Chapter 30. Shoulder and Axilla

Big Picture

The combined joints connecting the scapula (scapulothoracic joint), clavicle (sternoclavicular and acromioclavicular joints), and humerus (glenohumeral joint) form the shoulder complex and anchor the upper limb to the trunk. The only boney stability of the upper limb to the trunk is through the connection between the clavicle and the sternum. The remaining stability of the shoulder complex depends on muscles, and as a result, the shoulder complex has a wide range of motion.

Scapular Support

To best understand the actions of the scapula, it is important to understand the scapulothoracic, acromioclavicular, and sternoclavicular joints (Figure 30-1A).

• Scapulothoracic joint. Formed by the articulation of the scapula with the thoracic wall through the scapular muscles, including the trapezius and the serratus anterior muscles. The scapulothoracic joint is not considered a true anatomic joint; as such, it is frequently referred to as a “pseudo joint” because it does not contain the typical joint characteristics (e.g., synovial fluid and cartilage).
• Acromioclavicular joint. A synovial joint formed by the articulations of the scapula (acromion) and the clavicle.
• Sternoclavicular joint. A synovial joint formed by the articulations between the clavicle and sternum.

The scapulothoracic, sternoclavicular, and acromioclavicular joints are interdependent. For example, if motion occurs at one joint (e.g., the scapula elevates), the movement will directly affect the other two joints. Therefore, the motions produced frequently involve more than a single joint. Although the scapular movements include the scapulothoracic, acromioclavicular, and sternoclavicular joints, we will refer only to the scapula in the following text.

Movements of the Scapula

The following terms describe the movements of the scapula (Figure 30-1B):

• Protraction. Anterior movement of the scapula on the thoracic wall (e.g., reaching in front of the body).
• Retraction. Posterior movement of the scapula on the thoracic wall (e.g., squeezing the shoulder blades together).
• Elevation. Raising the entire scapula in a superior direction without rotation.
• Depression. Lowering the entire scapula in an inferior direction without rotation.
• Upward rotation. Named according to the upward rotation and direction that the glenoid fossa faces.
• Downward rotation. Named according to the downward rotation and direction that the glenoid fossa faces.

Actions of the Glenohumeral Joint

The glenohumeral joint is a synovial, ball-and-socket joint. The “ball” is the head of the humerus, and the “socket” is the glenoid fossa of the scapula. The glenohumeral joint is considered to be the most mobile joint in the body and produces the following actions (Figure 30-1C and D):

• Flexion. Movement anterior in the sagittal plane.
• Extension. Movement posterior in the sagittal plane.
• Abduction. Movement away from the body in the frontal plane.
• Adduction. Movement toward the body in the frontal plane.
• Medial rotation. Movement toward the body in the transverse or axial plane.
• Lateral rotation. Movement away from the body in the transverse or axial plane.
• Circumduction. A combination of glenohumeral joint motions that produce a circular motion.

Big Picture

The musculature of the scapulothoracic joint is responsible primarily for the stability of the scapula to provide a stable base for the muscles acting on the glenohumeral joint. In other words, the upper limb must have proximal stability to have distal mobility. In addition, the scapulothoracic joint works in conjunction with the glenohumeral joint to produce movements of the shoulder. For example, the available range of motion for shoulder abduction is 180 degrees. This motion is produced by approximately 120 degrees from abduction at the glenohumeral joint and by approximately 60 degrees of upward rotation from the scapulothoracic joint.

Muscles of the Scapula

The following are muscles of the scapula (Figure 30-2A–C and Table 30–1):

• Trapezius muscle. Attaches to the occipital bone, nuchal ligament, spinous processes of C7–T12, spine of the scapula, acromion, and the clavicle. The trapezius muscle is a triangular shape and has the following muscle fiber orientations:
  • Superior fibers. Course obliquely from the occipital bone and upper nuchal ligament to the scapula, producing scapular elevation and upward rotation.
  • Middle fibers. Course horizontally from the lower nuchal ligament and thoracic vertebrae to the scapula, producing scapular retraction.
  • Inferior fibers. Course superiorly from the lower thoracic vertebrae to the scapula, producing scapular depression and upward rotation.

The multiple fiber orientations of the trapezius muscle stabilize the scapula to the posterior thoracic wall during upper limb movement, which is innervated by the spinal accessory nerve [cranial nerve (CN) XI]. The vascular supply is provided by the superficial branch of the transverse cervical artery.

• Levator scapulae muscle. Located deep to the trapezius muscle and superior to the rhomboid muscles. The levator scapula muscle attaches to the cervical vertebrae (C1–C4) and the superior angle of the scapula, producing elevation and downward rotation of the scapula. The nerve supply is provided by branches of the ventral rami from spinal nerves C3 and C4 and occasionally by C5 via the dorsal scapular nerve. The vascular supply is provided by the deep branch of the transverse cervical artery.
• Rhomboid major and minor muscles. The rhomboid minor is superior to the rhomboid major, with both positioned deep to the trapezius muscle. The rhomboid minor muscle attaches to the spinous processes of C7–T1. The rhomboid major muscle attaches to the spinous processes of T2–T5. Both muscles attach to the medial border of the scapula, resulting in scapular retraction. They are innervated by the dorsal scapular nerve (ventral ramus of C5) and the vascular supply from the deep branch of the transverse cervical artery. In some instances, the dorsal scapular artery will replace the deep branch of the transverse cervical artery.
• Pectoralis minor muscle. Attaches anteriorly on the thoracic skeleton to ribs 3 to 5 and superiorly to the coracoid process of the scapula. The pectoralis minor muscle protracts, depresses, and stabilizes the scapula against the thoracic wall. The axillary vessels and the brachial plexus travel posteriorly to the pectoralis minor muscle. The deltoid and pectoral branches of the thoracoacromial trunk and the superior and lateral thoracic arteries provide the vascular supply to this muscle. The medial pectoral nerve (C8–T1) provides innervation to this muscle.
• Serratus anterior muscle. Attaches to ribs 1 to 8 along the midaxillary line and courses posteriorly to the medial margin of the scapula. The serratus anterior muscle primarily protracts and rotates the scapula and stabilizes the medial border of the scapula against the thoracic wall. The lateral thoracic arteries provide the vascular supply to the serratus anterior muscle, and the long thoracic nerve provides innervation (ventral rami of C5–C7).
• Subclavius muscle. Attaches to the first rib and clavicle. The subclavius muscle depresses the clavicle and provides stability to the sternoclavicular joint. The nerve to the subclavius muscle (C5–C6) innervates this muscle.

Muscles of the Glenohumeral Joint

The following muscles and muscle groups comprise the muscles of the glenohumeral joint (Figure 30-3A–C and Table 30–2)

• Deltoid muscle. The deltoid muscle attaches to the spine of the scapula, acromion, clavicle, and the deltoid tuberosity of the humerus. It has a triangular shape, with the muscle fibers coursing anteriorly to the glenohumeral joint producing flexion, laterally producing abduction, and posteriorly producing extension. The deltoid muscle is innervated by the axillary nerve (C5–C6) and receives its blood supply from the thoracoacromial trunk (deltoid and acromial branches), the anterior and posterior humeral circumflex arteries, and the subscapular artery.
• Intertubercular groove muscles. This group of muscles is named because of their common insertion into the intertubercular sulcus of the humerus.
  • Pectoralis major muscle. Attaches to the sternum, clavicle, and costal margins and laterally attaches over the long head of the biceps brachii tendon to insert into the lateral lip of the intertubercular groove of the humerus. The pectoralis major muscle is a prime flexor, adductor, and medial rotator of the humerus. The medial and lateral pectoral nerves (ventral rami of C5–T1) provide innervation. The pectoral branch of the thoracoacromial trunk provides most of the blood supply to the pectoralis major muscle.
  • Latissimus dorsi muscle. A broad, flat muscle of the lower region of the back. The latissimus dorsi muscle attaches to the spinous processes of T7 inferiorly to the sacrum via the thoracolumbar fascia, and inserts laterally into the intertubercular groove of the humerus. The latissimus dorsi muscle acts on the humerus (arm), causing powerful adduction, extension, and medial rotation of the arm. It is innervated by the thoracodorsal nerve (ventral rami of C6–C8) and receives its blood supply from the thoracodorsal artery (branch off the axillary artery).
  • Teres major muscle. Attaches to the inferior angle of the scapula and the medial lip of the intertubercular sulcus. The teres major muscle medially rotates the humerus and is innervated by the lower subscapular nerve (C5–C6).
• Rotator cuff muscles. The rotator cuff muscles consist of four muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) that form a musculotendinous cuff around the glenohumeral joint. The cuff provides muscular support primarily to the anterior, posterior, and superior aspects of the joint (the first letter of each muscle forms an acronym known as SITS).
  • Supraspinatus muscle. Attaches to the supraspinous fossa and courses under the acromion to attach to the greater tubercle of the humerus. Humeral abduction is the primary action of the supraspinatus muscle. The suprascapular nerve (C5–C6) and the suprascapular artery provide innervation and blood supply to the supraspinatus muscle.
  • Infraspinatus muscle. Attaches to the infraspinous fossa of the scapula and courses posteriorly to the glenohumeral joint to attach to the greater tubercle of the humerus. The primary action of the infraspinatus muscle is lateral rotation of the humerus. The suprascapular nerve (C5–C6) provides innervation, and the suprascapular artery provides the vascular supply to the infraspinatus muscle.
  • Teres minor muscle. Attaches to the lateral border of the scapula and greater tubercle of the humerus. The teres minor muscle courses posteriorly to the glenohumeral joint and produces lateral rotation of the humerus. The axillary nerve (C5–C6) supplies innervation, and the circumflex scapular artery supplies blood to the teres minor muscle.
  • Subscapularis muscle. Attaches to the subscapular fossa on the deep side of the scapula. The subscapularis muscle crosses the anterior glenohumeral joint to attach to the lesser tubercle of the humerus, producing medial rotation of the humerus with contraction. The muscle is innervated by the upper and lower subscapular nerves (C5–C6). The suprascapular, axillary, and subscapular arteries supply blood to the subscapularis muscle.

Axillary Borders

The axilla is a passageway that connects the structures that run from the trunk to the arm. The borders of the axillary region are as follows:

• Anterior border. Pectoralis major and minor muscles and the clavicopectoral fascia.
• Posterior border. Subscapularis, teres major, and latissimus dorsi muscles.
• Lateral border. Intertubercular groove of the humerus and the coracobrachialis muscle.
• Medial border. Serratus anterior (ribs 1–4).
• Apex. Rib 1, the clavicle, and the proximal edge of the subscapularis muscle.
• Base. Skin, subcutaneous tissue, and axillary fascia that spans from the arm to the thorax.

Big Picture

The upper limb is innervated by the ventral rami from nerve roots C5–T1, which form a network of nerves referred to as the brachial plexus. The brachial plexus is divided into five regions consisting of roots, trucks, divisions, cords, and terminal branches.

Roots

The roots of the brachial plexus are a continuation of the ventral rami of C5–T1 and pass between the anterior and middle scalene muscles with the subclavian artery (Figure 30-4A). The following nerves branch off the roots of the brachial plexus:

• Dorsal scapular nerve (C5). Branches off the C5 nerve root. The dorsal scapular nerve pierces the middle scalene muscle and descends deep to the levator scapulae and rhomboid major and minor muscles, along with the deep branch of the transverse cervical artery. The nerve supplies both rhomboid muscles and occasionally supplies the levator scapulae.
• Long thoracic nerve (C5–C7). Branches off the C5–C7 roots, descends posteriorly to the roots of the plexus and the axillary artery and along the lateral surface of the serratus anterior muscle, with the lateral thoracic artery, while supplying the muscle. The long thoracic nerve is one of the few nerves found superficial to the serratus anterior muscle.

Trunks

The trunks of the brachial plexus (upper, middle, and lower) emerge laterally between the anterior and middle scalene muscles and descend toward the clavicle (Figure 30-4A). The following nerves branch off the trunks of the brachial plexus:

• Suprascapular nerve (C5, C6). This branch from the upper trunk passes through the posterior triangle of the neck and the suprascapular foramen, inferior to the transverse scapular ligament (the suprascapular artery and vein pass superior to the transverse scapular ligament), entering the supraspinous fossa to supply the supraspinatus muscle. The suprascapular nerve then continues inferiorly through the greater scapular notch to the infraspinous fossa to supply the infraspinatus muscle.
• Nerve to the subclavius (C5, C6). Branches off the upper trunk and descends anteriorly to the brachial plexus and subclavian artery and posteriorly to the clavicle to the subclavius muscle.

Divisions

The divisions of the brachial plexus (anterior and posterior) consist of three anterior divisions that travel anteriorly to the axillary artery and give rise to the nerves that innervate the anterior compartments of the limb (flexor muscles). In addition to the three anterior divisions, three posterior divisions travel posteriorly to the axillary artery. The posterior divisions give rise to the nerves that innervate the posterior compartments of the limb (extensor muscles) (Figure 30-4A and B).

Cords

The cords of the brachial plexus (medial, lateral, and posterior) pass posteriorly to the pectoralis minor muscles. The medial and lateral cords arise from the anterior divisions and are named for their relation to the axillary artery; the posterior cord arises from the posterior division and runs posteriorly to the axillary artery. The cords give rise to the terminal branches of the plexus. The following nerves are branches from the medial, lateral, and posterior cords of the brachial plexus (Figure 30-4A–C):

• Medial cord
  • Medial pectoral nerve (C8, T1). Branches off the medial cord, receiving a contribution from the lateral pectoral nerve. The medial pectoral nerve innervates the pectoralis minor muscle as it pierces it and continues on to innervate the pectoralis major muscle.
  • Medial cutaneous nerve of the arm (C8–T1). Provides cutaneous innervation to the medial side of the arm.
  • Medial cutaneous nerve of the forearm (C8–T1). Provides cutaneous innervation to the medial side of the forearm.
• Lateral cord
  • Lateral pectoral nerve (C5–C7). Branches off the lateral cord, sending a branch to the medial pectoral nerve anteriorly to the axillary artery and passing proximally to the pectoralis minor muscle to reach the pectoralis major muscle.
• Posterior cord
  • Upper subscapular nerve (C5, C6). Branches off the posterior cord and enters the anterior surface of the subscapularis muscle.
  • Thoracodorsal nerve (middle subscapular) (C6–C8). Branches off the posterior cord and descends to the posterolateral thorax to supply the latissimus dorsi muscle.
  • Lower subscapular nerve (C5, C6). Branches off the posterior cord and bifurcates, sending one branch to the anterior surface of the subscapularis muscle and one branch to the anterior surface of the teres major muscle.

Terminal Branches

The terminal branches are the most distal region of the brachial plexus (Figure 30-4A–C). They travel distally to provide motor and sensory innervation to both the anterior (flexor) and posterior (extensor) compartments of the arm and forearm.

• Musculocutaneous nerve (C5–C7). Formed from the lateral cord. The musculocutaneous nerve frequently pierces the coracobrachialis muscle, as it continues distally to provide motor innervation to the anterior compartment of the arm and sensory innervation to the lateral forearm.
• Median nerve (C6–T1). Formed from the lateral and medial cords. There are no branches off the median nerve until it reaches the anterior forearm.
• Ulnar nerve (C7–T1). Formed from the medial cord. The ulnar nerve gives off no branches until it reaches the anterior forearm.
• Axillary nerve (C5, C6). Terminal branch off the posterior cord. The axillary nerve wraps posteriorly through the quadrangular space and then bifurcates into anterior and posterior branches. The posterior branch innervates the teres minor muscle. The anterior branch wraps anteriorly around the humerus, deep to the deltoid, while sending fibers into the muscle. Both branches contribute to the superior lateral cutaneous nerve of the arm, supplying the shoulder joint tissues and most of the skin surrounding the deltoid muscle.
• Radial nerve (C5–T1). Formed from the posterior cord. The radial nerve travels posteriorly around the humerus, through the radial groove, to provide motor innervation to the posterior compartment of the arm and sensory innervation to the inferior lateral skin of the arm.

• Musculocutaneous nerve. Weakness in elbow flexion and supination as a result of paralysis of the biceps brachii and brachialis muscles. Sensory loss may be noted on the lateral surface of the forearm.
• Radial nerve. Inability to extend the wrist (wrist drop) as a result of paralysis of forearm extensors. Sensory loss may be noted on the dorsal surface of the hand and digits 1, 2, 3 and radial half of digit 4.
• Median nerve. Inability to flex the 1st to 3rd proximal interphalangeal joints, and 2nd and 3rd distal interphalangeal joints due to paralysis of the flexor digitorum superficialis and medial slips of the flexor digitorum profundus muscles. Flexion of the 4th and 5th distal interphalangeal joints is unaffected because the lateral slips of the flexor digitorum profundus muscle is supplied by the ulnar nerve. Flexion of the 2nd and 3rd metacarpophalangeal joints is also lost due to paralysis of the 1st and 2nd lumbrical muscles. Therefore, when a patient with a median nerve injury attempts to make a fist the 2nd and 3rd fingers remain partially extended (“sign of benediction”). Strength in the thumb is also affected due to innervation of the flexor pollicus longus and thenar muscles (“carpal tunnel syndrome”). Sensory loss may be noted on the palmar surface of hand, digit 1, 2, 3 and radial half of digit 4.
• Ulnar nerve. Weakness with finger abduction due to paralysis of interinsic hand muscles such as the dorsal interossei muscles. Sensory loss may be noted on the palmar and dorsal surface of the hand, digit 5 and the ulnar half of digit 4.
  

Big Picture

Blood supply to both upper limbs is provided by the subclavian arteries. The right subclavian artery is a branch from the brachiocephalic artery, and the left subclavian artery is a branch from the aortic arch. The subclavian artery becomes the axillary artery as it crosses over the lateral border of the first rib. The axillary artery continues distally and becomes the brachial artery at the inferior border of the teres major muscle. The brachial artery continues distally, passing over the elbow, and becomes the ulnar and radial arteries.

Subclavian Artery

The subclavian artery branches directly from the aortic arch on the left and from the brachiocephalic artery on the right. The subclavian artery passes between the anterior and middle scalene muscles and then posteriorly to the clavicle (the subclavian vein travels anteriorly to the anterior scalene) (Figure 30-5A–C).

• Thyrocervical trunk. Branches from the subclavian artery, medially to the anterior scalene.
  • Transverse cervical artery. Branches from the thyrocervical trunk and crosses anteriorly to the anterior scalene muscle. At the lateral border of the anterior scalene, the transverse cervical artery branches into the deep and superficial transverse cervical arteries.
    • Superficial transverse cervical artery. Travels deep between the trapezius and the rhomboid muscles.
    • Deep transverse cervical artery (dorsal scapular artery). Travels deep to the trapezius and levator scapulae muscles and along the medial border of the scapula, deep to the rhomboid muscles. The deep transverse cervical artery forms collateral circuits with the circumflex scapular and suprascapular arteries.
      • Dorsal scapular artery. Occasionally, the dorsal scapular artery will branch directly off the subclavian artery. When this occurs, the deep transverse cervical artery may be absent.
  • Suprascapular artery. Branches from the thyrocervical trunk, and courses superior to the transverse scapular ligament. The suprascapular artery courses from the supraspinous fossa through the greater scapular notch to the infraspinous fossa and forms collateral circuits with the circumflex scapular and dorsal scapular arteries.

Axillary Artery

The axillary artery is a continuation of the subclavian artery and is divided into three parts (Figure 30-5A–C):

• First part (one branch). Lateral border of the first rib to the medial border of the pectoralis minor muscle.
  • Superior thoracic artery. Supplies the upper part of the anterior and medial axillary walls.
• Second part (two branches). Runs posteriorly to the pectoralis minor muscle.
  • Thoracoacromial artery (trunk). Wraps around the proximal border of the pectoralis minor muscle and then branches into the pectoral, acromial, clavicular, and deltoid branches (you can remember this by thinking of the acronym PACD). Each arterial branch of the thoracoacromial artery is named for the region it supplies.
  • Lateral thoracic artery. Courses with the long thoracic nerve along the lateral surface of the thorax, supplying the serratus anterior muscle and the surrounding tissue.
• Third part (three branches). Lateral border of the pectoralis minor muscle to the inferior border of the teres major muscle.
  • Subscapular artery. Courses along the anterior surface of the subscapularis muscle and branches into the circumflex scapular artery and the thoracodorsal artery.
    • Circumflex scapular artery. Courses through the triangular space to the posterior side of the scapula, forming an anastomosis with the suprascapular and dorsal scapular (deep transverse cervical) arteries.
    • Thoracodorsal artery. Courses with the thoracodorsal nerve along the posterolateral thorax, supplying the latissimus dorsi muscle.
  • Anterior circumflex humeral artery. Courses anteriorly around the surgical neck of the humerus and forms an anastomosis with the posterior circumflex humeral artery.
  • Posterior circumflex humeral artery. Passes through the quadrangular space with the axillary nerve, wraps around the surgical neck of the humerus, and forms anastomoses with the anterior circumflex humeral. Supplies the surrounding muscles and the glenohumeral joint.

Anatomic Spaces

The following spaces are helpful in locating the neurovascular structures in the posterior scapular region (Figure 30-5B):

• Quadrangular space
  • Borders. Teres major and teres minor muscles, long head of the triceps brachii muscle, and the humerus.
  • Contents. Axillary nerve and the posterior circumflex humeral artery.
• Triangular space
  • Borders. Teres major and teres minor muscles and the long head of the triceps brachii muscle.
  • Contents. Circumflex scapular artery.

Big Picture

Lymphatic vessels and nodes in the shoulder and axillary region drain excess interstitial fluid as well as having an immunologic function (Figure 30-6). Lymphatics from the right upper limb drain into the right subclavian vein via the right lymphatic duct. Lymphatics from the left upper limb drain into the left subclavian vein via the thoracic duct.

Lymph Nodes

The lymphatics of the shoulder are organized into the following axillary lymph nodes (Figure 30-6A):

• Humeral (lateral) nodes. Located posteriorly to the axillary vein and receive drainage from the arm.
• Pectoral (anterior) nodes. Located along the distal border of the pectoral minor muscles and receive drainage from the abdominal wall, thoracic wall, and the mammary gland.
• Subscapular (posterior) nodes. Located along the posterior axillary wall and receive drainage from the posterior axillary wall, neck, shoulder, and upper back.
• Central nodes. Embedded in axillary fat and receive drainage from the humeral, pectoral, and subscapular nodes.
• Apical nodes. Surrounds the axillary vein near the pectoralis minor muscle. The apical nodes drain all other axillary nodes and lymphatic vessels from the mammary gland into the subclavian veins.

Veins of the Shoulder and Axilla

The veins of the scapular region generally follow the arteries and have similar names (e.g., the axillary vein and the axillary artery) (Figure 30-6B and C). The veins of the scapular region generally drain into veins of the back, neck, axilla, and arm and eventually reach the superior vena cava. The upper limb contains a deep and a superficial venous system.

• Deep venous system. Follows the arteries and usually consists of two or more veins.
• Superficial venous system. Consists of the basilic and cephalic veins. Both veins originate in the hand. The basilic vein travels along the medial side of the arm until it reaches the inferior border of the teres major muscle and becomes the axillary vein. The cephalic vein travels more laterally along the arm and remains superficial as it travels along the anterior border of the deltoid muscle and deep through the clavipectoral triangle to join with the axillary vein. After the axillary vein crosses the lateral border of the first rib, it becomes the subclavian vein.

Big Picture

The glenohumeral joint allows for a considerable amount of range of motion; more than any other joint in the body. The glenohumeral joint is a ball-and-socket synovial joint that produces a great deal of freedom, including flexion and extension, abduction and adduction, and medial and lateral rotation of the humerus. Because of the large range of motion in the glenohumeral joint it must depend upon ligaments and muscles for structural support. To minimize friction, bursae (synovial sacs) are positioned between the rotator cuff muscles and the joint capsule.

Structure of the Glenohumeral Joint

The articulating surface of the glenoid cavity is approximately one-third the size of the articulating surface of the humeral head (Figure 30-7A). This disproportionate articulation results in increased range of motion. However, it also results in a joint that is not as stable as other ball-and-socket joints (e.g., hip). To compensate for this lack of stability, a cartilaginous cuff called the glenoid labrum enhances and deepens the articulating surface of the glenoid fossa. The capsule of the glenohumeral joint is loose, which enables a major degree of motion.

Ligamentous Support to the Glenohumeral Joint

The following ligaments provide support to the glenohumeral joint (Figure 30-7B):

• Joint capsule (glenohumeral ligaments). Reinforce the superior, anterior, posterior, and inferior regions of the joint capsule. The inferior region of the joint capsule possesses an axillary fold to allow slack when the joint is fully abducted.
• Coracoacromial ligament. Supports the superior aspect of the glenohumeral joint to prevent superior dislocation by forming an arch over the superior aspect of the humeral head.

Bursae

Bursae help to decrease the friction between two moving structures such as tendon and bone (Figure 30-7C). In the shoulder complex, and especially the glenohumeral joint, the bursae are important because of the complexity of the muscular stability and the high degree of mobility. The two most important bursae in the shoulder complex are as follows:

• Subacromial bursa. Separates the supraspinatus tendon and the head of the humerus from the acromion.
• Subdeltoid bursa. Separates the deltoid muscle from the joint capsule. The subdeltoid bursa is continuous with the subacromial bursa.

Muscular Support

Any muscle that crosses the glenohumeral joint and produces a compressive force between the head of the humerus and the glenoid cavity will produce muscle stability (Figure 30-7D). Muscle stability is best exemplified by the rotator cuff muscles, which provide support to all sides, except the inferior aspect of the glenohumeral joint.

• Supraspinatus muscle. Provides superior support.
• Infraspinatus muscle. Provides posterior support.
• Teres minor muscle. Provides posterior support.
• Subscapularis muscle. Provides anterior support.

In addition to the support of the rotator cuff musculature, the long head of the biceps brachii and deltoid muscles assist in the support of the glenohumeral joint:

• Long head of the biceps brachii muscle. Provides superior and anterior support.
• Deltoid muscle. Provides superior support.

A combination of ligamentous and dynamic muscle support of multiple joints is critical for the stability of the shoulder complex because of the laxity of the capsule and the high degree of mobility.