1923; Bokor & Fritsch, 2010). Hill‐Sachs lesion is a bony defect that is commonly associated with Bankart lesions (Provencher et al., 2012), caused by a repetitive anterior dislocation of an abducted and externally rotated humeral head resulting in abrasion of the posterosuperolateral humeral head against the anterior glenoid edge (Provencher et al., 2012). Furthermore, two more distinct varieties of these lesions have been described which are Anterior Labro‐ligamentous Periosteal Sleeve Avulsion lesion (ALPSA) and Posterior Labro‐capsular Periosteal Sleeve Avulsion (POLPSA), where the detached labrum is accompanied by an avulsion of the periosteum without its rupture (Neviaser, 1993; Simons et al., 1998) (Table 1).

Depending on the extent and severity of the injury, treatment of labral tears can be either conservative, such as physical therapy, or surgery (Frangiamore et al., 2021). The goal of labral tear management is to alleviate pain, restore range of movement and facilitate a return to normal activities (Frangiamore et al., 2021). Labral repair, tenotomy and tenodesis are the treatment options for SLAP tears (Ren et al., 2019; Sullivan et al., 2019). Tenotomy is a surgical release of the long head biceps tendon from its labral attachment. Long head of biceps tenodesis, on the other hand, is a tenotomy procedure accompanied by a fixation of the distal cut end of the tendon to the humerus (Boileau et al., 2009; Gausden et al., 2016; Gupta et al., 2015). Both of these procedures have shown good improvements in terms of pain relief and patient satisfaction (Boileau et al., 2009; Ren et al., 2019). However, these treatment options are generally preferred for older and less active patients or for a failed labral repair (Frangiamore et al., 2021). This is attributed to the decreased vascularity in the superior aspect of the glenoid bone in older patients, as well as the biomechanical force of the long head biceps’ tendon over the labrum. These findings could serve to justify long head of biceps release over labral repair to avoid the detachment of a reattached labrum to the glenoid bone (Boileau et al., 2009; Gottschalk et al., 2014).

Bankart lesion management, after failed conservative treatment, requires surgical reattachment of the labrum alone, or with remplissage of a humeral bony defect ‘Hill‐Sachs lesion’ (DeFroda et al., 2017). The main concern following Bankart lesion treatment is recurrent instability and/or redislocation. A recent meta‐analysis has identified three articles that compared these two surgical techniques in patients with Hill‐Sachs lesions (Camus et al., 2018). The authors found that Bankart repair alone led to a significantly higher rate of recurrent instability (p = 0.003) and redislocation (p = 0.04) than combined Bankart repair and Hill‐Sachs remplissage (Camus et al., 2018). Conversely, another study was conducted to evaluate the efficacy of isolated Bankart repair in lowering recurrent instability and redislocation (Thomazeau et al., 2019) where, based on an Instability Severity Index Score, patients who scored from 0 to 2 had a recurrence rate of 10% compared with 36% in patients who scored 3 and 4. These findings suggest that the management of Bankart lesions needs to be individually tailored on a patient‐by‐patient basis in order to lower recurrent instability and redislocation.

7. CONCLUSIONS

The glenoid labrum anatomically enhances the glenoid fossa's concavity and congruity to better house the larger and more curved humeral head. This substantially contributes to glenohumeral stability through concavity‐compression stability as well as intra‐articular pressure maintenance. Moreover, the labrum helps in centralising the humeral head within the glenoid cavity during joint movement, thereby, limiting excessive translation of the humeral head. Furthermore, the labrum decreases contact area and pressure of the humeral head over the chondral plate. Importantly, the presence of direct anatomical interactions with surrounding capsuloligamentous and tendonous structures significantly influences the functions of the labrum, as well as its propensity to damage and injury. This can, therefore, affect its ability to perform key biomechanical roles in the shoulder region. A more detailed understanding of the microanatomy of the labrum, and the role of its fibrocartilaginous content in dissipating angular stress, is required in order to understand the mechanisms of injury and optimal strategies for repair.

CONFLICT OF INTEREST

The authors have no conflicts of interest to declare.

ACKNOWLEDGEMENTS

Work by the authors related to this review has been supported by funding from the Prince Sultan bin Abdulaziz College for EMS Research Centre at King Saud University (Saudi Arabia) and Anatomy@Edinburgh at the University of Edinburgh (UK).

Almajed, Y.A. , Hall, A.C. , Gillingwater, T.H. & Alashkham, A. (2022) Anatomical, functional and biomechanical review of the glenoid labrum. Journal of Anatomy, 240, 761–771. 10.1111/joa.13582

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