Medical School · Year 1 · Anatomy Msk · includes a quiz and discussion video

Lecture 6: Upper Limb - Shoulder Region

Unit 1.3: Human Gross Anatomy I - Musculoskeletal System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the bones of the pectoral girdle (clavicle and scapula) and their features
2. Identify the joints of the shoulder region (sternoclavicular, acromioclavicular, glenohumeral)
3. Describe the rotator cuff muscles, their attachments, and functions
4. Explain the structure and clinical significance of the glenohumeral joint
5. Identify additional muscles acting on the shoulder and their innervation
6. Describe the boundaries and contents of the axilla

Bones of the Pectoral Girdle

The pectoral girdle, also called the shoulder girdle, consists of two bones on each side: the clavicle and the scapula. Together, these bones connect the upper limb to the axial skeleton and provide the mobile platform from which the arm operates. Unlike the pelvic girdle, which is rigidly attached to the vertebral column, the pectoral girdle is only connected to the axial skeleton at one point, the sternoclavicular joint, allowing remarkable mobility at the expense of stability.

The clavicle is an S-shaped bone that extends horizontally across the anterior thorax, connecting the sternum to the scapula. It is the only bony link between the upper limb and the axial skeleton. The clavicle serves several functions: it transmits forces from the upper limb to the trunk, acts as a strut holding the shoulder away from the body to allow free arm movement, and protects the neurovascular structures passing between the neck and the arm.

The sternal end of the clavicle is enlarged and articulates with the manubrium of the sternum and the first costal cartilage at the sternoclavicular joint. The acromial end is flattened and articulates with the acromion of the scapula at the acromioclavicular joint. The medial two-thirds of the clavicle is convex anteriorly, while the lateral one-third is concave anteriorly, creating the characteristic S-curve. On the inferior surface of the lateral third are the conoid tubercle and the trapezoid line, which provide attachment for the coracoclavicular ligament. The subclavian groove on the inferior surface of the medial third marks the attachment of the subclavius muscle.

The clavicle is the most commonly fractured bone in the body. Fractures typically occur at the junction of the middle and lateral thirds, where the bone changes curvature and is thinnest. The medial fragment is elevated by the sternocleidomastoid muscle, while the lateral fragment drops due to the weight of the limb. The subclavian vessels and brachial plexus lie posterior to the clavicle and are protected by it, though they may be damaged in severely displaced fractures.

The scapula is a flat, triangular bone on the posterior thorax, overlying ribs 2 through 7. It provides attachment for numerous muscles that act on the shoulder and arm and serves as a mobile base for the glenohumeral joint.

The scapula has two surfaces. The anterior or costal surface faces the ribcage and contains the subscapular fossa, a large concave area providing attachment for the subscapularis muscle. The posterior surface is divided by the spine of the scapula into two smaller fossae: the supraspinous fossa above the spine houses the supraspinatus muscle, while the infraspinous fossa below the spine houses the infraspinatus muscle.

The scapula has three borders. The superior border is the shortest and includes the suprascapular notch, which transmits the suprascapular nerve (the suprascapular artery passes over the transverse scapular ligament bridging the notch). The medial or vertebral border runs parallel to the vertebral column and provides attachment for the rhomboid muscles and serratus anterior. The lateral or axillary border is the thickest and extends from the inferior angle to the glenoid cavity.

The scapula has three angles. The superior angle is at the junction of the superior and medial borders. The inferior angle is at the junction of the medial and lateral borders and is easily palpable. The lateral angle is expanded to form the glenoid cavity, the shallow socket that articulates with the head of the humerus.

Several processes project from the scapula. The spine of the scapula is a prominent ridge on the posterior surface that provides attachment for the trapezius and deltoid muscles. The acromion is the lateral extension of the spine, forming a protective roof over the glenohumeral joint and articulating with the clavicle. The coracoid process is a hook-like projection from the superior border that provides attachment for muscles (pectoralis minor, coracobrachialis, biceps short head) and ligaments.

The glenoid cavity is the shallow, pear-shaped articular surface at the lateral angle that receives the humeral head. The supraglenoid tubercle, just above the glenoid cavity, gives origin to the long head of the biceps. The infraglenoid tubercle, just below, gives origin to the long head of the triceps.

<image>Panel A: Anterior costal view showing subscapular fossa with subscapularis attachment area, coracoid process projecting anteriorly, glenoid cavity with supraglenoid and infraglenoid tubercles. Panel B: Borders and angles including medial and lateral borders, superior border with suprascapular notch, superior, inferior, and lateral angles marked. Panel C: Posterior view showing spine of scapula crossing the bone, supraspinous fossa above spine, infraspinous fossa below spine, acromion as lateral extension. Panel D: Lateral view inset of glenoid cavity with labrum, muscle attachment sites color-coded for subscapularis, supraspinatus, and infraspinatus.</image>

Proximal Humerus

The humerus is the bone of the arm, extending from the shoulder to the elbow. Its proximal end articulates with the scapula at the glenohumeral joint and provides attachment for the rotator cuff muscles.

The head of the humerus is a smooth, hemispherical surface that faces medially, superiorly, and posteriorly to articulate with the glenoid cavity. It represents approximately one-third of a sphere. The anatomical neck is the groove immediately adjacent to the smooth articular surface of the head, marking the junction between the head and the rest of the proximal humerus.

The greater tubercle is a large projection on the lateral aspect of the proximal humerus. It has three facets on its surface that provide insertion for three of the four rotator cuff muscles: the supraspinatus inserts on the superior facet, the infraspinatus on the middle facet, and the teres minor on the inferior facet. The lesser tubercle is a smaller projection on the anterior aspect, providing insertion for the fourth rotator cuff muscle, the subscapularis.

The intertubercular groove, also called the bicipital groove, lies between the greater and lesser tubercles. It transmits the tendon of the long head of the biceps from its origin at the supraglenoid tubercle down into the arm. The lateral lip of the groove provides insertion for pectoralis major, the floor for latissimus dorsi, and the medial lip for teres major.

The surgical neck is the narrowed region just below the tubercles, where the proximal humerus transitions to the shaft. This is a common site of fracture, and the axillary nerve, which wraps around the surgical neck posteriorly, may be damaged in such fractures.

The deltoid tuberosity is a roughened elevation on the lateral surface of the humeral shaft, approximately halfway down, where the deltoid muscle inserts.

<image>Panel A: Anterior view showing smooth hemispherical head, anatomical neck as groove around head, lesser tubercle with subscapularis insertion, greater tubercle with supraspinatus insertion on superior facet. Panel B: Intertubercular groove between tubercles with tendon of long head of biceps passing through, surgical neck as narrowed region below tubercles, deltoid tuberosity on lateral shaft. Panel C: Posterior view showing greater tubercle with all three facets labeled for supraspinatus, infraspinatus, and teres minor insertions. Panel D: Surgical neck with axillary nerve path wrapping posteriorly, superior view inset showing relationship of head, tubercles, and groove with color-coded rotator cuff insertion sites.</image>

Sternoclavicular Joint

The sternoclavicular joint is the only joint between the upper limb and the axial skeleton. It is formed by the articulation of the sternal end of the clavicle with the clavicular notch of the manubrium and the cartilage of the first rib.

Although classified as a saddle joint based on the shape of its articular surfaces, the sternoclavicular joint functions more like a ball-and-socket joint, allowing movement in multiple planes. An articular disc divides the joint cavity into two compartments, improving the congruence of the articular surfaces and allowing the wide range of motion. The disc also absorbs forces transmitted from the upper limb through the clavicle.

The joint permits elevation and depression of the clavicle (and with it the shoulder), protraction and retraction (forward and backward movement), and rotation of the clavicle on its long axis. These movements of the clavicle are essential for the full range of shoulder motion.

The joint is reinforced by several ligaments. The anterior and posterior sternoclavicular ligaments strengthen the joint capsule anteriorly and posteriorly. The interclavicular ligament connects the sternal ends of the two clavicles across the jugular notch, preventing excessive lateral depression of the shoulder. The costoclavicular ligament runs from the first rib to the inferior surface of the clavicle and is the primary restraint against elevation of the clavicle.

Despite the considerable forces transmitted through it, dislocation of the sternoclavicular joint is uncommon because of its strong ligamentous support. When dislocation does occur, it may be anterior (more common) or posterior. Posterior dislocation is potentially dangerous because the clavicle may compress the trachea, esophagus, or great vessels lying immediately behind the joint.

Acromioclavicular Joint

The acromioclavicular joint is formed by the articulation of the lateral end of the clavicle with the medial margin of the acromion. It is a plane synovial joint that allows gliding movements and rotation of the scapula on the clavicle.

The joint capsule is weak, and the acromioclavicular ligament, which reinforces the capsule, is also relatively weak. The primary stabilizers of the joint are the coracoclavicular ligaments, which connect the clavicle to the coracoid process of the scapula. These ligaments consist of two parts: the conoid ligament, which is cone-shaped and oriented vertically from the coracoid to the conoid tubercle of the clavicle, and the trapezoid ligament, which is quadrilateral and oriented more horizontally from the coracoid to the trapezoid line of the clavicle.

The acromioclavicular joint is commonly injured in falls onto the shoulder, resulting in acromioclavicular joint separation, often called a "separated shoulder." These injuries are classified by severity. Type I injuries involve strain of the acromioclavicular ligament without displacement. Type II injuries involve rupture of the acromioclavicular ligament with the coracoclavicular ligaments intact. Type III injuries involve rupture of both the acromioclavicular and coracoclavicular ligaments, allowing the clavicle to ride superiorly. Higher grades involve more severe displacement.

The scapulothoracic articulation is not a true anatomical joint but describes the relationship between the scapula and the posterior thoracic wall. The scapula glides on the serratus anterior and subscapularis muscles that lie between it and the ribcage. Movements at this articulation include elevation, depression, protraction, retraction, and upward and downward rotation of the scapula. These movements significantly amplify the range of motion of the shoulder complex.

<image>Panel A: Anterior view of shoulder girdle articulating with sternum, sternoclavicular joint detail showing clavicle articulating with manubrium and articular disc dividing joint. Panel B: Sternoclavicular ligaments including anterior ligament, costoclavicular ligament from first rib, interclavicular ligament crossing between clavicles. Panel C: Acromioclavicular joint showing clavicle articulating with acromion, coracoclavicular ligaments with conoid vertical/medial and trapezoid horizontal/lateral. Panel D: AC joint separation grades I, II, III with increasing displacement, scapulothoracic movement arrows showing elevation/depression and protraction/retraction.</image>

Glenohumeral Joint

The glenohumeral joint, commonly called the shoulder joint, is the articulation between the head of the humerus and the glenoid cavity of the scapula. It is a ball-and-socket synovial joint and is the most mobile joint in the body. This exceptional mobility comes at the cost of stability, making the glenohumeral joint the most commonly dislocated major joint.

The humeral head is much larger than the glenoid cavity, which covers only about one-third of the articular surface of the head at any given position. The glenoid labrum, a ring of fibrocartilage attached to the rim of the glenoid cavity, deepens the socket and somewhat improves the fit between the surfaces. The labrum also serves as an attachment site for the glenohumeral ligaments and the long head of the biceps tendon.

The joint capsule is loose and redundant, allowing the wide range of motion characteristic of this joint. It attaches to the anatomical neck of the humerus and to the rim of the glenoid cavity and the labrum. The capsule has synovial recesses, particularly inferiorly, that unfold during movement. The inferior part of the capsule is the weakest because it lacks reinforcement from ligaments or muscles, which is why most dislocations occur in an inferior direction before the head slides anteriorly.

The glenohumeral ligaments are thickenings of the anterior capsule that reinforce the joint. The superior glenohumeral ligament is a weak structure that limits inferior translation of the humeral head. The middle glenohumeral ligament is variable in size and limits external rotation. The inferior glenohumeral ligament is the strongest and most important, limiting external rotation especially when the arm is abducted. It has anterior and posterior bands with a thin axillary pouch between them.

The coracohumeral ligament runs from the coracoid process to the greater tubercle and supports the weight of the hanging arm. The transverse humeral ligament bridges the intertubercular groove, holding the biceps tendon in place.

The coracoacromial arch is formed by the coracoid process, the coracoacromial ligament, and the acromion. This arch forms a rigid roof over the supraspinatus tendon and the subacromial bursa. The subacromial space beneath this arch is clinically important as the site of shoulder impingement syndrome, where repetitive compression of the supraspinatus tendon and bursa causes pain and inflammation.

Several bursae facilitate movement around the shoulder. The subacromial-subdeltoid bursa lies between the supraspinatus tendon and the coracoacromial arch, reducing friction during abduction. The subscapular bursa lies between the subscapularis tendon and the scapular neck and usually communicates with the joint cavity. These bursae may become inflamed in bursitis.

The blood supply to the glenohumeral joint comes from the anterior and posterior circumflex humeral arteries (branches of the axillary artery), the suprascapular artery, and branches of the subscapular artery. The nerve supply follows Hilton's Law, coming from the nerves that cross the joint: the suprascapular nerve, axillary nerve, and lateral pectoral nerve.

<image>Panel A: Coronal section showing humeral head articulating with glenoid cavity, glenoid labrum deepening socket, joint capsule attaching to anatomical neck and labrum with inferior recess. Panel B: Coracoacromial arch as roof with coracoid process, coracoacromial ligament, acromion, subacromial bursa beneath arch, supraspinatus tendon in subacromial space. Panel C: Anterior view showing glenohumeral ligaments as thickenings of anterior capsule with superior, middle, and inferior bands in different colors. Panel D: Coracohumeral ligament, transverse humeral ligament over bicipital groove with biceps tendon, blood supply including circumflex humeral and suprascapular arteries.</image>

Movements at the Shoulder

The shoulder complex permits the greatest range of motion of any joint region in the body. This is achieved through coordinated movement at the glenohumeral joint and the scapulothoracic articulation, as well as smaller contributions from the sternoclavicular and acromioclavicular joints.

Flexion moves the arm anteriorly and superiorly in the sagittal plane, with a range of 0 to 180 degrees. The primary flexors are the anterior deltoid, the clavicular head of pectoralis major, and the coracobrachialis. The biceps brachii assists, especially when the elbow is extended.

Extension moves the arm posteriorly in the sagittal plane, with a range of 0 to approximately 60 degrees. The primary extensors are the posterior deltoid, latissimus dorsi, and teres major. Extension beyond the anatomical position is called hyperextension.

Abduction moves the arm laterally away from the body in the coronal plane. The supraspinatus initiates abduction, producing the first 15 degrees of movement. The middle deltoid becomes the primary abductor for the remainder of the range. Full abduction to 180 degrees requires rotation of the scapula to turn the glenoid cavity upward.

Adduction returns the arm to the side and is produced by the pectoralis major, latissimus dorsi, and teres major. Adduction beyond the midline requires the arm to pass anterior or posterior to the trunk.

External rotation turns the anterior surface of the arm laterally, with a range of approximately 60 degrees. The primary external rotators are the infraspinatus and teres minor, with assistance from the posterior deltoid.

Internal rotation turns the anterior surface of the arm medially, with a range of approximately 90 degrees. The primary internal rotator is the subscapularis, with assistance from the pectoralis major, latissimus dorsi, teres major, and anterior deltoid.

The scapulohumeral rhythm describes the coordinated movement of the glenohumeral joint and scapula during abduction. During the first 30 degrees of abduction, movement is primarily at the glenohumeral joint while the scapula stabilizes. Beyond 30 degrees, movement occurs at both articulations in a ratio of approximately 2:1, meaning that for every 3 degrees of abduction, 2 degrees occur at the glenohumeral joint and 1 degree through upward rotation of the scapula. Full abduction of 180 degrees consists of approximately 120 degrees of glenohumeral movement and 60 degrees of scapular rotation.

<image>Panel A: Flexion in sagittal plane (0-180 degrees) with anterior deltoid and pectoralis major highlighted, extension (0-60 degrees) with posterior deltoid and latissimus highlighted. Panel B: Abduction in coronal plane (0-180 degrees) with supraspinatus initiating first 15 degrees and deltoid continuing, adduction moving toward body. Panel C: External/internal rotation in transverse plane with arms at side, external rotation by infraspinatus and teres minor, internal rotation by subscapularis. Panel D: Scapulohumeral rhythm diagram showing arm at 0, 90, and 180 degree abduction with 2:1 ratio labeled showing 120 degrees glenohumeral plus 60 degrees scapular movement.</image>

Rotator Cuff Muscles

The rotator cuff is a group of four muscles whose tendons blend with and reinforce the glenohumeral joint capsule. Their primary function is dynamic stabilization of the humeral head in the glenoid cavity during shoulder movement. Without the rotator cuff, the stronger superficial muscles would pull the humeral head superiorly out of the socket during arm elevation.

The rotator cuff muscles can be remembered with the mnemonic SITS: Supraspinatus, Infraspinatus, Teres minor, and Subscapularis.

The supraspinatus originates from the supraspinous fossa of the scapula and passes laterally beneath the coracoacromial arch to insert on the superior facet of the greater tubercle. It is innervated by the suprascapular nerve (C5-C6). The supraspinatus initiates abduction of the arm, producing the first 15 degrees of movement, and stabilizes the humeral head against the glenoid. Its tendon is the most commonly torn of the rotator cuff tendons because it passes through the relatively narrow subacromial space, where it is subject to compression and wear. The "painful arc" sign, pain during abduction between 60 and 120 degrees when the supraspinatus tendon is compressed beneath the acromion, suggests supraspinatus pathology.

The infraspinatus originates from the infraspinous fossa and inserts on the middle facet of the greater tubercle. It is also innervated by the suprascapular nerve (C5-C6). The infraspinatus is a powerful external rotator of the arm and contributes to stabilization of the humeral head.

The teres minor originates from the upper portion of the lateral border of the scapula and inserts on the inferior facet of the greater tubercle. It is innervated by the axillary nerve (C5-C6). The teres minor assists the infraspinatus in external rotation and stabilization.

The subscapularis originates from the subscapular fossa on the anterior surface of the scapula and inserts on the lesser tubercle. It is innervated by the upper and lower subscapular nerves (C5-C6). The subscapularis is the only internal rotator of the rotator cuff group and is the only one that attaches to the lesser tubercle. It forms the anterior part of the rotator cuff and protects the joint anteriorly.

<image>Panel A: Posterior view showing supraspinatus filling supraspinous fossa and inserting on superior facet of greater tubercle, infraspinatus filling infraspinous fossa inserting on middle facet. Panel B: Teres minor from lateral border inserting on inferior facet, proximal humerus with tendon insertions on greater tubercle demarcated by color. Panel C: Anterior view showing subscapularis covering subscapular fossa and crossing anteriorly to insert on lesser tubercle. Panel D: Superior view showing all four tendons blending with capsule forming complete cuff, chart listing each muscle with origin, insertion, action, innervation, supraspinatus highlighted as most commonly torn.</image>

Other Muscles of the Shoulder Region

Several additional muscles act on the shoulder joint and girdle beyond the rotator cuff.

The deltoid is the large muscle forming the rounded contour of the shoulder. It originates from the lateral third of the clavicle, the acromion, and the spine of the scapula, and inserts on the deltoid tuberosity of the humerus. The muscle is functionally divided into three parts. The anterior deltoid flexes and medially rotates the arm. The middle deltoid is the primary abductor of the arm beyond the first 15 degrees. The posterior deltoid extends and laterally rotates the arm. The deltoid is innervated by the axillary nerve (C5-C6), which also supplies sensation to the skin over the lateral shoulder. Damage to the axillary nerve results in weakness of abduction and a characteristic flattening of the shoulder contour.

The teres major originates from the inferior angle and lower lateral border of the scapula and inserts on the medial lip of the intertubercular groove, just below the subscapularis. It adducts, medially rotates, and extends the arm. It is innervated by the lower subscapular nerve (C5-C7). The teres major works synergistically with the latissimus dorsi and is sometimes called "lat's little helper."

The pectoralis major is a large fan-shaped muscle covering the anterior chest. It has two heads: the clavicular head from the medial half of the clavicle and the sternocostal head from the sternum and costal cartilages 1-6. The fibers converge to insert on the lateral lip of the intertubercular groove. The muscle adducts and medially rotates the arm. The clavicular head assists flexion. The pectoralis major is innervated by both the medial pectoral nerve (C8-T1) and the lateral pectoral nerve (C5-C7).

The pectoralis minor lies deep to pectoralis major, originating from ribs 3-5 and inserting on the coracoid process. It depresses and protracts the scapula and is innervated by the medial pectoral nerve (C8-T1). The pectoralis minor is a key landmark in the axilla, dividing the axillary artery into three parts.

The serratus anterior originates from the lateral surfaces of ribs 1-8 and inserts on the costal surface of the medial border of the scapula. It protracts the scapula, holds it against the thoracic wall, and rotates the scapula upward to allow elevation of the arm above the horizontal. The serratus anterior is innervated by the long thoracic nerve (C5-C7). Injury to this nerve results in paralysis of the serratus anterior and a characteristic "winged scapula," where the medial border of the scapula protrudes posteriorly, especially when the patient pushes against a wall. Weakness in overhead movements and in pushing activities occurs.

<image>Panel A: Anterior view showing deltoid with three parts (anterior/clavicular, middle/acromial, posterior/spinal) converging to deltoid tuberosity, pectoralis major with clavicular and sternocostal heads. Panel B: Pectoralis minor deep to pectoralis major from ribs to coracoid process, posterior view showing teres major from inferior scapular angle to humerus medial lip. Panel C: Lateral view showing serratus anterior on lateral chest wall from ribs wrapping to insert on medial scapular border. Panel D: Winged scapula demonstration with patient pushing against wall and scapula protruding due to serratus anterior weakness, each muscle with innervation and action noted.</image>

The Axilla

The axilla is a pyramidal space located between the upper limb and the thoracic wall. It serves as a passageway for the neurovascular structures traveling between the neck and the arm. Understanding the boundaries and contents of the axilla is essential for physical examination, axillary surgery, and understanding the spread of infection or malignancy.

The axilla has an apex, a base, and four walls. The apex, also called the cervicoaxillary canal, is the passage from the neck into the axilla. It is bounded by the clavicle anteriorly, the first rib medially, and the superior edge of the scapula posteriorly. All major structures entering or leaving the axilla pass through this narrow aperture.

The base of the axilla is formed by the skin and axillary fascia stretching between the anterior and posterior folds of the axilla. The base is concave and forms the "armpit."

The anterior wall is formed by the pectoralis major and minor muscles and the clavipectoral fascia. The pectoralis minor divides the axillary artery into three parts based on their relationship to this muscle.

The posterior wall is formed by the subscapularis, teres major, and latissimus dorsi muscles. The subscapularis lies against the scapula, while teres major and latissimus dorsi form the posterior axillary fold.

The medial wall is formed by the lateral surface of the thoracic wall, consisting of the upper ribs and the intercostal muscles, covered by the serratus anterior muscle.

The lateral wall is narrow, formed by the floor of the intertubercular groove of the humerus where the biceps tendon and the convergent tendons of the anterior and posterior walls meet.

The contents of the axilla include the axillary artery and its branches, the axillary vein and its tributaries, the cords and branches of the brachial plexus, and the axillary lymph nodes. Fat and loose connective tissue fill the space between these structures. The axillary lymph nodes are clinically important because they drain the breast and the upper limb, and may be involved in breast cancer or infection of the arm.

<image>Panel A: Pyramidal axilla shape with apex (cervicoaxillary canal) bounded by clavicle, first rib, and scapula, base inferiorly formed by skin and fascia. Panel B: Anterior wall formed by pectoralis major superficially and pectoralis minor deep, posterior wall formed by subscapularis, teres major, and latissimus dorsi. Panel C: Medial wall formed by ribs 1-4 with serratus anterior covering, lateral wall at intertubercular groove, pectoralis minor dividing axillary artery into three parts. Panel D: Contents showing axillary artery surrounded by brachial plexus cords, axillary vein medially, axillary lymph nodes scattered throughout with central, lateral, pectoral, and subscapular groups.</image>

Clinical Correlations

Rotator cuff tears are among the most common shoulder problems. The supraspinatus tendon is most frequently affected because it passes through the narrow subacromial space, where it experiences compression and friction. Tears may result from acute trauma or chronic degeneration. Patients present with pain, weakness of abduction and external rotation, and difficulty with overhead activities. Partial tears may be managed conservatively, while complete tears often require surgical repair.

Shoulder dislocation most commonly occurs in an anterior direction, with the humeral head displaced anteroinferiorly. The typical mechanism involves forced abduction and external rotation of the arm. The axillary nerve wraps around the surgical neck of the humerus and may be injured during dislocation, resulting in weakness of the deltoid (impaired abduction) and numbness over the lateral shoulder in the "regimental badge" distribution. The glenoid labrum may be torn from the anterior glenoid rim (Bankart lesion), predisposing to recurrent dislocations. The posterolateral humeral head may be compressed against the glenoid rim during dislocation, creating an impaction fracture called a Hill-Sachs lesion.

Adhesive capsulitis, commonly called frozen shoulder, is characterized by progressive loss of both active and passive range of motion at the glenohumeral joint. The capsule becomes thickened and adherent, limiting movement in all directions. The condition is painful and may take months to years to resolve, though most patients eventually recover with conservative treatment.

Clavicle fractures typically result from falls onto an outstretched hand or direct blows to the shoulder. The middle third is most commonly affected. The medial fragment is pulled superiorly by the sternocleidomastoid muscle, while the lateral fragment drops due to the weight of the arm. Most clavicle fractures heal well with conservative treatment, though severely displaced fractures may require surgical fixation.

Scapular winging results from paralysis of the serratus anterior following injury to the long thoracic nerve. The medial border of the scapula protrudes posteriorly, especially when the patient pushes forward against resistance. Patients have weakness in reaching and pushing movements. The long thoracic nerve may be injured during axillary surgery, by trauma, or by viral neuritis (Parsonage-Turner syndrome).

Summary

The pectoral girdle consists of the clavicle and scapula, connecting the upper limb to the axial skeleton only at the sternoclavicular joint. The scapula is suspended from the axial skeleton by muscles, allowing great mobility. The glenohumeral joint is a ball-and-socket joint between the humeral head and the glenoid cavity, offering the greatest range of motion but the least inherent stability of any major joint.

The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) dynamically stabilize the humeral head in the glenoid during movement. The supraspinatus initiates abduction and is the most commonly injured. The sternoclavicular and acromioclavicular joints permit movement of the clavicle and scapula, amplifying total shoulder motion.

The scapulohumeral rhythm describes the 2:1 ratio of glenohumeral to scapulothoracic movement during arm elevation. The axilla is a pyramidal space transmitting nerves, vessels, and lymphatics between the neck and the upper limb. Common shoulder pathologies include rotator cuff tears, anterior dislocation with possible axillary nerve injury, adhesive capsulitis, clavicle fractures, and scapular winging from long thoracic nerve injury.

Key Terms

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