The concavity of the glenoid represents a functional center of the scapula. The thin body of the scapula expands or broadens laterally to form the glenoid, which comprises a surface area three to four times smaller than the humeral head. The radius of curvature, however, is larger than that of the humeral head.¹ It is this size mismatch that generates the tremendous range of motion from the glenohumeral joint. The glenoid surface is nearly perpendicular to the plane of the scapula, with an average of 1.23° of retroversion.² Additionally, the glenoid is oriented 10° to 15° superior to the medial border of the scapula with a mean inclination of 7°. Primary features of the scapular anatomy adjacent to the glenoid are the spinoglenoid notch medially, the scapular spine superiorly, and the acromion expansion laterally. Medial to the glenoid vault, the scapular body is remarkably thin and encased in rotator cuff musculature.

The bony anatomy of the proximal humerus has four parts: the humeral head, the greater tubercle, the lesser tubercle and the shaft. The sphere of the articular surface of the humeral head is directed posteriorly and superiorly. The superior inclination of the humeral head relative to the humeral shaft ranges from 30° to 55° and the retroversion relative to the transepicondylar axis of the elbow ranges from 0° to 55° (mean, 30°).³ This retrotorsion allows the humeral head to remain oriented in the plane of the scapula.

The bony anatomy around the articular surface provides specific attachment points for the ligamentous and tendinous structures stabilizing the shoulder (Figure 1).
The lesser tubercle is anterior to the humeral head and the greater tubercle is lateral to the humeral head, with these bony protuberances separated by the intertubercular groove. This bicipital groove represents an important surgical landmark for both arthroscopic and open shoulder surgery.

The clavicle is the first bone to begin ossification in embryologic development and serves as the connection for the shoulder to the axial skeleton. The osseous anatomy of the clavicle is complex and can vary substantially.⁴ The clavicle is identifiable by its S-shaped curvature and is cephalad to the caudad bow. Medially, the clavicle articulates with the clavicular facet of the sternum, which together create the sternoclavicular joint. Laterally, the clavicle articulates with the acromion at the acromioclavicular joint.

There is a wealth of literature on the complex interplay of the glenohumeral joint and the critical balance between mobility and stability. When all parts of the joint work as intended, the balance between the inherent instability of a shallow concavity and the various stabilizing structures facilitates maximum function. The static and dynamic stabilizers are critical components of the glenohumeral joint. Static stabilizers consist of articular congruity and bony version, the glenoid labrum, the glenohumeral ligaments, and negative intra-articular pressure. Dynamic stabilizers consist of the rotator cuff muscles, the rotator interval, and the periscapular muscles.

It is vital to understand the normal osseous anatomy of the proximal humerus and glenohumeral joint as it pertains to reconstruction of this anatomy with anatomic shoulder arthroplasty. The aim of anatomic shoulder arthroplasty is to restore normal glenohumeral geometry; restoring the following relationships is particularly important: humeral head to tubercle height, acromiohumeral interval, and the anatomic center of rotation (Figure 2). A best fit or perfect circle has been described to illustrate variability between the anatomic
and prosthetic centers of rotation⁵ (Figure 3). Multiple studies have demonstrated that a change in the center of rotation of greater than 3 mm negatively affects postoperative range of motion and shoulder biomechanics.⁶,⁷

The acromioclavicular joint is a diarthrodial joint and represents the primary articulation connecting the upper extremity to the axial skeleton. The joint is therefore subjected to repetitive stress and strain and ultimately is at risk of degenerative disease. Scapulothoracic movement requires 40° to 50° of rotation from the acromioclavicular and sternoclavicular joints, but only 5° to 8° occurs at the acromioclavicular joint.⁸ The acromioclavicular ligaments and the coracoclavicular ligaments are the two primary groups of stabilizing structures for the acromioclavicular joint. The acromioclavicular ligaments primarily resist posterior translation and posterior axial rotation. The coracoclavicular ligaments are critical stabilizers that provide vertical stability and resist superior and anterior translation as well as anterior axial rotation.⁹

The sternoclavicular joint is the articulation of the clavicle with the manubrium of the sternum and the superior aspect of the first costal cartilage. The posterior sternoclavicular ligament connects the posterior aspect of the medial clavicle to the posterosuperior manubrium and is the primary anterior-posterior stabilizer. The anterior sternoclavicular ligament connects the medial clavicle to the superior anterior edge of the manubrium and prevents superior displacement. The costoclavicular ligament connects the inferior aspect of the clavicle to the first rib and acts as the primary restraint for the sternoclavicular joint. Multiple vital structures rest posterior to the sternoclavicular joint and are at risk in traumatic injuries. The vascular structures found posterior to the sternoclavicular joint include the common carotid artery, internal jugular vein, and the brachiocephalic trunk. Additionally, the phrenic nerve, vagus nerve, trachea, and esophagus are found posterior to the medial clavicle and sternoclavicular joint.

The scapular body articulates with the thorax where it rests over the posterolateral aspect of ribs 2 through 7.¹⁰ Over a large area of the scapular body, the bony thickness is remarkably thin, measuring between 10 and 26 mm.¹¹ Although no bony or ligamentous structure connects the scapula and the thorax, there are many muscular attachments to the scapula that serve to stabilize this unique osseous anatomy. The muscles connecting the scapula to the axial skeleton that serve as scapular stabilizers include the trapezius, serratus anterior, rhomboid major, rhomboid minor, levator scapulae, subclavius, and pectoralis minor muscles. The muscles connecting the scapula to the remainder of the upper extremity serve to position the arm in space and include the supraspinatus, infraspinatus, teres minor, teres major, deltoid, long and short heads of the biceps brachii, long head of the triceps brachii, and coracobrachialis.¹²

The muscular anatomy of the shoulder is perhaps more critically important than other functional human anatomy, given the inherent instability of the glenohumeral joint. These muscles serve to stabilize the shoulder and provide its motion to position the arm in space.

Shoulder pain is a relatively common presenting concern in the general population and therefore in a general medical practice. In a 2020 study, the prevalence of shoulder pain was found to be 42%, similar to that of low back pain (44%) or knee pain (48%).¹⁵ The lifetime prevalence has been reported to be as high as 66%.¹⁶ Shoulder disorders vary by age with certain conditions seen more commonly in youth, middle age, or older age groups.