Keywords

After the human race evolved to be bipedal, the scapulohumeral complex also adapted. The bone continuity required in the weight-bearing joints is compromised to perform more complex movements with the upper limb and increase the range of motion. This adaptation in bony structures of shoulder complex increased the importance of soft tissue in joint stability [1, 2]. Thus, more unstable but the most flexible joint in our body has been formed. This is called “mobility-stability trade-off” [3].

The shoulder complex consists of four joints: glenohumeral joint (GH), acromioclavicular joint (AC), sternoclavicular joint (SC) joint, and scapulothoracic (ST) joint.

The GH joint is the main component of the shoulder complex. It connects the humerus and the scapula and is the joint with the widest range of motion in the human body. The mismatch between the humeral head and the relatively smaller glenoid creates instability, which provides a wide range of motion [4]. The GH joint can perform 180° of vertical abduction and 40° of vertical adduction (a), 180° of flexion and 55° of extension in the sagittal plane (b), 130° of horizontal abduction and 40° of horizontal adduction (c), 70° of internal rotation and 90° of external rotation movements around the long axis of the humerus (d). The glenohumeral joint also allows translation in all directions, which also increases the shoulder range of motion [5] (Fig. 4.1).

Fig. 4.1
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Glenohumeral Joint movements

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Although the shoulder complex constitutes most of the upper limb, they are connected to the axial skeleton by a single joint, the sternoclavicular (SC) joint [6]. Keeping the shoulder complex steady in the trunk is done mainly with muscle strength than this single joint. The sternoclavicular joint is a plane synovial joint that allows elevation/depression, protraction/retraction, and axial rotation movements. The position of the lateral end of the clavicle defines elevation/depression and protraction/retraction movements; the rotation is around the long axis of the clavicle. Besides, the medial end of the clavicle can translate in the anterior/posterior, superior/inferior, and medial/lateral directions on the sternum. Stability is provided by a synovial capsule, joint disc, and three major ligaments [6, 7]. Since the clavicle is connected laterally to the scapula with the acromioclavicular (AC) joint, the SC joint is also involved in the movement of the scapula [8, 9].

The acromioclavicular joint is the synovial plane joint between the lateral end of the clavicle and the acromion. Similar to the SC joint, stability is ensured by capsule, ligaments, and joint disc [10]. The acromioclavicular joint helps scapula move in harmony with the thorax that changes shape during shoulder movements [11]. It also allows the forces applied to the upper limb to be transferred to the trunk through the clavicle and are more susceptible to injuries. Joint movements are limited because the joint surfaces between the scapula and the clavicle are incongruent. The number of studies describing the movements of this joint is limited.

The scapulothoracic joint forms the connection between the scapula and thorax. Still, the ST joint is not a real joint where the bone segments are connected by fibrous, synovial, and cartilage tissue. Thus, the ST joint is often referred to as “functional joint” in the literature [12]. Accordingly, the shoulder is mainly kept stable on the thorax by muscle contractions. The scapula is attached to the clavicle with an AC joint. Therefore, every movement of the scapula affects the AC joint and SC joint [12,13,14]. The scapula is located on the thorax between the second and seventh ribs. It is positioned in 35–45° internal rotation, 10–15° anteriorly tilted, and 10° upward rotation [8]. The glenohumeral joint forms two-thirds of the total range of motion of the shoulder, and scapula movements create one third. The regular movement of the scapula includes three components: upward and downward rotation around a horizontal axis perpendicular to the plane of the scapula (a), abduction and adduction (b), elevation and depression(c). During these movements, protraction and retraction occur with the help of the clavicle and acromioclavicular joint [15, 16] (Fig. 4.2).

Fig. 4.2
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Scapular movements

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The subacromial space, which is part of the glenohumeral joint, can also be considered another “functional joint.” The movements of this joint are essential in shoulder functions [12].

The shoulder complex allows more complicated movements than other parts of the body as different types of joints work together in harmony. This large range of motion is allowed by a balanced interaction between static and dynamic stabilizers.

1 Shoulder Stability

Stability is the state that remains unchanged in the presence of forces that would change the current situation [17]. Shoulder stability can be analyzed in two parts: glenohumeral stability and scapulothoracic stability.

1.1 Glenohumeral Stability

Glenohumeral stability is that the humeral head remains in the glenoid and maintains its anatomic alignment during and after shoulder movements. Glenohumeral joint instability has been the most studied shoulder problem since the time of Hippocrates [18]. The stabilization of the joint is analyzed in two parts: static stabilization and dynamic stabilization [19].

1.1.1 Static Stabilization

1.1.1.1 Bony Static Stabilizers

Although the continuity between the humeral head and glenoid is low, bony structures are essential in ensuring shoulder stability. During rest, the inferior surface of the humeral head touches only a small area in the inferior part of the glenoid. Only 30% of the humeral articular surface is in contact with the glenoid articular surface at any time [20, 21]. Abduction increases the glenohumeral contact, and the pressure in the joint decreases [22]. When the pressure increases at the glenohumeral contact point, the humeral chondral surface can penetrate the glenoid chondral surface up to 1.2 mm. However, it is still controversial in which movements the pressure increases [23].

The humeral head forms the distal joint surface. The humeral head faces medially, superiorly, and posteriorly with regard to the humeral shaft and the condyles. The humeral head is retroverted on average 19° (range 9–31°) and inclined on average 41° (range 34–47°); head radius measures 23 mm (range 17–28 mm), and medial and posterior head center offsets are on average 7 mm (range 4–12 mm) and 2 mm (range 1–8 mm), respectively [24, 25].

Since the scapula is in internal rotation in the resting position, humeral retroversion increases the congruence of the glenohumeral joint by directing the humeral head toward the glenoid. Increased retroversion also increases the amount of external rotation of the humerus while decreasing its internal rotation. This mechanism explains the increased humeral retroversion of the dominant shoulders of the overhead athletes that have forced external rotation of the humerus during pitching [26, 27].

The shape of the glenoid fossa, which forms the proximal part of the joint, is also crucial in glenohumeral stability. The glenoid is a shallow socket that holds the humeral head; its mean depth is 2.5 mm on the anteroposterior direction and 9 mm in the superior-inferior direction. Therefore, different amounts of displacing forces must be applied to dislocate the shoulder in different directions [28, 29]. It is retroverted on average, 1.23° (range 9.5° of anteversion to 10.5° of retroversion), and inclined superiorly, on average 4.2° (range, 7° of inferior inclination to 15.8° of superior inclination) [30]. More than 10° of anteversion and more than 15° of retroversion is related to increased anterior and posterior instability, respectively [31,32,33]. Friedman and Kessler reported that its bending radius is greater than the humeral head radius in 93% of examined joints; the remainders have the glenoid and humeral head with the same bending radius [34].

Moroder et al. and Peltz et al. showed that the loss in glenoid concavity is related to glenohumeral instability. And patients with traumatic or atraumatic shoulder instability have a flatter glenoid cavity with a higher radius of curvature than healthy controls [35,36,37]. Weishaupt et al. mentioned that the dysplastic glenoid could also cause shoulder instability due to bone defects in the posterior glenoid rim. They defined three different glenoid forms according to bone defects in the posterior glenoid rim: pointed form (without any deficiency), rounded glenoid deficiency (“lazy J” form), and the triangular bony deficiency (“delta” form) [38, 39].

Bone loss is also an important factor in shoulder instability. It usually occurs traumatically. In most cases, forced abduction and external rotation force cause the humeral head to dislocate anterior-inferiorly [31]. Most important bony lesions that result in instability occur after traumatic events and involve the anterior-inferior glenoid rim (Bony Bankart lesion) and the posterolateral aspect of the humeral head (Hill-Sachs lesion).

Bony Bankart lesions are significant if they involve more than 20% of the length of the glenoid. In this case, if the correct soft tissue repair is not performed, there is a high probability of recurrence. If Bony Bankart involves more than 50% of the glenoid, there will be more than a 30% reduction in shoulder stability [40]. Bony Bankart lesions are classified according to Bigliani et al.: type I, a displaced avulsion fracture with attached capsule; type II, a medially displaced fragment mal-united to the glenoid rim; type III, an erosion of the glenoid rim lower than 25% (III A) and more than 25% (III B) [41]. The PICO method suggested by Baudi et al. could be used to calculate glenoid bone defect [42].

Hill-Sachs lesion occurs after anterior shoulder dislocation due to a compression fracture involving the posterior-lateral part of the humeral head. The effect of the lesion on shoulder stability depends on the size and location. There are different classification methods. Calandra classification, which uses arthroscopy to measure the depth of the lesion, is the most frequently used method [43]. Apart from this, classification can be made according to radiography or magnetic resonance imaging [44, 45].

It is necessary to evaluate bone defects that cause instability in glenoid and humerus together and not to ignore injuries in soft tissue other than bone defects [46]. Glenoid track concept and its association with the concept of “engaging” and “non-engaging” lesions showed that the relationship between the humerus and glenoid lesions determines stability [47, 48].

Posterior shoulder dislocations are much rarer. It usually occurs after direct trauma or seizure. It usually occurs after direct trauma or seizure. In this case, a compression fracture occurs in the anterior superior of the humeral head (Reverse Hill-Sachs Lesion or McLaughlin lesion), and another fracture may occur in the posteroinferior rim of the glenoid (Reverse Bankart Lesion) [49,50,51].

1.1.1.2 Soft Tissue Static Stabilizers

Soft tissue static stabilizers include glenoid labrum, glenohumeral capsule, glenohumeral ligaments, rotator interval, negative intracapsular pressure, and adhesion cohesion mechanism.

1.1.1.2.1 Glenoid Labrum

The glenoid labrum is a triangular section ring around the glenoid rim, deepening the relatively flat glenoid cavity. The upper part is more mobile than the lower part, which is more tightly attached to the glenoid rim [52]. The superior part joins the structure of the biceps anchor, and the long head of the biceps tendon.

The glenoid labrum increases the depth of the glenoid cavity by 50% and increases its congruity with the humeral head and contributes to the negative pressure required for shoulder stability [28]. It increases the contact surface between the humerus and the glenoid by 2 mm anteroposteriorly and 4.5 mm supero-inferiorly [53].

The negative pressure in the glenohumeral joint is 32 mmHg. This pressure is particularly effective against traction force, while it is less effective against shear forces [54]. The contribution of negative pressure to joint stability is higher in the hanging arm position, while it decreases with shoulder abduction [55]. Loss of intracapsular negative pressure can manifest itself as an anterior translation of the humeral head. The labrum creates an attachment site around the glenoid rim for the glenohumeral ligaments and joint capsule. It also acts as an anti-shear bumper during mid-range movements [21].

When defining lesions in the labrum, it is necessary to analyze anatomical variants such as sublabral foramen, meniscoid labrums, and cord-like middle glenohumeral ligament do not require surgery [56].

The most common glenoid labrum injury is Bankart lesion. It accompanies 90% of traumatic anterior shoulder instability [57]. It is defined as a detachment of the anteroinferior aspect of the labrum and capsule. It occurs due to the detachment of the middle glenohumeral ligament and inferior glenohumeral ligament from the glenoid. Despite its frequency, it cannot be considered as an isolated cause of instability [58].

Green and Christensen classified Bankart lesions in five arthroscopic types: type 1 refers to the entire labrum; type 2 is a simple detachment of labrum with no other significant lesions; type 3 is an intra-parenchymal labrum tear; type 4 and 5 are complex tears with significant or complete degeneration of inferior glenohumeral ligament, respectively [59]. This classification also has a prognostic value: type 4 and 5 lesions have a high chance of recurrent instability after arthroscopic Bankart procedure of 87%.

Another lesion involving the anteroinferior aspect of the labrum is the ALPSA lesion (anterior labroligamentous periosteal sleeve avulsion). The anterior labroligamentous complex rolls up in a sleeve-like fashion and becomes displaced medially and inferiorly on the glenoid neck [60].

The redislocation rate in ALPSA lesions and the probability of engaging the Hill-Sachs lesion are higher than those of Bankart lesions. Besides, the external rotation limitation developed after ALPSA lesion repair is another crucial problem [61].

Specular lesions can be described for the posterior aspect of the labrum. Reverse Bankart lesion involves the posterior labrum and the posterior band of the inferior glenohumeral ligament. POLPSA is the posterior labroligamentous sleeve avulsion. In chronic conditions, Bennett lesions may occur (an extra-articular calcification along the posteroinferior glenoid neck close to the posterior band of the glenohumeral ligament) [62, 63].

Reverse Bankart lesion is frequent in athletes, such as rugby players, with a 20% incidence reported in a study of 142 elite rugby player shoulder arthroscopy [64]. The injury mechanism could be traced to a direct blow to the anterior and lateral aspects of the shoulder, while the arm is adducted; a rare mechanism of injury is a posterior blow to the arm while holding a tackle shield [65].

Concerning superior labrum, a prevalent lesion in throwing overhead athletes is SLAP (superior labrum anterior and posterior) tear. This lesion is described for the first time by Snyder et al. [66]. Snyder classified SLAP tears into four types. Type 2 and type 4 are more likely to create instability as they involve both the labrum and the long head of the biceps.

Moreover, SLAP lesions are common in contact sports. Funk and Snow have reported a 35% incidence of SLAP tears, arthroscopically diagnosed, in 51 rugby players’ shoulders [67].

1.1.1.2.2 Capsuloligamentous Structures

Capsuloligamentous structures include joint capsule and glenohumeral ligaments (superior, middle, and inferior). There are many cadaveric and clinical studies investigating the biomechanical properties of these structures.

The constitutional trait of laxity facilitates extensive motion in multiple planes and may be essential to athletic performance. On the other hand, capsular stretching is noted along with a Bankart lesion is up to 28% of patients with recurrent anterior instability [68].

Superior and middle glenohumeral ligaments, together with coracohumeral ligament, long head of the biceps, and a thin layer of capsule, help to form rotator interval, and they will be treated in detail later.

The inferior glenohumeral ligament is also called the inferior glenohumeral ligament complex (IGHLC). It comprises three parts: two thicker bands on anterior and posterior and a thinner recess. During the abduction and external rotation, extension IGHLC moves anteriorly, forming a restraint to anterior translation of the humeral head.

During adduction, flexion, and internal rotation, IGHLC moves posteriorly, forming a restraint to posterior translation. IGHLC suffers from initial plastic deformation during the initial dislocation, but the damage becomes more critical after several episodes [69]. The lesion could more frequently occur at the glenoid insertion (anteroinferior glenoid rim) and in the middle part or at the humeral insertion [70].

Capsular stretching is often noted along with a Bankart lesion in up to 28% of patients with recurrent anterior instability [68]. The posterior capsule can also be injured; repetitive subluxations may lead to posterior instability by causing posterior capsular redundancy and increased joint volume.

1.1.1.2.3 The Rotator Interval

The rotator interval is a triangular space in the anterosuperior of the shoulder. It was first described by Neer in 1970 [71]. It creates resistance against extreme flexion, extension, adduction, and external rotation movements, limits inferior translation of the humeral head during adduction, and limits posterior translation of the humeral head during flexion or external rotation with abduction [72].

Furthermore, the synovial fluid provides to generate adhesion cohesion mechanism. The force formed between the wet surfaces of the humeral head and the glenoid contributes to stability [4].

1.1.2 Dynamic Stabilization

Dynamic stabilization provides a wide range of motion while securing stability during the normal function of the joint. There is a delicate balance between stability in the shoulder and range of motion. The muscles surrounding the shoulder and the neuromuscular balance between them ensure the dynamic stability of the joint. The muscles surrounding the shoulder and the neuromuscular balance between them provide the dynamic stability of the joint.

1.1.2.1 Proprioception

We know that capsuloligamentous structures also contribute to shoulder stability with their sensorimotor properties in addition to their mechanical functions. There are mechanoreceptors, especially in the anterior-inferior of the glenohumeral joint capsule. Proprioceptive information obtained from these structures contributes to shoulder stability by coordinating motor movements, reflexes, and joint stiffness.

As a result of the injuries in these structures, the decrease in proprioceptive information causes shoulder instability [73, 74]. Besides direct injury, capsular laxity has also been shown to cause a decrease in proprioception, leading to instability [75, 76].

Repairing of the capsuloligamentous structures restores the mechanical functions and tension of these tissues [77]. Retention allows joint capsule and ligamentous structures to sense mechanical stimulation and to facilitate proprioceptive feedback [74, 78].

1.1.2.2 Rotator Cuff Muscles

The rotator cuff is the common name of the structure consisting of muscles and tendons that contributes to shoulder stability. The rotator cuff consists of four muscles. These are supraspinatus (SSP), infraspinatus (ISP), teres minor (TM), and subscapularis (SSC).

Rotator cuff muscles provide fine control of shoulder movement. They play an essential role in dynamic stability, as well as contribute to proprioception [21].

Rotator cuff muscles compress the humeral head toward the glenoid and make an essential contribution to dynamic stabilization during shoulder movements. While symmetric rotator cuff contraction provides concavity compression, asymmetric contractions during shoulder movements rotate the humeral head. Joint reaction force decreases in rotator cuff tears [29, 79]. This stabilizing effect depends on the force couple formed by coordinated activation of the anterior and posterior fibers of the rotator cuff [80]. They act as an anti-shear force with the help of their mechanoreceptors. During the abduction, the rotator cuff tendon acts as a depressor for the humeral head and balances the pull of the deltoid muscle superiorly. Since this balance is disrupted after rotator cuff tears, the humeral head may be migrated superiorly [81]. A 50% reduction in rotator cuff force increases anterior dislocation by 46% and posterior dislocation by 31% [82].

The SSC is larger than the other three rotator cuff muscles and alone creates as much force as the sum of SSP, ISP, and TM [83]. The attachments of the muscles can be as tendons or muscle bodies [84, 85]. Therefore, the symptoms vary depending on the location and size of the rupture [86].

1.1.2.3 Long Head of the Biceps

The long head of the biceps (LHB) is a secondary stabilizer with a predominant role in the rotator cuff or capsuloligamentous deficiency. This tendon, originating from the supraglenoid tubercle and passing through the bicipital groove, acts as an anterior stabilizer during external rotation. During the late throwing phase, LHB reduces anterior translation, helping to prevent excessive torsion of the glenohumeral joint with a flexing elbow. These concepts can explain why type II or IV SLAP lesions are widespread in throwing athletes. Also, patients with rotator cuff insufficiency have hypertrophy in the tendon due to increased tension [87].

1.2 Scapulothoracic Joint Stability

The contribution of the scapula to upper extremity movements is better understood, especially in the last two decades [88]. The scapula provides a base to support the glenohumeral joint for regular upper limb movements [89, 90]. Since the scapulothoracic joint is not a real joint, its stability is provided only by dynamic stabilizers. The agonist, antagonist, and synergist contraction of the muscles adhering to both the thorax and the scapula ensures scapulothoracic joint stability. Scapular muscles dynamically coordinate the position of the glenoid, helping to create an effective glenohumeral joint movement. This harmonious relationship between the scapula and the humerus is called “scapulohumeral rhythm” [90, 91].

Upper and lower trapezius muscles, the serratus anterior and rhomboids (major and minor) are the structures that contribute most to scapulothoracic stability [15, 92].

Trapezius, together with the serratus anterior, initiates the upward rotation and posterior tilt movement of the scapula. Lower fibers of the trapezius contribute to the stability of the scapulothoracic joint during the descending of the arm from maximum elevation [15].

The serratus anterior muscle pulls the scapula toward the thoracic wall and makes a protraction movement. It provides stability, especially during abduction and pushing or punching type activities [91].

The rhomboids (major and minor) are especially active during adduction and retraction. They control the medial border of the scapula. It is quite active during swimming strokes and pulling [88]. It also takes part in the overhead throwing, both by reducing the stress on the anterior structures by fully retracting the scapula and braking by contracting eccentrically during the follow-through phase of pitching [93, 94].

Most abnormal biomechanics and overuse injuries in the shoulder girdle can be attributed to scapulothoracic joint instability [95, 96]. Alterations in joint movements due to weakness in scapular stabilizing muscles are called scapular dyskinesis [15, 97].

2 The Thrower’s Shoulder

Throwing consists of six stages: the windup, early cocking, late cocking, acceleration, deceleration, and follow-through. During throwing, large muscle groups work together [98]. The transition between late cocking and acceleration is critical, and most of the injuries occur in this segment. During the late cocking, the shoulder is in abduction, and external rotation, the anterior capsule, and the coracohumeral ligament are under tension. Repetitive stress may cause stains or tensile failure in these structures, causing anterior shoulder instability [99, 100]. When the shoulder is in the 90°–90° position, the posterosuperior rotator cuff can be trapped between the greater tuberosity and the glenoid labrum, causing internal impingement. Shear forces also act on the posterosuperior labrum and biceps anchor in this position [101, 102]. In late cocking, structures in the posterior contract, when leading from late cocking to acceleration, the opposite happens. The anterior structures contract rapidly, allowing energy to be transferred to the ball. In acceleration, mainly the pectoralis major, latissimus dorsi, triceps, and serratus anterior muscles contract. The rotator cuff contracts during deceleration. During follow-through, the posterior capsule and the posterior rotator cuff are under eccentric stress. In repetitive stress, posterior rotator cuff failure, thickening in the capsule, and decrease in compliance may occur [103, 104].