Glenohumeral joint

Articular surfaces

The glenohumeral joint is the articulation between the spherical head of the humerus and the concave glenoid fossa of the scapula. Being a synovial joint, both articular surfaces are covered with hyaline cartilage.

The glenoid fossa is a shallow pear-shaped pit on the superolateral angle of scapula. The concavity of the fossa is less acute than the convexity of the humeral head, meaning that the articular surfaces are not fully congruent. Congruency is increased somewhat by the presence of a glenoid labrum, a fibrocartilaginous ring that attaches to the margins of the fossa. The labrum acts to deepen the glenoid fossa slightly, it is triangular in shape and thicker anteriorly than inferiorly. The surface of the humeral head is three to four times larger than the surface of glenoid fossa, meaning that only a third of the humeral head is ever in contact with the fossa and labrum.

This incongruent bony anatomy allows for the wide range of movement available at the shoulder joint but is also the reason for the lack of joint stability. Instead, joint security is provided entirely by the soft tissue structures; the fibrous capsule, ligaments, shoulder muscles and their tendons.

Joint capsule

The shoulder joint is encircled by a loose fibrous capsule. It extends from the scapula to the humerus, enclosing the joint on all sides. The internal surface of the capsule is lined by a synovial membrane.

On the humerus, the capsule attaches to its anatomical neck. Extending only at its medial margin, where the fibers protrude by around 1 cm. On the scapula, the capsule has two lines of attachments. The first is on its anterior and inferior sides where the capsule inserts into the scapular neck, posterior to the glenoid labrum. The second is on its superior and posterior aspects, where the capsular fibers blend directly with the glenoid labrum. Here the capsule arches over the supraglenoid tubercle and it’s long head of biceps brachii muscle attachment, thus making these intra-articular structures.

The capsule remains lax to allow for mobility of the upper limb. It relies on ligaments and muscle tendons to provide reinforcement. The anterior capsule is thickened by the three glenohumeral ligaments while the tendons of the rotator cuff muscles spread over the capsule blending with its external surface. These tendons form a continuous covering called the rotator capsule. It is comprised of the supraspinatus superiorly, infraspinatus and teres minor posteriorly, subscapularis anteriorly and the long head of triceps brachii inferiorly.

Two weak spots exist in this reinforced capsule. The first is the rotator interval, an area of unreinforced capsule that exists between the subscapularis and supraspinatus tendons. The second is the inferior capsular aspect, this is the point where the capsule is the weakest. The loose inferior capsule forms a fold when the arm is in the anatomical position. It becomes stretched, and least supported, when the arm is abducted.

The capsule has two openings;

•  Between the greater and lesser tubercles of humerus, through which the tendon of the long head of biceps brachii passes.
• Between the superior and middle glenohumeral ligaments, via which the subscapular bursa communicates with the glenohumeral joint cavity.

Synovial fluid filled bursae assist with the joint’s mobility. The subdeltoid-subacromial (SASD) bursa is located between the joint capsule and the deltoid muscle or acromion, respectively. Similarly the subcoracoid bursae are found between the capsule and the coracoid process of the scapula. The subscapular bursa sits between the capsule and the subscapularis tendon, while the coracobrachial bursa is located between the subscapularis and coracobrachialis muscles. These bursae allow the structures of the shoulder joint to slide easily over one another.

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Ligaments

Several ligaments limit the movement of the GH joint and resist humeral dislocation. These are the coracohumeral, glenohumeral and transverse humeral ligaments. Glenohumeral and transverse humeral are capsular ligaments while coracohumeral is an accessory ligament.

The transverse humeral ligament extends horizontally between the tubercles of the humerus. It covers the intertubercular sulcus and the long head tendon of the biceps brachii muscle, preventing displacement of the tendon from the sulcus. The coracohumeral ligament extends between the coracoid process of the scapula to the tubercles of the humerus and the intervening transverse humeral ligament, supporting the joint from its superior side. It acts to limit inferior translation and excessive external rotation of the humerus.

The superior, middle and inferior glenohumeral ligaments support the joint from the anteroinferior side. They have a weak stabilizing function, each acting to limit the maximum amplitude of certain arm movements;

•  All three ligaments become taut during external (lateral) rotation of humerus, while they relax in internal (medial) rotation. They also resist anterior translation of the humeral head.
•  The middle and inferior ligaments tense during abduction, while the superior is relaxed.

The superior glenohumeral ligament extends from the supraglenoid tubercle of scapula to the proximal aspect of the lesser tubercle of humerus. Along with the coracohumeral ligament, it supports the rotator interval and prevents inferior translation of the humeral head, particularly during shoulder adduction.

The middle glenohumeral ligament attaches along the anterior glenoid margin of the scapula, just inferior to the superior GH ligament. It extends to the lesser tubercle of humerus. This wide ligament lies deep to, and blends, with the tendon of subscapularis muscle. It stabilizes the anterior capsule, limiting external rotation, particularly when the arm is in an abducted position (45o – 60o abduction).

The inferior glenohumeral ligament is a sling-like ligament extending between the glenoid labrum and the inferomedial part of the humeral neck. It is split into anterior and posterior bands, between which sits the axillary pouch. This is the strongest of the three GH ligaments, being thicker and longer than the other two. Both bands stabilize the humeral head when the arm is abducted above 90°. The anterior band limits external rotation of the arm, while the posterior band limits internal rotation.

Innervation

The glenohumeral joint is innervated by the subscapular nerve (C5-C6), a branch of the posterior cord of brachial plexus. The joint capsule is supplied from several sources;

• Suprascapular nerve supplies the posterior and superior aspects
• Axillary nerve innervates the anteroinferior part of the capsule
• Lateral pectoral nerve supplies the anterosuperior part and the rotator capsule

Blood supply

Blood supply to the shoulder joint comes from the anterior and posterior circumflex humeral, circumflex scapular and suprascapular arteries.

Movements

The glenohumeral joint has a greater range of movement (RoM) than any other body joint. Being a ball-and-socket joint, it allows movements in three degrees of freedom (average maximum glenohumeral active RoM is shown in brackets);

1. Flexion (110°) - extension (60°)
2. Abduction (120°) - adduction (0°)
3. Internal rotation (90°) - external rotation (90°)

Combination of these movements gives circumduction.

Activities of the arm rely on movement from not only the glenohumeral joint but also the scapulothoracic joint (acromioclavicular, sternoclavicular and scapulothoracic articulations). Together these joints can change the position of the glenoid fossa, relative to the chest wall. Thus repositioning the glenohumeral joint, and upper limb, within space. This provides for a greater range of motion available within the greater shoulder complex;

1. Flexion (180°) - extension (90°)
2. Abduction (180°) - adduction (30°)
3. Internal rotation (90°) - External rotation (90°)

The close-packed position of the glenohumeral joint is abduction and external rotation, while open packed (resting) position is abduction (40-50°) with horizontal adduction (30°). The joints’ capsular pattern is external rotation, followed by abduction, internal rotation and flexion. The additional accessory movements of spin, roll and slide (glide) are also available within the glenohumeral joint.

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Muscles acting on the shoulder joint

The scapulohumeral and thoracohumeral muscles are responsible for producing movement at the glenohumeral joint.

The prime flexors of the glenohumeral joint are the deltoid (anterior fibers) and pectoralis major (clavicular fibers) muscles. While coracobrachialis and the long head of biceps brachii assist as weak flexor muscles.

Extension is performed by the latissimus dorsi, teres major, pectoralis major (sternocostal fibers) and long head of triceps brachii muscles. Of note, is that these muscles have a stronger action when acting to extend the flexed arm.

The prime abductors of the arm are the supraspinatus and deltoid muscles. It is believed that the supraspinatus is important for movement initiation and early abduction, while the deltoid muscle is engaged from approximately 20° of abduction and carried the arm through to the full 180° of abduction. Contraction of the deltoid muscle applies a strong superior translation force to the humerus, this is countered by the action of the rotator cuff muscles, preventing superior humeral dislocation.

Adduction is produced by the pectoralis major, latissimus dorsi and teres major muscles. Together these three are known as the “climbing muscles”, as they are powerful adductors, alternatively they can lift the trunk up towards a fixed arm. The coracobrachialis, teres minor, short head of biceps, long head of triceps brachii and deltoid (posterior fibers) muscles are also active during this movement, depending on the position of the arm. In particular, accessory adductor muscles serve to counter the strong internal rotation produced by pectoralis major and latissimus dorsi.

Internal rotation is primarily performed by the subscapularis and teres major muscles. Pectoralis major, deltoid (anterior fibers) and latissimus dorsi are also capable of producing this movement. The main lateral rotators are the infraspinatus and teres minor muscles, with help from the posterior fibers of the deltoid muscle. External rotation of the humerus moves the greater tubercle out from under the acromial arch, allowing uninhibited arm abduction to occur.

Rotator cuff

The rotator cuff muscles are four muscles that form a musculotendinous unit around the shoulder joint. These are the supraspinatus, infraspinatus, teres minor and subscapularis muscles. The function of this entire muscular apparatus is to produce movement at the shoulder joint while keeping the head of humerus stable and centralized within the glenoid cavity.

Memorize the rotator cuff muscles using the mnemonic given below!

Rotator cuff SITS on the shoulder

• Supraspinatus
• Infraspinatus
• Teres minor
• Subscapularis

All four muscles are firmly attached around the joint in such a way that they form a sleeve (rotator capsule). Individually, each muscle has its own pulling axis that results in a certain movement (prime mover), while together they create a concavity compression. This is a stabilizing mechanism in which compression of the humerus into the concavity of glenoid fossa prevents its dislocation by translating forces.

Take the following custom quiz for a rotator cuff workout! You can even add and remove individual muscles if you like.