Anatomy, Shoulder and Upper Limb, Glenohumeral Joint

Structure and Function

The glenohumeral joint is a ball-and-socket joint of the upper limb formed by the dynamic articulation between the scapula's glenoid cavity and the humeral head. Articular cartilage covers the articulating surfaces of these bones. The glenoid cavity is a shallow osseous structure deepened by a fibrocartilaginous rim, the glenoid labrum, which spans the vault’s osseous periphery. The labrum is continuous with the tendon of the long head of the biceps brachii (long head of biceps tendon or LHBT) at its superior aspect. The loose joint capsule and the humeral head’s disproportionate size relative to the shallow glenoid fossa (a 4:1 surface area ratio) contribute to the shoulder joint’s high mobility and susceptibility to dislocation.

The glenohumeral joint capsule is a fibrous sheath that forms a continuous support structure around the articulating elements, encircling the region from the anatomical neck of the humerus to the rim of the glenoid fossa. Within this capsule, the capsulolabral complexes contain distinct thickened bands, known as the glenohumeral ligaments, first described in 1829. These ligaments become taut at varying degrees of abduction and humeral rotation, unlike traditional ligaments that carry a pure tensile force along their length.[5][6] A synovial membrane also lines the joint capsule's inner surface, secreting synovial fluid to minimize friction between the articular surfaces.[7]

Multiple synovial bursae help reduce friction within the glenohumeral joint by cushioning between joint structures such as tendons. The shoulder bursae include the subacromial (subdeltoid), subcoracoid, and subscapular bursae.

The subacromial bursa lies between the deltoid muscle and the joint capsule in the superolateral aspect of the glenohumeral joint, superficial to the supraspinatus tendon. This bursa reduces friction underneath the deltoid muscle, allowing a greater range of motion. The subacromial bursa does not usually communicate with the shoulder joint. The subscapular bursa lies between the subscapularis tendon and the capsule, reducing frictional damage to the subscapularis muscle during internal shoulder rotation. The subcoracoid bursa is situated between the coracoid process and the subscapularis. The subacromial and subscapular bursae are clinically significant due to their vulnerability to injury.

Shoulder ligaments include the glenohumeral, coracoglenoid, coracohumeral, transverse humeral, coracoacromial, coracoclavicular, and acromioclavicular ligaments. The glenohumeral ligaments are composed of the superior, middle, and inferior ligaments. These fibrous structures are intrinsic to the glenohumeral region, reinforcing the glenohumeral capsule and preventing anterior shoulder dislocation. The glenohumeral ligaments serve as the primary stabilizers of the joint.

The coracoglenoid ligament is a component of the anterior limb of the superior glenohumeral ligament. This ligament originates from the middle of the coracoid process and inserts posterior to the supraglenoid tubercle, covering the superior glenoid rim, labrum, and LHBT. This ligament contributes to the anterosuperior stabilization of the shoulder joint.[8]

The coracohumeral ligament is an intrinsic band that supports the superior aspect of the glenohumeral joint capsule. This dense fibrous structure extends from the base of the coracoid process to the greater and lesser tuberosities. The ligament is thin and broad at its origin, measuring about 2 cm in diameter at the coracoid base. Laterally, this ligament separates into 2 distinct bands that envelop the LHBT at the proximal end of the bicipital groove.

The transverse humeral ligament secures the LHBT within the bicipital groove, passing between the greater and lesser tuberosities over the tendon sheath. Research suggests the absence of a distinct, separate transverse humeral ligament. Instead, the roof of the intertubercular groove comprises fibers from the subscapularis tendon, with contributions from the supraspinatus tendon and the coracohumeral ligament. Additionally, the transverse humeral ligament is closely associated with the biceps pulley.

The coracoacromial ligament is a robust, fibrous triangular band that forms a crucial part of the coracoacromial arch. This osteoligamentous arch is extrinsic to the glenohumeral region, shielding the humeral head and rotator cuff tendons from trauma. This ligament contributes to the roof of the glenohumeral joint, limiting the space available to the rotator cuff tendons, subacromial subdeltoid bursa, and the LHBT. Dysfunction or compression of these structures due to coracoacromial ligament abnormalities can lead to subacromial impingement syndrome and subacromial bursitis.

The coracoclavicular ligament consists of the conoid and trapezoid ligaments and spans from the coracoid process to the clavicle. This ligament preserves vertical stability at the acromioclavicular joint, preventing superior clavicular dislocation. Strong forces causing acromioclavicular joint injuries can rupture this ligament or its components. The acromioclavicular ligament comprises superior and inferior parts. The superior ligament spans from the upper acromion to the clavicle's edge, stabilizing the joint horizontally. The inferior acromioclavicular ligament is thinner than its superior counterpart and covers the lower joint portion, attaching to the adjoining surfaces of both bones.[9][10] Acromioclavicular joint injuries often present with glenohumeral damage due to the proximity of these structures.[11]

Static stabilizers of the glenohumeral joint include osseous articular anatomy, joint congruity, the glenoid labrum, glenohumeral ligaments, the joint capsule, and negative intraarticular pressure. Dynamic stability relies on the LHBT, rotator cuff muscles, rotator interval, and periscapular muscles.

Soft-Tissue Pulley System and the Long Head of the Biceps Tendon

The subscapularis consists of superficial and deep fibers that surround the bicipital groove, forming its "roof" and "floor," respectively. These fibers merge with those from the supraspinatus and the superior glenohumeral ligament/coracohumeral ligament complex. These structures attach intimately at the lesser tuberosity, forming the proximal and medial aspects of the pulley system, with soft tissue extensions enveloping the LHBT within the bicipital groove. The LHBT exits the groove and takes a 30º to 40º turn as it heads toward the supraglenoid tubercle and glenoid labrum. Thus, the proximal soft tissue elements of the groove play a critical role in maintaining the overall stability of the biceps complex.[12][13]

Glenohumeral Joint Motion

The glenohumeral joint permits a vast range of motion across multiple planes. Understanding the contribution of various muscles to these movements is essential for differentiating shoulder pathologies.[14]

Flexion moves the upper limb anteriorly in the sagittal plane. The typical range of motion is 180º. The shoulder's main flexors are the anterior deltoid, coracobrachialis, and pectoralis major. The biceps brachii weakly assists in this action. Extension displaces the upper limb posteriorly in the sagittal plane. The normal motion range is 45º to 60º. The shoulder's principal extensors are the posterior deltoid, latissimus dorsi, and teres major.

Internal rotation involves limb rotation toward the midline along a vertical axis. The normal range of motion is 70º to 90º. The shoulder's internal rotators are the subscapularis, pectoralis major, latissimus dorsi, teres major, and the anterior deltoid. External rotation entails limb rotation away from the midline along a vertical axis. The normal range of motion is 90º. The infraspinatus and teres minor are chiefly responsible for this action.

Adduction moves the upper limb toward the midline in the coronal plane, mainly driven by the latissimus dorsi, teres major, and pectoralis major. Abduction displaces the upper limb away from the midline in the coronal plane. The normal range of motion is 150º. The supraspinatus abducts the shoulder for the first 0º to 15º.[15] The deltoid's middle fibers abduct the shoulder from 15º to 90º.[16] Scapular rotation from activating the trapezius and serratus anterior enables further abduction beyond 90º.

Rotator Cable and Rotator Interval

The rotator cable and interval are crucial joint stabilizers that have generated tremendous interest in recent years. Although plain radiographs cannot visualize these structures, their pathologies are believed to contribute significantly to degenerative shoulder injuries.

The rotator cable originates from the coracohumeral ligament and divides into superficial and deep layers. This structure encircles the rotator cuff and inserts into the lesser and greater humeral tuberosities. The deeper layer forms a crescent-shaped configuration, extending posteriorly from the anterior insertion of the supraspinatus to the inferior border of the infraspinatus. This anatomical arrangement contributes significantly to shoulder stability. Research indicates that rotator cable thickness varies among individuals, with older shoulders tending to exhibit thicker cables, suggesting a shift toward greater reliance on this structure for superior rotator cuff stability with age.[17][18]

The rotator interval consists of anterior and posterior segments, though the anterior portion holds greater clinical relevance. This triangular space is bordered superiorly by the supraspinatus tendon, inferiorly by the subscapularis tendon, laterally by the coracohumeral ligament, and medially by the superior glenohumeral ligament. The rotator interval houses the LHBT, coracohumeral ligament, and superior glenohumeral ligament. This structure functions as a soft tissue restraint, securing the LHBT within the bicipital groove and preventing anteromedial subluxation during external rotation and anteroinferior subluxation during shoulder abduction. Additionally, the rotator interval restricts excessive inferior movement of the shoulder joint. Anatomical variations in this region may contribute to shoulder pathology.[19]

Embryology

Skeletal shoulder development involves both intramembranous and endochondral ossification. The clavicle forms through intramembranous ossification, where bone develops directly within the mesenchyme. The remaining shoulder structures arise via endochondral ossification.[20] The mesoderm germ layer gives rise to nearly all connective tissues within the musculoskeletal system, including the glenohumeral joint. Congenital musculoskeletal and limb anomalies, influenced by both genetic and environmental factors, represent a significant subset of developmental disorders.

Blood Supply and Lymphatics

The glenohumeral joint receives vascular supply from the posterior (PCHA) and anterior (ACHA) circumflex humeral arteries, both branches of the axillary artery. The PCHA contributes more significantly (64%) than the ACHA (34%) to the humeral head's blood supply. Distribution varies, with the PCHA perfusing the humeral head's lateral, superior, and inferior quadrants, while the ACHA primarily supplies a portion on the inferior aspect.[21][22] The arcuate artery extends from the ACHA's ascending branch and enters the bicipital groove. The subscapular artery, arising from the thyrocervical trunk, sends branches to the shoulder.[23] Veins accompanying these arteries drain the glenohumeral region.[24]

Most upper extremity lymphatic vessels drain toward the axilla. Axillary nodes are categorized into 5 groups based on location: pectoral, subscapular, humeral, central, and apical. Efferent vessels from the apical axillary nodes traverse the cervicoaxillary canal and merge to form the subclavian lymphatic trunk, which either drains into the right venous angle on the right or empties directly into the thoracic duct on the left. Assessing axillary lymph node involvement is vital for breast cancer staging. Disrupting lymphatic drainage can result in lymphedema, where subcutaneous lymph accumulation leads to painful upper limb swelling.[25]

Nerves

The suprascapular, lateral pectoral, and axillary nerves supply the glenohumeral joint. These nerves originate from the brachial plexus, a nerve network formed by the ventral rami of C5 to T1. Axillary nerve anatomy is critical, as it is close to the glenohumeral joint. The axillary nerve originates from the posterior cord of the brachial plexus, courses along the subscapularis to its inferior border, and travels closely along the inferior glenohumeral joint capsule. The nerve then transits posterior to the humerus, wraps around the surgical humeral neck alongside the PCHA, and enters the quadrangular space before running within the deep deltoid fascia.[26]

Muscles

The 4 muscles that constitute the rotator cuff are the supraspinatus, infraspinatus, subscapularis, and teres minor. The primary biomechanical function of the rotator cuff is to stabilize the glenohumeral joint by compressing the humeral head against the glenoid. The rotator cuff muscles thus act as the glenohumeral joint's dynamic stabilizers.[27][28] The LHBT also has a debated contribution to glenohumeral joint stability. Current consensus suggests that the LHBT's stabilizing role becomes more significant in the presence of rotator cuff dysfunction.[29]

The supraspinatus primarily abducts the shoulder and initiates the first 15° of abduction. The infraspinatus and teres minor contribute to external rotation, while the subscapularis facilitates internal rotation. The suprascapular nerve innervates the supraspinatus and infraspinatus muscles. The axillary nerve supplies the teres minor. The subscapular nerve innervates the subscapularis.

Physiologic Variants

Glenohumeral joint variations may arise from differences in glenoid cavity and humeral head shape and size. The glenoid cavity may be pear-shaped, oval, or inverted comma-shaped. The pear shape is the most common, while the inverted comma shape is the least prevalent. The oval shape provides the greatest stability, whereas the inverted comma shape is the least stable. These anatomical variations influence prosthetic design after shoulder arthroplasty and affect glenohumeral movement in sports such as javelin throwing and tennis.[30]

Humeral head shape variations influence the LHBT reflection pulley's movement and stability. Humeral head shape may be flat, convex, spiculated, or mixed. Flat humeral heads are prevalent in men and younger individuals, while convex ones are more common among women and older individuals. The mixed type occurs least frequently.[31]

Surgical Considerations

Shoulder osteoarthritis narrows the glenohumeral joint space due to joint capsule thickening and degeneration of the articular cartilage and subchondral bone. Osteoarthritis-related shoulder pain is a common and debilitating condition. Nonsurgical treatments include oral analgesics, intraarticular injections, physical therapy, and acupuncture, though evidence supporting their effectiveness in shoulder osteoarthritis is inconclusive. Unresponsiveness to conservative measures warrants surgical intervention.

Joint replacement surgery for shoulder osteoarthritis involves removing damaged bone and cartilage and releasing tight soft tissues contributing to contractures. Prosthetic materials replace the resected structures to restore shoulder joint anatomy and function. This procedure is the primary surgical treatment for shoulder osteoarthritis and may entail replacing only the humeral head (hemiarthroplasty) or both the humeral head and glenoid (total shoulder replacement). Newer techniques, particularly reverse total shoulder replacement, are gaining popularity, especially among younger patients and those with less advanced disease (see Image. Reverse Total Shoulder Arthroplasty).[32]

Clinical Significance

Shoulder Dislocations

The glenohumeral joint is by far the most frequently dislocated joint of the body, accounting for up to 45% of dislocations. Anterior dislocation comprises 96% of cases and is often the result of a force directed to the shoulder joint while the arm is abducted and externally rotated. Posterior dislocation is the 2nd most common, accounting for 2% to 4% of cases. Posterior dislocations usually arise from indirect mechanisms, such as electric shock or convulsions, causing contraction of the relatively stronger internal shoulder rotators, the latissimus dorsi, pectoralis major, and subscapularis.

The incidence of glenohumeral joint dislocation is approximately 17 per 100,000 annually, with peak occurrences among men aged 21 to 30 and women aged 61 to 80. Treatment typically involves analgesics and closed reduction, with some patients requiring subsequent surgical correction, especially those with recurrent shoulder dislocations from concurrent soft tissue injuries.[33] The axillary nerve, which wraps around the humeral surgical neck, can sustain damage during dislocation or reduction attempts. Axillary nerve injury may lead to deltoid paralysis and loss of sensation over the lateral shoulder.

Hill-Sachs lesions result from impaction fractures of the posterolateral humeral head against the anteroinferior glenoid. Bankart lesions stem from the detachment of the anteroinferior labrum with or without an avulsion fracture. Anterior dislocation can cause both conditions. The recurrence rate for glenohumeral joint dislocation averages 50%, with a higher risk in individuals who experience their first dislocation at a younger age.[34]

Adhesive Capsulitis

Adhesive capsulitis is characterized by glenohumeral capsule inflammation and stiffness, leading to significant motion restriction and chronic pain that typically worsens at night. The condition affects approximately 3% of the general population and is more common in adult women aged 40 to 70.[35] Risk factors include diabetes mellitus, connective tissue diseases, thyroid disease, and heart disease, with highly active antiretroviral therapies also implicated.[36]

Management includes physical and occupational therapy, analgesic medications, intraarticular steroid injections, and, in some cases, surgical intervention. Surgical options may involve manipulation under general anesthesia to disrupt joint adhesions and scar tissue. Most individuals eventually regain around 90% of their shoulder range of motion.

Rotator Cuff Injuries

The rotator cuff tendons are often injured due to their crucial role in stabilizing the glenohumeral joint. Rotator cuff conditions, including subacromial bursitis, tendinitis, impingement, and tears, are the primary source of shoulder pain in primary care settings.[37] Patients with impingements or subacromial bursitis often experience pain during overhead activities. Most cases improve with physical therapy, but persistent symptoms warrant an evaluation by an orthopedic surgeon or sports medicine specialist. Rotator cuff tears are often associated with tendon degeneration or shoulder impingement and may be classified as chronic or acute, partial- or full-thickness.