Shoulder Anatomy and Biomechanics

Osteology

A variety of bony structures including the scapula, humerus, clavicle, and sternum are connected via soft tissue throughout the shoulder complex. The scapula is a flat, triangular bone that forms the posterior aspect of the pectoral girdle. Numerous muscles originate at and insert on the scapula. The four rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis), teres major, triceps, deltoid, and several other muscles all originate from the scapula ( Fig. 14.1A and B ).

The scapula has three borders named by their location: the superior border, medial border, and lateral (axillary) border. The superior border is the shortest and extends from the superior angle of the scapula to the suprascapular notch at the base of the coracoid. The medial border is the longest of the three and runs parallel to the spinous processes of the thoracic vertebrae. It extends from the superior angle to the inferior angle of the scapula and serves as the insertion site of the levator scapulae, serratus anterior, and rhomboid muscles. The lateral border is thick and runs obliquely downward from the lateral angle to the inferior angle of the scapula. It is the attachment site for the teres minor, teres major, subscapularis, and long head of the triceps brachii. The borders of the scapula meet at three distinct angles: superior, lateral, and inferior. The superior angle serves as the insertion site for the levator scapulae and serratus anterior. The inferior angle is located at the junction of the lateral and medial borders. It serves as the origin of the teres major and latissimus dorsi muscles and the insertion of the serratus anterior. The superior and lateral borders form the lateral angle, which gives rise to one of the scapula's most important structures, the glenoid.

The glenoid is a shallow, pyriform recess, which is wider inferiorly. Overall, females have smaller glenoids than males. The mean glenoid height ranges from 31.7 to 39 mm, with females having, on average, 2.9–3.18 mm smaller glenoid heights than males. ^{, , ,} The mean glenoid anteroposterior width ranges from 23 to 29 mm, with females having 3.4 mm smaller glenoid widths than males. ^{, ,} Additionally, males tend to have slightly more retroverted glenoids than females. ^, The glenoid has, on average, 1–3 degrees of retroversion, but can vary from 14 degrees of anteversion to 12 degrees of retroversion. It has been reported that males have between 1 and 8.4 degrees more glenoid retroversion than females. ^, Glenoid retroversion can theoretically help prevent anterior translation of the humeral head.

The scapula has three processes: the spine, the acromion, and the coracoid. The spinous process is a prominent, horizontal ridge that divides the dorsal scapula into the supraspinatus and infraspinatus fossae. The spine of the scapula suspends the acromion and functions as a lever arm to aid in deltoid motion. The acromion is a lateral and anterior continuation of the spine that forms the summit of the shoulder. Three acromion types have been described: flat (Type I), curved (Type II), and hooked (Type III). Types II and III acromions are associated with subacromial impingement, whereas Type I has the lowest risk for impingement ( Fig. 14.2 ). The lateral border of the acromion provides an origin for the deltoid muscle, while the medial border acts as an insertion point for the trapezius muscle. Additionally, the acromion articulates with the distal end of the clavicle at the acromioclavicular (AC) joint (ACJ). The apex, located at the anteroinferior aspect of the acromion, serves as the attachment site for the coracoacromial ligament, which extends between the acromion and the coracoid process of the scapula and helps provide stability to the glenohumeral joint (GHJ). The coracoid process is a bony projection that arises from the superior border of the scapula. It projects superiorly and anteriorly above the glenoid fossa. The coracoid process works to stabilize the scapula and provides the attachment for the coracoclavicular (CC) ligament, the short head of the biceps brachii, the coracobrachialis the pectoralis minor, the coracoacromial ligament, and the coracohumeral ligament (CHL). The coracoid measures approximately 15.9 mm in width and 10.4 mm in thickness. The CC ligament varies in length from 22 to 28 mm measuring from the tip of the coracoid, and the normal CC distance is approximately 11–13 mm. ^,

The proximal humerus consists of the humeral head, which articulates with the glenoid fossa; the greater and lesser tuberosities, which serve as attachment sites for the rotator cuff muscles; and the humeral shaft. The humeral head is covered by hyaline cartilage. The thickness of the cartilage on the humeral head ranges from 0.2 to 2.0 mm, with the thickest portion located at the center of the head. ^, The humeral head articular surface has a radius of curvature ranging from 22 to 55 mm. ^, The most superior aspect of the humeral head lies 8 mm superior to the greater tuberosity. The narrow groove separating the articular cartilage of the humeral head and the tuberosities is referred to as the anatomic neck of the humerus. The greater tuberosity provides the insertion site for the supraspinatus, infraspinatus, and teres minor. The lesser tuberosity serves as the insertion site for the subscapularis tendon. Below the level of the tuberosities, the humerus narrows into the surgical neck of the humerus where the shaft begins ( Fig. 14.3 ). The average angle of the humeral head and neck ranges from 130 to 150 degrees, and the shaft is retroverted about 20–30 degrees. ^,

The clavicle is an S-shaped bone that consists of an anteriorly convex greater medial curve and a posteriorly convex lesser lateral curve. The medial end of the clavicle connects to the sternum, the first rib, and its costal cartilage via the sternoclavicular (SC) joint. ^, The lateral end of the clavicle articulates with the acromion at the ACJ. The clavicle acts as a site of muscle attachment and plays an important role in maintaining the positioning and kinematics of the scapula. Muscles that originate from the clavicle include the deltoid, pectoralis major, sternocleidomastoid, and sternohyoid, while the trapezius and the subclavius insert on the clavicle ( Fig. 14.4A and B ).

Arthrology

The shoulder complex is composed of four different articulations: glenohumeral (GH), acromioclavicular (AC), strenoclavicular (SC), and scapulothoracic. The GH jointt is the most complex articulation in the body. Together, the four articulations allow the shoulder to function normally in space.

The GH joint is a multiaxial spheroidal joint, which has minimal inherent stability. It allows for the largest range of motion in the human body, with multiple degrees of freedom including flexion/extension, abduction/adduction, translation, and internal/external rotation ( Fig. 14.5 ). ^{, ,} The humerus is stabilized in the concave glenoid fossa by dynamic and static stabilizers. As noted earlier, the radius of curvature of the humeral head ranges from 22 to 55 mm. ^, The radius of curvature of the glenoid is slightly larger than that of the humeral head, which allows the humeral head to be compressed into the glenoid. The glenoid only covers 25%–30% of the humeral head in any given position, which further demonstrates the importance of the static and dynamic stabilizers for shoulder stability. ^,

The AC joint is an articulation between the convex, lateral end of the clavicle and the concave acromion process of the scapula. Additionally, it is a synovial joint that consists of an intra-articular disk and a joint capsule, which is thickest superiorly and posteriorly. The AC joint is supported by two sets of ligaments: the AC ligaments and the CC ligaments. The CC ligaments consist of the conoid and the trapezoid. The conoid attaches approximately 45 mm from the distal clavicle, whereas the trapezoid attaches approximately 25 mm from the distal clavicle ( Fig. 14.6 ).

The SC joint consists of the proximal end of the clavicle and the upper sternum and is the only joint connecting the shoulder complex to the axial skeleton. The clavicular articular surface is covered by fibrocartilage, and the joint is divided by an articular disk. The articular surface of the sternum is small and lacks congruency with the irregular surface of the clavicle. Consequently, the SC joint has minimal bony stability. Several ligamentous structures, including the SC ligaments, the interclavicular ligament, and the costoclavicular ligaments, provide stability to the joint ( Fig. 14.7 ). The SC ligaments connect the clavicle and sternum and stabilize the joint against anteroposterior movement of the clavicle. The interclavicular ligament runs across the sternum and connects the medial aspects of the bilateral clavicles, providing joint stabilization against superior translation. The costoclavicular ligaments, which are attached to the inferior surface of the clavicle and the first rib, resist medial and lateral displacement of the clavicle relative to the thorax. The SC joint allows elevation, depression, retraction, and rotation of the clavicle.

The scapulothoracic articulation is a collective unit composed of the AC joint, the SC joint, and the fascial spaces between the anterior surface of the scapula and the thorax. It serves as an important insertion site for several stabilizing muscles and contributes largely to scapular motion. The scapulothoracic articulation allows for elevation by the trapezius and levator scapulae, depression by the serratus anterior and pectoralis minor, protraction by the serratus anterior and pectoralis minor, retraction by the trapezius and rhomboids, lateral rotation by the trapezius and serratus anterior, and medial rotation by the levator scapulae and rhomboids. Smooth movement at this articulation is crucial for proper shoulder mechanics.

Static Stabilizers

There are three main static stabilizers of the GH joint: the labrum, the glenohumeral ligaments, and the capsule. The labrum is a fibrocartilaginous extension of the glenoid, which increases the depth of the glenoid by 50%. The labrum resists translation forces of the humeral head on the glenoid and serves as an attachment for the glenohumeral ligaments. The inferior labrum is attached more firmly to the glenoid, whereas the anterosuperior labrum is more loosely attached. The glenohumeral ligaments are thickened areas of the capsule and consist of the superior glenohumeral ligament (SGHL), the middle glenohumeral ligament (MGHL), the inferior glenohumeral ligament (IGHL), and the CHL ( Fig. 14.8 ). These ligaments are important for protecting against instability during extremes of joint motion as well as during midranges of motion. Furthermore, the capsule, the SGHL, and the CHL make up the rotator interval. The size of the rotator interval varies, and the larger the interval, the greater the potential for inferior and posterior instability. The SGHL arises from the anterosuperior labrum and inserts onto the lesser tuberosity. It is located in the rotator interval, which is the area between the superior border of the subscapularis and the anterior margin of the supraspinatus. The MGHL has been described as having the most variation in size of all the glenohumeral ligaments, and it may be absent in up to 40% of individuals. ^, It has a wide attachment and extends from the SGHL along the anterior margin of the glenoid fossa to the anatomic neck of the humerus. The IGHL is the thickest and runs from the anterior, inferior, and posterior glenoid margins to the humeral metaphysis. It is composed of three distinct portions: the anterior band (AIGHL), the posterior band (PIGHL), and the axillary pouch. The AIGHL and axillary pouch act as anterior stabilizers, while the PIGHL acts as a posterior stabilizer. The CHL originates from the base and lateral border of the coracoid process, and it makes up the rotator interval, along with the SGHL. As it descends obliquely, it separates into two bands that insert onto the lesser and greater tuberosities. The capsule surrounds the GHJ and is loose, allowing for a large range of humeral motion. It is attached medially to the glenoid fossa and laterally to the circumference of the anatomic neck. The capsule is relatively thin, varying anywhere between 1.3 and 4.5 mm, with the thickest portion in the anteroinferior quadrant and the thinnest in the rotator interval and posterior quadrant. ^,