The Glenohumeral Joint – Anatomy, Nerve Supply, Movement

The glenohumeral joint is structurally a ball-and-socket joint and functionally is considered a diarthrodial, multiaxial, joint. The glenohumeral articulation involves the humeral head with the glenoid cavity of the scapula, and it represents the major articulation of the shoulder girdle.[rx] The latter also includes minor articulations of the sternoclavicular (SC), acromioclavicular (AC), and scapulothoracic joints.[rx][rx][rx] The glenohumeral joint ranks as the most mobile joint of the human body.[rx][rx] The static and dynamic stabilizing structures allow for extreme degrees of motion in multiple planes of the body that predisposes the joint to instability events.

Structure of The Glenohumeral Joint

The glenohumeral joint is a ball and socket joint that includes a complex, dynamic, articulation between the glenoid of the scapula and the proximal humerus. Specifically, it is the head of the humerus that contacts the glenoid cavity (or fossa) of the scapula. The articulating surfaces of both have a lining of articular cartilage. The glenoid cavity is a shallow osseous element that is structurally deepened by a fibrocartilagenous rim, the glenoid labrum,  that spans the osseous periphery of the vault. The labrum is continuous with the tendon of the biceps brachii at its superior aspect.[rx]

Due to the loose joint capsule, and the relative size of the humeral head compared to the shallow glenoid fossa (4:1 ratio in surface area), it is one of the most mobile joints in the human body. This increased mobility contributes to it being the most commonly dislocated joint.[rx]

The glenohumeral joint is enclosed by a joint capsule that encapsulates the structures of the joint in a fibrous sheath. Structurally the joint capsule wraps around the anatomic neck of the humerus to the rim of the glenoid fossa.  While the joint capsule itself is a contiguous supportive structure surrounding the articulating elements, the capsulolabral complexes include important characteristic thickened bands that constitute the glenohumeral ligaments. First described in 1829, the glenohumeral ligaments do not act as traditional ligaments that carry a pure tensile force along their length, but rather, the glenohumeral ligaments become taut at varying positions of abduction and humeral rotation.[rx][7] A synovial membrane forms the lining of the inner surface of the joint capsule. This membrane produces synovial fluid to reduce friction between the articular surfaces.[rx]

In addition to the synovial fluid reducing friction within the joint, there are multiple synovial bursae present as well. These bursae functionally act as a cushion between joint structures, such as tendons. The most clinically significant are the subacromial and subscapular bursae. There are numerous, including:

• Subacromial/subdeltoid bursa – This structure lies between the deltoid muscle and joint capsule in the superolateral aspect of the joint. It is superficial to the supraspinatus tendon. This bursa reduces friction underneath the deltoid muscle, allowing an increased range of motion. This bursa, excluding anatomic variants, does not usually communicate with the shoulder joint itself.
• Subcoracoid bursa –  This bursa is between the coracoid process and the subscapularis.
• Subscapular bursa – is located between the tendon of the subscapularis muscle and the capsule. It functions to reduces frictional damage to the subscapularis muscle during movement of the glenohumeral joint, particularly during internal rotation.

Static

It stabilizing structures include the osseous articular anatomy and joint congruity, the glenoid labrum, the glenohumeral ligaments, joint capsule, and negative intraarticular pressure [rx]:

• Glenohumeral ligaments– Composed of a superior, middle, and inferior ligament, these three ligaments combine to form the glenohumeral joint capsule connecting the glenoid fossa to the humerus. Due to their location, they protect the shoulder and prevent it from dislocating anteriorly — this group of ligaments functions as the primary stabilizers of the joint.
• Coracoclavicular ligament – This ligament is composed of the conoid and trapezoid ligaments and spans from the coracoid process to the clavicle. It functions to maintain the position of the clavicle in conjunction with the acromioclavicular ligament. Strong forces can rupture these ligaments during acromioclavicular joint injuries.
• Coracohumeral ligament – This ligament supports the superior aspect of the joint capsule. It is a dense fibrous structure connecting the base of the coracoid process to the greater and lesser tuberosities. At its origin, the ligament is thin and broad, measuring about 2 cm in diameter at the base of the coracoid. Laterally, the CHL separates into two distinct bands that envelope the Long Head Biceps tendon at the proximal extent of the bicipital groove.

Dynamic

It stabilizing structures include the Long head biceps tendon, rotator cuff muscles, the rotator interval, and the periscapular muscles.

Soft tissue pulley system and Long head of the biceps tendon (LHBT) [rx][rx]

• The subscapularis has superficial and deep fibers that envelope the bicipital groove, creating the “roof” and “floor,” respectively. These fibers also coalesce with those from the supraspinatus and superior glenohumeral ligament/coracohumeral ligament complex.  These structures attach intimately at the lesser tuberosity to create the proximal and medial aspect of the pulley system, with soft tissue extensions serving to further envelope the LHBT in the bicipital groove. Once the LHBT exits the groove, it takes a 30- to 40-degree turn as it heads toward the supraglenoid tubercle and glenoid labrum. Thus, the proximal soft tissue elements of the groove are especially critical for the overall stability of the entire biceps complex.

The glenohumeral joint possesses the capability of allowing an extreme range of motion in multiple planes.[rx]

• Flexion – Defined as bringing the upper limb anterior in the sagittal plane. The usual range of motion is 180 degrees. The main flexors of the shoulder are the anterior deltoid, coracobrachialis, and pectoralis major. Biceps brachii also weakly assists in this action.
• Extension—Defined as bringing the upper limb posterior in a sagittal plane. The normal range of motion is 45 to 60 degrees. The main extensors of the shoulder are the posterior deltoid, latissimus dorsi, and teres major.
• Internal rotation—Defined as rotation toward the midline along a vertical axis. The normal range of motion is 70 to 90 degrees. The internal rotation muscles are the subscapularis, pectoralis major, latissimus dorsi, teres major, and the anterior aspect of the deltoid.
• External rotation – Defined as rotation away from the midline along a vertical axis. The normal range of motion is 90 degrees. Primarily infraspinatus and teres minor are responsible for the motion.
• Adduction – Defined as bringing the upper limb towards the midline in the coronal plane. Pectoralis major, latissimus dorsi, and teres major are the muscles primarily responsible for shoulder adduction.
• Abduction – Defined as bringing the upper limb away from the midline in the coronal plane. The normal range of motion is 150 degrees. Due to the ability to differentiate several pathologies by the range of motion of the glenohumeral joint in this plane of motion, it is essential to understand how different muscles contribute to this action.[rx]

I. The supraspinatus is responsible for the first 0 to 15 degrees of abduction[rx]

II. The middle fibers of the deltoid are responsible for approximately 15 to 90 degrees of abduction following[rx]

III. Scapular rotation due to the actions of the trapezius and serratus anterior allow for abduction beyond 90 degrees

Blood Supply of The Glenohumeral Joint

The glenohumeral joint receives vascular supply via the posterior and anterior circumflex humeral arteries, both of which are branches of the axillary artery. The predominant arterial blood supply to the humeral head is via the posterior humeral circumflex artery.[rx][rx] The arcuate artery is the extension/continuation of the ascending branch of the anterior humeral circumflex. It enters the bicipital groove and supplies most of the humeral head. A branch of the thyrocervical trunk, the subscapular arteries, and its branches, also contribute to the blood supply of the shoulder.[rx]

The majority of the lymph nodes in the upper extremity are located within the axilla. These can be divided based on location into five main groups: pectoral, subscapular, humeral, central, and apical. Efferent vessels coming from the apical axillary nodes travel through the cervico-axillary canal and then converge to form the subclavian lymphatic trunk. This trunk will either continue to enter the right venous angle or drain directly into the thoracic duct on the right and left, respectively. Removal and analysis of axillary lymph nodes is often an essential tool in the staging of breast cancers. The interruption of lymphatic drainage from the upper limb can, however, result in lymphoedema, a condition where accumulated lymph in the subcutaneous tissue leads to painful swelling of the upper limb.[rx]

Nerves

Innervation of the glenohumeral joint is a function of the suprascapular, lateral pectoral, and axillary nerves. All of the nerves supplying the glenohumeral joint originate from the brachial plexus, which is a network of nerves formed by the ventral rami of the lower four cervical nerves and the first thoracic nerve (C5, C6, C7, C8, and T1). The anatomy of the axillary nerve is critical as it is close to the glenohumeral joint. The axillary nerve arises from the posterior cord of the brachial plexus, courses along the subscapularis to its inferior edge, and then passes closely along the inferior glenohumeral joint capsule. It then courses posterior to the humerus, wraps around the surgical neck of the humerus with the posterior circumflex artery, running in the deep deltoid fascia.

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