The shoulder joint is a complex structure consisting of the sternoclavicular, clavicular, acromioclavicular, scapular, glenohumeral, proximal humerus, and scapulothoracic wall joints. This article describes the functional anatomy and biomechanical properties of the shoulder joint, which are closely related to clinical work. Sternoclavicular joint Joint motion: The sternoclavicular joint moves in six directions around the horizontal, vertical, and anterior-posterior axes. These are forward rotation, backward rotation, forward extension, backward extension, supination and compression. The supination can reach 35°, the anterior and posterior extension 35°, and the axial rotation of the long axis of the clavicle can reach 45-50°. Shoulder clavicle joint 1, ligamentous structure: the ligamentous structure of the distal end of the clavicle is very familiar to orthopedic surgeons. The most important rostroclavicular ligament consists of two parts: the conical ligament and the oblique ligament. The oblique ligament is more robust. In addition, the upper acromioclavicular joint capsule is thickened to form the acromioclavicular ligament. Joint motion: The motion of the acromioclavicular joint may include the movement of the clavicle relative to the scapula in three directions, i.e., anterior-posterior motion, up-and-down motion, and axial-rotational motion. The range of motion in the anterior-posterior direction is the largest: about three times as large as the range of motion in the up-and-down direction. There have been few studies on the angle of motion of the acromioclavicular joint in all directions. The focus has been on the limiting effect of ligamentous structures on the motion of the acromioclavicular joint. The limiting effect on the rotational motion of the clavicle in the anterior-posterior direction relative to the acromion comes mainly from the anterior and posterior fibers of the acromioclavicular capsule. The rostroclavicular ligaments, primarily the conical ligaments, limit the upward motion of the clavicle relative to the acromion. There are virtually no ligamentous structures that limit the downward motion of the clavicle. The ligaments that limit the axial rotational motion of the clavicle have been reported differently by various authors. Rockwood et al. suggested that posterior axial rotation mainly strains the oblique ligament but also the conchoclavicular ligament and the acromioclavicular ligament complex, while Fukuda et al. investigated the role of different ligamentous structures in restricting the degree of acromioclavicular joint dislocation. He suggested that the acromioclavicular ligament mainly limits mild acromioclavicular joint displacement, while the rostral acromioclavicular ligament limits larger acromioclavicular joint displacements more significantly. It is also clinically evident that if the acromioclavicular joint is radiographically dislocated (>3 degrees of acromioclavicular joint dislocation), the conotruncal ligament must be compromised. Motion of the clavicle The potential range of motion of the clavicle exceeds the range of motion achieved during actual activity. It is difficult to accurately characterize the movements of the clavicle on a three-dimensional model. During upper extremity lifting, the clavicle can be lifted up to a maximum of 30°, which occurs when the upper extremity is lifted to approximately 130°. The clavicle extends anteriorly with respect to the acromion by 10° during the first 40° of supination, and then there is no further anterior extension of the clavicle until 130° of supination. There is no further anterior extension of the clavicle: if the upper limb continues to be lifted to its limit thereafter, there is a further 15-20 degrees of anterior extension of the clavicle. The significance of the axial movement of the clavicle in the mobilization of the upper limb is different. However, actual clinical experience supports Rockwood and Green’s view that the axial rotation of the clavicle in relation to the acromion does not exceed 10° during the entire process of upper extremity lifting, and therefore patients with acromioclavicular fusion can be seen to have no significant limitation of upper extremity lifting function. For upper extremity motion, the more important axial rotation of the clavicle occurs at the sternoclavicular joint. Screwing the clavicle to the rostral process does not significantly affect shoulder lifting: however, if the sternoclavicular joint is ankylosed, the upper extremity cannot be lifted more than 90°. Scapular motion The entire scapular girdle has a range of motion that exceeds the range of motion of any other joint in the body, and the upper extremity can be lifted in abduction by almost 180°: internal and external rotation combined exceed 150°. The range of motion of the entire scapular girdle exceeds that of any other joint in the human body, with an abduction and supination of nearly 180°: internal and external rotation combined exceed 150°: and forward and backward flexion and extension around the horizontal axis of motion combined approach 170°. This large range of motion is achieved by combining the range of motion of the sternoclavicular, acromioclavicular, glenohumeral, and scapulothoracic joints. The main movements occur at the glenohumeral and scapulothoracic joints, while at the limit of the range of motion, the movement of the sternoclavicular joint is also important. 1. Resting position: The resting position of the scapula is a 30° forward rotation relative to the coronal plane of the trunk. In addition, when viewed from the posterior, the long axis of the scapula is rotated 3° superiorly relative to the long axis of the trunk. Finally, from the lateral view, the scapula is resting with 20° of anterior flexion relative to the coronal plane of the trunk. The humeral head is centered on the glenoid at rest. Both the humeral head and the humeral stem lie within the plane of the scapula. The articular surface of the humeral head has a 30° posterior tilt relative to the humeral stem. The articular surface of the humeral head occupies about 1/3 of the surface area of the entire globe and is 120° rounded. Relative to the long axis of the humeral stem, the articular surface of the humeral head has an upward inclination of 45°. The articular surface of the humeral head has a 30° posterior inclination relative to the line between the two condyles of the distal humerus. The shape of the glenoid resembles a reverse comma. In general, the glenoid articular surface has an upward inclination of approximately 5° with respect to the medial border of the scapula, and the glenoid articular surface has an average posterior inclination of approximately 7° with respect to the scapula. 3. Upper Extremity Lifting: The most important function of the shoulder joint is to lift the upper extremity, so this movement has been studied in detail. The focus of the study is, in the process of upper extremity lifting, the glenohumeral joint and scapulothoracic wall joints of the respective range of motion, that is, often referred to as the scapula, humerus rhythm of the problem. Bergmann summarized the results of previous studies and concluded that the glenohumeral joint has a greater proportion of range of motion during the first 30° of the lift, and that the glenohumeral and scapulothoracic wall joints have essentially equal degrees of motion during the last 60° of the lift. Ultimately, the ratio of total glenohumeral to scapulothoracic wall motion is approximately 2:1 throughout the entire upper arm lift, and studies of patients who have undergone unrestricted total shoulder replacement surgery have shown that the ratio of glenohumeral to scapulothoracic wall motion becomes 1:2 during the postoperative lift of the affected limb. There is also an anteroposterior rotation of the scapula against the chest wall with the lifting of the upper limb when viewed from the lateral side. During the first 90° of lifting, the scapula rotated forward about 6° relative to the chest wall; during the subsequent lifting of the upper limb, the scapula rotated backward by 16°. Thus, at the limit of upper extremity supination, the scapula was in a position of 10° of posterior rotation relative to the resting position. 4, the upper limb external rotation: experiments have proved that in the extreme upper limb lifting must be accompanied by the external rotation of the humeral head so that the humeral tuberosity can avoid the rostral shoulder arch to avoid impingement. In addition, the external rotation of the humerus during supination also relaxes the ligamentous structures below the glenohumeral joint to maximize supination of the upper arm. As the upper limb can be lifted in different positions, the description of the lifting activity needs to specify the angle L between the plane of the upper limb and the plane of the scapula on the one hand, and the angle of lifting in the plane of the limb on the other hand. Browne designed experiments to illustrate the relationship between the angle of supination and the angle of external rotation of the humerus during supination of the upper arm on a scapula-fixed model. He found that maximal supination of the upper arm occurred when the plane of humeral mobility was located 23° anterior to the plane of the scapula. Supination with the humerus in either angular position anterior to the plane of the scapula was accompanied by external rotation of the humeral stem. The humeral stem externally rotates up to 35° during maximal supination. In the internal rotation of the humeral stem, the maximal lifting of the upper arm is located in the plane of 20°-30° behind the plane of the scapula, and the maximal lifting of the upper arm at this time is only 115°. 5. Center of rotation: Studies of shoulder motion have shown that the center of rotation of the glenohumeral joint is located within (6+2) mm of the geometric center of the humeral head. This indicates that there is minimal displacement of the humeral head during glenohumeral rotation. The humeral head is displaced upward only about 4 mm during the entire upper arm lift; therefore, excessive upward displacement of the humeral head may indicate the presence of a rotator cuff defect or a rupture of the long head of the biceps tendon. The center of rotation of the scapula during supination is located at the tip of the acromion. 6. Relevant Clinical: The above biomechanical knowledge is helpful in guiding clinical work. For example, according to the relative position of the scapula and the thorax, we should adjust the patient’s position accordingly when we take the scapula front and side view film. Since shoulder elevation is always accompanied by the external rotation of the humerus, it can be explained that for patients with frozen shoulder, due to the obvious restriction of external rotation of the shoulder joint, which results in the obvious restriction of the upper limb elevation. Knowing how these movements accompany each other is good for us to guide the patient’s functional exercises after surgery. Joint fusion is an effective means of addressing shoulder problems. The choice of fusion position is extremely important for the patient’s postoperative function, and the optimal fusion position is still controversial. The choice is based on the range of motion of the scapulothoracic wall joint and the range of motion of the shoulder required for daily living. The motion of the scapulothoracic wall joint is a good explanation for why patients with frozen shoulders and fused joints continue to have a certain amount of mobility in the shoulder joint. In addition, scapulothoracic wall motion allows the deltoid muscle to maintain the proper length for optimal function throughout upper extremity lifting. Because the center of rotation of the glenohumeral joint is close to the geometric center of the humeral head, there is minimal displacement of the humeral head during glenohumeral rotation, which confirms the design rationale for the unrestricted glenohumeral prosthesis currently used. In addition, the optimal value for the mismatch in radius between the humeral head and the glenoid seems to be 3-4 mm, as this replicates the small displacement of the humeral head during normal joint motion. Stability of the Shoulder Joint The shoulder joint is the joint with the greatest range of motion in the body, and its stability is maintained by both static and dynamic stabilization. Its stability is mainly maintained by static and dynamic stabilizing effects. Static stabilizing structures include soft tissues, rostro-shoulder ligament, glenohumeral ligament, glenoid labrum, articular capsule, and mutual contact of articular surfaces, tilting of the scapula, and intra-articular pressure. 1. Articular factors: Anatomically there is a 30° posterior inclination of the articular surfaces of the humeral head, which is obviously significant for balancing the muscular forces around the joint. Current research on the influence of the correspondence of the articular surfaces on the degree of stability of the joint has focused on the glenoid side. It is generally accepted that the size and anatomical shape of the glenoid are significant for the stability of the joint. This can be confirmed by the fact that patients with glenoid dysplasia are prone to recurrent shoulder instability. On the other hand, the glenoid labrum is important for enlarging the area of the glenoid and increasing the depth of the glenoid. In the presence of the labrum, the articular surface of the glenoid accounts for approximately 1/3 of the articular surface of the humeral head, whereas with the removal of the labrum, this percentage is reduced to 1/4. However, the extent to which glenoid labral tissue can increase shoulder stability is still debated. The glenoid surface has a 5° upward slope,9 which, together with the superior capsule and superior glenohumeral ligament, is important in preventing downward dislocation of the humeral head. Intra-articular pressure is another important stabilizing factor. It has been demonstrated that there is always a negative pressure in the normal shoulder joint, and if this negative pressure is counteracted by incision of the joint capsule or pumping of air into the joint, the shoulder is very susceptible to downward subluxation. In fact, negative intra-articular pressure plays an important role in stabilizing the shoulder in many directions, but is never limited to inferior stabilization. The amount of negative pressure varies according to the relative position of the glenohumeral joint and the extra-articular load. Studies have shown that intra-articular negative pressure is minimal when the upper arm is lightly elevated and maximal when the upper arm is extremely elevated. 2, the role of the joint capsule and ligamentous tissue: the biological composition of the shoulder joint capsule is consistent with the joint capsule of other joints in the body, including the elbow joint. Tests have shown that for a young person under 40 years of age, a dislocation of the shoulder requires an external force of 2000 N, compared to 1500 N for a dislocation of the elbow, and that the force required decreases with age, but this decrease is more pronounced in the shoulder. The capsule of the shoulder joint is very thin and redundant, and the degree of redundancy of the capsule is genetically related: it varies from person to person. As a result: each person has a different degree of laxity in the joint, and if the joint is too lax, this may lead to a prevalence of shoulder instability. The ligaments of the shoulder joint include the superior, middle, inferior, and rostro-humeral ligaments, and these structures were first described in detail by Flood in 1892. 3, rostro-humeral ligament: The rostro-humeral ligament originates from the anterolateral portion of the base of the rostral process and is divided into two bundles: one bundle is woven into the joint capsule; the other ends at the greater tuberosity of the humerus. The role of the rostro-humeral ligament is controversial. Some studies have suggested that the ligament is strained during external rotation of the shoulder joint and that the rostro-humeral ligament resists downward subluxation of the shoulder joint, but other studies have shown the opposite result. Another opinion is that the rostro-humeral ligament is an important lower stabilizing structure in external rotation, but not in neutral or internal rotation. The rotator cuff space, the space between the supraspinatus and subscapularis muscles, is covered by the joint capsule and reinforced by the rostro-humeral ligament. Cutting the joint capsule and the rostro-humeral ligament from the rotator cuff space resulted in significant inferior and posterior instability of the shoulder joint, whereas cutting the capsule and leaving the rostro-humeral ligament in place did not result in inferior instability. 4. Supraglenohumeral ligament: The supraglenohumeral ligament originates from the anterior aspect of the long head of the biceps muscle from the origin of the superior glenoid tubercle: it ends proximal to the base of the lesser tubercle of the humerus. This ligament, along with the upwardly tilting glenoid, prevents the humeral head from dislocating or subluxing in a downward direction. 5. Middle glenohumeral ligament: The middle glenohumeral ligament originates from the superior glenoid tuberosity and the superior rim of the glenoid as well as the anterior superior glenoid labrum and travels downward and outward: it weaves into the subscapularis muscle about 2 mm medial to the stopping point of the subscapularis muscle at the tuberosity. The ligament is very thick: up to 2 cm wide and 4 mm thick, and it is considered to be an important structure that prevents the humeral head from dislocating anteriorly. When the upper extremity is abducted or externally rotated, the glenohumeral ligament becomes tense, and selective severance of the glenohumeral ligament does increase the degree of displacement of the humeral head but does not result in instability. Therefore, the glenohumeral ligament does play a role in preventing anterior instability of the shoulder joint, but is not sufficient by itself to prevent anterior subluxation of the humeral head in abduction and external rotation of the affected extremity. Recent studies have shown that in upper extremity abduction and external rotation, the glenohumeral ligament is tense at small angles of abduction, remains tense at 90° of abduction, and decreases in tension as the angle of abduction increases. In the neutral or internally rotated position of the upper extremity, regardless of the angle of abduction, the tension is almost zero. 6. Inferior glenohumeral ligament: Almost the entire anterior glenoid labrum is the origin of the inferior glenohumeral ligament. The ligament originates from this point and travels inferiorly and externally to the inferior border of the humeral head and the anatomical neck. o’Brien, after careful study of the inferior glenohumeral ligament, termed the anterior portion of the ligament as the anterior fascicle and the posterior portion of the ligament as the posterior fascicle. The inferior glenohumeral ligament is important in maintaining the anterior stability of the shoulder joint when the upper arm is in abduction or external rotation. On the other hand, the posterior bundle of the inferior glenohumeral ligament, as well as the posterior and inferior capsules, are important in maintaining the posterior stability of the shoulder joint when the upper arm is in the flexed and internally rotated position. Recognizing the importance of the inferior glenohumeral ligament can help us to solve many clinical problems. Recurrent anterior instability of the shoulder joint is often caused by an incomplete inferior glenohumeral ligament. In conclusion: The shoulder capsule and ligamentous tissues are important static stabilizing structures around the shoulder joint. The inferior glenohumeral ligament is the most important of these. The entire capsule and ligament complex as a whole works synergistically to maintain the stability of the shoulder joint. Dynamic stabilizing structures Dynamic stabilizing structures include the rotator cuff, biceps and deltoid muscles. The muscles around the shoulder joint contract during movement to produce dynamic stabilization. Its mechanism of action is reflected in four aspects: 1, the size and tension of the muscle itself. 2, the contraction of the muscle results in the contraction between the joint surfaces. 3, the contraction of the shoulder joint. 2. Muscle contraction leads to increased pressure between the joint surfaces. 3. The movement of the joint can indirectly tighten the surrounding static stabilizing structures. 4. The contracted muscle itself has a barrier effect. 1, rotator cuff: the rotator cuff muscles due to its own muscle volume and tension: that is, to help maintain the stability of the shoulder joint, subscapularis is an important barrier in front of the shoulder joint: to prevent the humeral head from dislocating anteriorly, and the supraspinatus, infraspinatus, and teres minor are also very important in maintaining the stability of the posterior aspect of the shoulder joint. On the other hand, active contraction of the rotator cuff muscles has been suggested to contribute to the stability of the shoulder joint. The supraspinatus has been reported to be an important lower stabilizing structure, while other studies have emphasized the subscapularis as the most important anterior stabilizing structure of the shoulder joint. Electromyography has shown that the subscapularis and infraspinatus contract significantly in the intermediate range of shoulder elevation, and in patients with anterior shoulder instability: electromyography has shown that the supraspinatus and biceps muscles contract more actively than in normal subjects. However, it has been suggested that active contraction of the rotator cuff muscles does not contribute to shoulder stabilization, so this is still a controversial issue. 2. Biceps: The long head tendon of the biceps is considered to be an important structure for the downward compression of the humeral head. Shoulder arthroscopy shows that the humeral head can be compressed into the glenoid when the biceps tendon is electrically stimulated. The stabilizing effect of the biceps tendon on the shoulder joint was most pronounced during external rotation of the upper arm and least pronounced during internal rotation. It has also been reported that in throwing athletes with anterior instability of the shoulder joint, electromyography showed that the contractile activity of the biceps muscle was significantly increased, suggesting that the biceps muscle may have a stabilizing effect on the shoulder joint. Itoi et al. concluded that the long head tendon of the biceps muscle stabilizes the shoulder joint inferiorly, anteriorly, and posteriorly, and that the long head tendon, along with the short head tendon, maintains anterior stabilization of the shoulder joint. Cadaveric studies have shown that for a stable shoulder joint: the stabilizing effect of the biceps is at the same level as that of the supraspinatus, infraspinatus and teres minor, but for an unstable shoulder joint: the stabilizing effect of the biceps is more significant. 3. Deltoid: There are fewer studies on the role of deltoid on shoulder stabilization, and some reports suggest that deltoid does not have a significant stabilizing effect on the shoulder joint. However, it has been suggested that the role of the deltoid muscle is highly differentiated in different regions: the anterior and posterior fibers of the deltoid muscle contribute to the stability of the shoulder joint. Interaction between static and dynamic stabilizing structures The roles of static and dynamic stabilizing structures are not mutually exclusive, and Blasier studied the relationship between the two in cadaveric studies, concluding that the glenohumeral ligament and the rostro-humeral ligament are more important in static stabilizing structures, whereas the rotator cuff muscles and the biceps muscle play a more important role in dynamic stabilizing structures. Dynamic stabilizing structures are more important when the humeral head is less displaced, and static stabilizing structures are more important when the humeral head is more displaced. The ligamentous tissues of the capsule sense position, motion, and tension, and these signals are transmitted from the static stabilizing structures to the dynamic stabilizing structures via a reflex arc, which is called proprioception Smith and Brunoli reported that this proprioception is disrupted in patients with recurrent anterior shoulder dislocations Murakami noted that when a backward force was applied to the upper arm at 90° of flexion, the EMG potentials of the infraspinatus muscle were significantly higher. potentials of the supraspinatus are markedly enhanced. Receptors for mechanical activity have been found on the rostro-humeral ligament, subacromial bursa, articular capsule, and glenoid labrum tissues in humans.Zuckerman examined the level of proprioception in the shoulder joint bilaterally in patients with recurrent anterior instability of the shoulder joints preoperatively, and then at six and twelve months postoperatively. It was found that preoperative proprioception was reduced on the affected side compared to the healthy side and eventually returned to normal levels after surgery.Gaunche in his experiments stimulated the anterior and inferior branches of the axillary nerve to cause contraction of the biceps and rotator cuff muscles while stimulation of the posterior branch of the axillary nerve caused contraction of the deltoid muscle. From the above studies, we can see that the static and dynamic stabilizing structures are closely related to each other, and together they respond to any adverse movement or displacement of the shoulder joint. Summary of shoulder stability The glenohumeral joint owes its great mobility to the complex interactions between the joint, the capsule, the ligamentous tissues and the dynamic stabilizing structures. The glenohumeral ligament system mainly prevents excessive external rotation of the shoulder joint, and its lower ligamentous structures are also the most important structures to prevent forward dislocation of the shoulder joint. The rotator cuff, biceps muscle and deltoid muscle form the dynamic stabilizing structures, and these different stabilizing mechanisms interact with each other through the proprioceptive system in order to improve the stability of the shoulder joint.