Keywords

1 Introduction

The shoulder motion is subject to the reaction to the shoulder structure stimuli. Due to the mismatch between the humeral head and the articular surface of the glenoid, the stabilization of the shoulder is compensated by static and dynamic stabilizers. The static stabilizers include the glenohumeral (GH) ligaments, the labrum, and the constrained capsule. The dynamic stability is provided by the rotator cuff (RC) muscles surrounding the shoulder joint. The combined effect of the stabilizers is to support the multiple degrees of motion within the GH joint. The scapulothoracic (ST) joint provides additional mobility and stability to the shoulder.

2 Musculoskeletal and Functional Anatomy of Shoulder

The human shoulder is composed of bones as well as associated muscles, ligaments, and tendons. It is the connection between the neck and the upper limb.

2.1 Bony Structure of the Shoulder

The skeletal structure of the shoulder includes the humerus, the scapula, and the clavicle.

2.1.1 Humerus

The humerus is located on the upper limb and is articulated with the glenoid fossa at the glenohumeral joint. The head of the humerus is elliptical and inclines between 120° and 145° [1] as well as a retroversion within a range of 60° [2]. There are two bulges on the lateral and anterior sides of the humeral head. They are greater and lesser tubercles. These two tubercles are separated by intertubercular groove and are origins to subscapularis, teres minor, infraspinatus, and supraspinatus muscles. The humerus has two necks: surgical and anatomical necks. The surgical neck is a constriction below the tubercles of the greater tubercle and lesser tubercle, and above the deltoid tuberosity. The anatomical neck is obliquely directed, forming an obtuse angle with the body. It provides the attachment to the articular capsule.

2.1.2 Scapula

The scapula is located on the posterior chest wall and is in an inverted triangular shape. It inclines approximately 40° anteriorly to the coronal plane [3]. There is a broad concavity on the anterior surface of the scapula and it forms the origin of the subscapularis muscle. The posterior surface of the scapula consists of the infraspinatus and supraspinal fossae, which is separated by the scapular spine provide the origins to the infraspinatus and supraspinatus muscles. Further, the scapula is composed of three corners and three sides, namely the lateral, superior, and inferior angles, and the medial, superior, and lateral borders, respectively. Next to the lateral angle, the glenoid is a shallow and pear-shape cavity. It inclines 3–5° superiorly [4] and 2.5–12.5° posteriorly [5].

2.1.3 Clavicle

The clavicle is an elongated bone in an S-shaped and connects the upper limb to the torso. The lateral and medial end of the clavicle is flat and thick, respectively. The clavicle transmits forces from the trunk to the upper limb. The articulation between the clavicle and the manubrium of the sternum is a sternoclavicular (SC) joint. The articulation between the clavicle and the acromion of the scapula is the acromioclavicular (AC) joint.

2.2 Articulation of the Shoulder

The shoulder joint is composed of four joints (SC joint, AC joint, GH joint, and ST articulation) with bony and soft-tissue structures. These articulations provide a high degree of shoulder motion. The arm can reach an elevation angle of 180°, internal and external rotations of approximately 150°, and flexion and extension of 170° [6].

2.2.1 Sternoclavicular Joint

The articulation between the manubrium sterni and the medial end of the clavicle is the SC joint. It connects the thorax to the upper limb and allows approximately 35° of elevation and 50° of axial rotation [6]. The intrinsic bony stability of the SC joint is mainly supported by the anterior and posterior SC ligaments, the interclavicular ligament, the costoclavicular ligament, and the articular disc.

2.2.2 Acromioclavicular Joint

The AC joint is composed of the acromion and the clavicle surfaces, which transmits forces from the upper extremity to the chest musculature. The AC articulation is stabilized by the coracoclavicular and the AC ligaments. The coracoacromial ligament spans between the lateral aspect of the coracoid process and the anterior facet of the acromion. It is to restraint the superior-inferior migration of the clavicle [7, 8]. The AC ligament is to restrain the anterior-posterior translation of the clavicle [7, 8] and provides scapular and synchronous clavicular rotation [9].

2.2.3 Glenohumeral Joint

The GH joint is a flexible and unstable articulation composed of the scapula glenoid fossa and humeral head. The glenoid cavity accounts for 25–30% of the humeral head and can move relatively to the humerus [9]. The glenoid-humerus contact area varies in degrees during the shoulder motion. Its stability is mainly supported by the articular capsule, the labrum, and the surrounding muscles [10]. The capsular-ligamentous complex includes the superior, the medial, and the inferior GH ligaments. The superior and the medial GH ligaments are both single-band structures and reach the lesser tubercle and the humeral neck, respectively. The inferior GH ligament complex consists of anterior and posterior bands and inserts on the humerus beyond the lesser tuberosity [11]. The glenoid labrum is a circumferential and fibrocartilaginous ring attaching to the glenoid rim. The glenoid labrum serves as a bumper during the shoulder motion and an attachment site of the GH ligaments. The muscles supplying the stability of the GH joint refers to the RC, which will be described in Sect. 2.3.

2.2.4 Scapulothoracic Articulation

The ST articulation is composed of the anterior surface of the scapula and the posterior surface of the thoracic [12]. The neurovascular, muscular, and bursal structures allow smooth motion of the scapula on the thorax. The ST articulation allows 30° of abduction internal rotation, respectively. It increases the shoulder movement after the initial 120°, which is supplied by the GH joint [12].

2.3 Functional Muscles of the Shoulder

The shoulder muscles supply athletic ability and dynamic stability. The function of muscles are subjected to its origin and endpoint. The muscles and their functions on the shoulder complex are described below.

The anterior outer layer muscles on the shoulder complex involve the pectoralis major and the deltoid muscles. The pectoralis major muscle has two heads (sternocostal and clavicular heads) and is located above the anterior chest wall. The sternocostal head originates from the anterior sternum surface, the superior six costal cartilages, and the external oblique muscle aponeurosis. In contrast, the clavicular head originates from the medial clavicle anterior surface. Both the sternocostal and clavicular heads insert to the crest of the greater tubercle of the humerus and serve the elevation and adduction of the arm. The deltoid muscle is composed of the anterior, the intermediate, and the posterior fibers. The anterior fibers originate from the upper surface and the anterior border of the lateral third of the clavicle and serve the flexion and internal rotation of the arm. The intermediate fibers originate from the acromion process and the spine of the scapula and serve the abduction of the arm after the initial 15° of arm rotation [6]. The posterior section of the deltoid originates from the spine of the scapula. It serves the external rotation and the extension of the humerus. All the fibers of the deltoid muscle insert into the deltoid tuberosity of the humerus.

The inner muscles involve the pectoralis minor, the subclavius, and the subscapularis muscles. The pectoralis minor plays an important stabilizing role on the scapula. The subclavian muscle is located underneath the clavicle and contributes to the clavicular movement. The subscapularis muscle, which is located in the anterior scapula, functions as the arm rotator.

The posterior outer layer muscles involve the latissimus dorsi, the trapezius, the serratus anterior, and the posterior deltoid muscles. The latissimus dorsi muscle arises from the thoracic vertebrae and inserts into the intertubercular groove of the humerus. It works in collaboration with the pectoralis major to contribute to the adduction and medial rotation of the humerus. The trapezius muscle is one of the broadest back muscles. It arises from the occipital bone, the ligamentum nuchae, and the spinous processes of T01–T12 and inserts into the third clavicle lateral, as well as the acromion and scapular spine. The trapezius muscle contributes to the shoulder elevation and rotation and also acts in head/neck extension [13]. The serratus anterior muscle arises from the anterior surfaces of the eighth upper ribs and inserts into the inner medial border of the scapula. The serratus anterior muscle allows the forward rotation of the arm and to pull the scapula forward and around the rib cage.

The posterior inner layer muscle below the superficial muscles (the trapezius and deltoid muscles) includes the supraspinatus, infraspinatus, teres minor, and teres major muscles. The origin of the supraspinatus muscle is the supraspinatus fossa. The muscle tracks laterally underneath the acromion and goes on the insertion at the greater tuberosity [14]. Thus, the supraspinatus muscle assists in the arm abduction and humerus stabilization [6]. The infraspinatus muscle originates from the infraspinous fossa of the scapula and inserts at the greater tuberosity. The origin of the teres minor muscle is at the lateral margin of the scapula. The muscle inserts at the most posterior and inferior facet of the greater tuberosity. Both the teres minor and the infraspinatus muscles assist the stability and rotation of the humerus. The subscapularis muscle is trapezoidal and origins from the anterior scapula aspect and inserts at the lesser tuberosity [15]. The teres major muscle arises from the inferior angle of the scapula and passes laterally and superiorly to the bicipital groove. It contributes to the extension and rotation of the humerus.

The RC is formed by the subscapularis, the infraspinatus, the teres minor, the supraspinatus muscles, and their associated tendons. The RC muscles contribute to the abduction and rotation of the humerus and also provide a compressive force to centralize the humeral head on the glenoid. In the case of a massive rotator cuff tear, the loss of enough passive muscle tension and dynamic contraction leads to excessive superior translation of the humerus to the glenoid cavity. The translation can reach 12 mm in some cases [9]. It may result in subacromial impingement and the erosions of surrounding bone and articulations (i.e., the glenoid, the acromioclavicular joint, and the anterior acromion) (the severe RC tear is described further in Sect. 4.2.1).

2.4 Summary

The GH joint is an enarthrodial socket-to-ball joint and supports the polyaxial arm motions. It includes abduction/adduction around the sagittal axis, flexion/extension around the frontal axis, and external/internal rotation around a longitudinal-humeral axis.

GH joint kinematics is not precisely equivalent to enarthrodial kinematics. Numerous studies have reported the exact determination of the GH joint kinematics and the founding is controversial. Due to different methodological approaches in different studies, it is difficult to compare the results. In general, the humeral head translates approximately 1.1 mm inferiorly during the whole abduction and 2.4 mm anteriorly before the abduction of 90° and 1.4 mm posteriorly during the abduction of 90–150° [16].

The GH joint maintains stability in utilizing static and dynamic restraints. These static stabilizers dynamic restraint involve ligamentous, capsular, cartilaginous, and bony structures, as well as musculature structure of the shoulder, respectively. The GH ligament stabilizes the GH joint by preventing excessive movements of the humeral head relative to the glenoid cavity in the extremes of motion [17]. A competent sealed capsule of appropriate volume, minimal joint fluid, and an intact congruent glenoid labrum provides the stability of the GH articulation [18]. Neuromuscular control primarily provides dynamic stability between the RC muscles and ST musculature. The functional ST musculature can ideally release the instability of the shoulder joint and the neural feedback from the GH ligaments and RC muscles, which are used to prevent pathologic translation of the GH joint.

Dynamic stabilizers may contribute to joint stability by the muscle contraction, which leads to compression of the articular surfaces, and the passive muscle tension from the bulk effect of the muscles [17]. The contraction of the RC muscles compresses the humeral head on the glenoid cavity and the asymmetric contraction leads to the humeral head rotation during the shoulder motion. The interaction of the RC muscles works in conjunction with other muscles in the shoulder girdle [19].

The shoulder joint kinematics relies on the ST and deltoid muscles, as well as the RC muscles interaction. The subscapularis and infraspinatus muscles act as a transverse force couple generating compressive forces. Also, the supraspinatus muscle plays a significant role in the concavity compression during early abduction [20].

3 Mechanical Properties and Glenoid Architecture

The mechanical parameters of the glenoid relate to bone architecture [21,22,23]. As illustrated in Fig. 6.1, the subchondral bone from the sagittal plane view is formed by the superoinferior, which oriented parallel to the trabeculae in the peripheral regions and honeycomb-fashion struts in the center of the glenoid [24]. In the transverse view, the trabeculae are plate-like and generally perpendicular to the glenoid surface (Fig. 6.1b). In the posterior and anterior regions, the thin rods directly connect the semi-circular cortical base to the subchondral plate and present low porosity, while the trabeculae in the middle portion distribute partially and present high porosity [21, 24].

Fig. 6.1
Two-part radiograph depicts the sagittal view of glenoid bone with posterior and anterior on left and right, superior and inferior at top and bottom. Transverse view shows lateral on top and medial at bottom.

The glenoid bone (a) sagittal view; (b) transverse view. L lateral, M medial, P posterior, A anterior, I inferior, S superior. (Adapted from Frich et al., 1998)

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As a result of the partial distribution of trabeculae in the center of the glenoid and high alignments of trabeculae in the anterior and posterior portions, the bone in the center of the glenoid presents a lesser strength, low stiffness, and isotropic characteristic with reference to the bone in the peripheral glenoid [22, 25]. The scapula bone from the lateral to the medial is orderly the subchondral bone, trabecular bone, and compact bone. In conformity with the architecture of the scapula, the strength of the scapula bone reduces significantly beneath the subchondral layer and then increases to the cortical bone layer [23]. In a regional test, the posterosuperior and anteroinferior glenoid shows the strongest and weakest bones, respectively [21, 22, 25, 26]. The porous trabecular bone at the center of the glenoid enables the heavy loads from the articular surface to be absorbed and conveyed, and the dense bone in the peripheral glenoid enables the compressive stresses generated during the arm motion to be absorbed [24]. The studies shows the individual glenoid present that the glenoid trabecular bone Young’s modulus varies between 99 and 264 MPa and the strength ranges from 26 to 110 MPa [21, 23, 25]. Young’s modulus and strength values are related to many factors such as health conditions, gender, and ages [23, 27, 28].

4 Shoulder Joint Arthroplasty

Shoulder joint arthroplasty is a surgical way to alleviate shoulder pain and restore the shoulder function. This section introduces the shoulder joint replacements from the following sections: (1) the types of shoulder joint arthroplasty; (2) clinical indications of shoulder joint arthroplasty; and (3) knowledge-based related to the shoulder joint arthroplasty.

4.1 Types of Shoulder Arthroplasty

The artificial shoulder joint arthroplasty can be classified into (1) anatomical and reverse shoulder arthroplasties, (2) total shoulder arthroplasty (TSA) and prosthetic humeral hemiarthroplasty (HA), and (3) stemmed and resurfacing shoulder arthroplasties.

4.1.1 Anatomical and Reverse Shoulder Arthroplasties

The structure of anatomical shoulder arthroplasty is similar to that of a natural shoulder joint and composed of a ball replacing the humeral head and a socket replacing the glenoid fossa. In contrast, the design of reverse shoulder arthroplasty (RSA) is different. Thus, in prosthesis type, the glenoid fossa is replaced with a ball and the humeral head is replaced with a socket. Anatomical shoulder arthroplasty is mainly used for the treatment of the incongruent osseous surface, moderate and severe osteoarthritis (OA), and inflammatory arthritis [29]. Primary indications of the reverse total shoulder arthroplasty (RTSA) are arthritis with massive RC tear. (Detail information on the advantages of RSA for severe RC treatment are shown in Sect. 4.2.1.)

4.1.2 Humeral Hemiarthroplasty and Total Shoulder Arthroplasty

In humeral HA (Fig. 6.2a), only the proximal humerus was replaced by an artificial device, while in the TSA, both the humerus and the glenoid are reconstructed. In the surgical technique of HA, the entire glenoid can be preserved and the complications associating with the glenoid component in the TSA can be avoided. HA is used for patients with intact articular cartilage surfaces. When the cartilage is damaged or the glenoid erosion is severe, TSA is usually preferred.

Fig. 6.2
A two-part radiograph depicts the long head of biceps tendon coupled to bursa in figure a and the bursa turned to the shoulder muscle in figure b.

(a) Radiograph of a hemiarthroplasty and (b) prosthesis resurfacing

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4.1.3 Resurfacing and Stemmed Shoulder Arthroplasties

Prosthesis resurfacing (Fig. 6.2b) refers to the surgical technique in which only the damaged surface of the humeral head is reconstructed. This technique can preserve the native inclination and offset of the humerus as well as the head shaft angle. Prosthesis resurfacing is usually used for the treatments of young and active patients [30, 31]. The contact between the glenoid bone and the stiff metallic humeral resurfacing device may lead to late glenoid arthrosis and need a revision to a TSA [30].

4.2 Clinical Indications of Shoulder Joint Arthroplasty

The indications for shoulder joint replacements involve cuff tear arthropathy (CTA), primary OA, rheumatoid arthritis, humeral fracture, and the failed shoulder joint arthroplasty [29].

4.2.1 Cuff Tear Arthropathy

CTA is a common shoulder disease referring to shoulder arthritis with RC tears [32]. Besides the loss of the shoulder cartilage function, severe CTA also leads to the dysfunction of RC tendons. This type of shoulder disease is often accompanied by synovial fluid leakage, GH articular cartilage atrophy, and subchondral osteoporosis on the humeral head [32]. In Sect. 2.3, the RC acts as a dynamic stabilizer. The dysfunction of these muscles may lead to the migration of the humeral head and subacromial impingement. Meanwhile, it may also induce pseudocyesis, pain, and loss of shoulder function. CTA may be developed after surgical treatments. It is reported that RC tears were observed in 47% of patients who had TSA from 2004 to 2007 [33] (Fig. 6.3).

Fig. 6.3
Figure a depicts the radiograph of the impingement and erosion of the superior glenoid and image of a lifted up shoulder.

(a) A radiograph of the RC arthropathy. (Adapted from http://www.djosurgical.com/patient/rsp/index.htm). (b) Photograph of a patient with CTA

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CTA is generally treated with RTSA or HA. If the CTA is mild and the shoulders are stable, HA is recommended [34, 35]. Further, if the shoulder is unstable, RSA treatment is generally used [29]. The application of TSA for CTA treatment is not an ideal approach [29]. Biomechanical analyses for these reasons are described below. In the case of severe CTA, the deltoid muscle force may lead to the superior movement of the humerus and could cause the subacromial impingement and other indications of CTA (Fig. 6.4a). When the TSA is applied for CTA treatment (Fig. 6.4b), the GH joint cannot be stabilized, and the humeral head may still be translated superiorly under the deltoid muscle force. Moreover, it may lead to eccentric loading on the glenoid socket and finally result in glenoid loosening. When the RSA is used for the treatment of CTA, the center of rotation of the humerus is secured on the glenoid and the arm can rotate under the deltoid muscle force (Fig. 6.4c). RSA is currently the only recommended treatment for the elderly adults (≥65 years) with pseudoparalysis and pain, due to the high long-term implant failure rate [36,37,38].

Fig. 6.4
Figure a: 2 upward arrows in deltoid muscle and point of centre of rotaion. Figure b: eccentric loading is labeled in the glenoid socket with an upward arrow. Figure c: centre of rotation marked near R S A and scapula.

Schematic diagram of the CTA in (a) the anatomical shoulder; (b) the TSA and (c) the RSA

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4.2.2 Osteoarthritis

OA is a disorder in which the biomechanical properties of the articular cartilage are gradually degraded. The OA starts from the articular cartilage wearing. When the wear leads to the direct bone contact of the two parts of the GH joint, OA may cause osteophytes, subchondral bone thickening, motion restriction, swelling, and pain [39]. The mild OA is usually treated with a nonsurgical method (i.e., local injections, medications, and physical therapy [40]). When OA is severe, HA or TSA is usually recommended [41,42,43].

4.2.3 Rheumatoid Arthritis

Rheumatoid arthritis is an autoimmune-related disease, which is associated with the breakdown of articular cartilage, synovial membranes thickening, and inflammation. Thus, patients with rheumatoid arthritis usually suffer from bone ossification or fusing and limited motion. The treatment depends on the integrity of the RC. When the RC is irreparable, RSA is recommended. If the RC is intact, TSA or resurfacing HA is used, depending on the degree of glenoid erosion [44].

4.2.4 Proximal Humerus Fractures

There are four types of proximal humeral fractures (Fig. 6.5). Type 1 (Fig. 6.5a) refers to the fracture with collapse or necrosis on the humeral head; Type 2 (Fig. 6.5b) refers to the fracture with irreducible dislocations; Type 3 (Fig. 6.5c) refers to the facture with completely broken surgical neck; Type 4 (Fig. 6.5d) is the fracture with severe tuberosity malunions. For Type 1 and Type 2, TSA treatment is recommended. For Types 3 and 4, the treatment depends on the degree of RC deficiency. HA treatment is recommended for patients with intact RC, and RSA treatment is used for patients over the age of 65-years-old with massive RC tear [29].

Fig. 6.5
A set of 7 diagrams depicts the different types of fractures of the proximal humerus.

(a) Type 1 proximal humeral fracture; (b) Type 2 proximal humeral fracture; (c) Type 3 proximal humeral fracture; (d) Type 4 proximal humeral fracture

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4.2.5 Revision

The revision of the shoulder arthroplasty is usually recommended for the treatments of implant problems (i.e., loosening, wearing, improper sizing, and malposition), osseous problem (i.e., bone loss, glenoid arthrosis), and soft-tissue deficiency. It is reported that the application of RSA for the failed HA can increase the American Shoulder and Elbow Surgeons (ASES) score to 30, and improve the forward elevation from 38.1° to 72.7° [45].

5 Conclusions

The GH joint is an articulation with the compensation of a series of static and dynamic stabilizers. The static and dynamic stabilizers cooperate with each other to support the stability of the whole shoulder joint and achieve the shoulder motion. Once the stabilizing structure is disrupted, it may lead to clinical manifestations of the shoulder pain or instability. Therefore, treatments to the shoulder diseases and disorders are subjected to a clear understanding of the etiology of the shoulder problem and the designs of the shoulder arthroplasties.