Bones 5
Flat triangular bone with spine, acromion and coracoid process. The glenoid cavity, small and shallow, receives the humeral head; the supraglenoid and infraglenoid tubercles give origin to the long heads of biceps and triceps. The suprascapular notch, closed by the superior transverse scapular ligament, transmits the suprascapular nerve. On radiographs, the "Y" view and the outlet view show the shape of the acromion (Bigliani types: flat, curved, hooked).
S-shaped bone connecting the axial skeleton to the scapula (sternoclavicular and acromioclavicular joints). The middle third is the most common fracture site; the conoid tubercle and the trapezoid line, on the lateral third, give attachment to the coracoclavicular ligaments.
Hemispherical head facing upward, medially and backward (retroversion of about 30°), separated from the tubercles by the anatomical neck. The greater tubercle (insertions of supraspinatus, infraspinatus and teres minor) and the lesser tubercle (subscapularis) bound the intertubercular groove for the long head of biceps. The surgical neck, below the tubercles, is the typical fracture site; the axillary nerve runs in contact with it.
Upper part of the rib cage, over which the scapula glides. The first rib, short and flat, is crossed by the grooves for the subclavian artery and vein; the anterior scalene inserts between the two grooves, and the brachial plexus passes behind the artery. The serratus anterior and pectoralis minor arise from the ribs.
The manubrium, with the jugular notch and the clavicular notches, articulates with the clavicles (sternoclavicular joint) and with the first costal cartilage; the sternal angle separates it from the body, at the level of the second costal cartilage.
Cartilage 4
They extend the ribs toward the sternum. The first joins the manubrium directly (synchondrosis); the others form synovial sternocostal joints (from the second to the seventh).
Covers the head, thicker centrally than at the margins, and ends at the anatomical neck. MR arthrography assesses thickness and continuity. After an anterior dislocation a Hill-Sachs lesion may occur: an osseous (and chondral) impaction of the posterosuperior head against the anteroinferior glenoid rim.
Covers the glenoid cavity; unlike the head, it is thinner centrally and thicker peripherally. The labrum surrounds it. In recurrent anterior instability the glenoid may lose bone (bony Bankart) and take on an inverted-pear shape.
Ring of fibrocartilage, triangular in cross-section, surrounding the glenoid rim and increasing its depth and surface. Superiorly it is continuous with the long head of biceps tendon, anteroinferiorly with the inferior glenohumeral ligament. MR arthrography looks for Bankart lesions (anteroinferior, after anterior dislocation), SLAP lesions (superior) and associated bone lesions.
- Origin
- Rim of the glenoid cavity (blends with the cartilage and with the periosteum of the scapular neck).
- Insertion
- Closed ring on the glenoid rim; superiorly it receives the long head of biceps tendon, inferiorly the inferior glenohumeral ligament.
- Function
- Deepens the glenoid cavity (concavity-compression stability) and anchors the glenohumeral ligaments.
Ligaments 11
Medial and posterior component of the coracoclavicular ligament, cone-shaped with its apex on the coracoid. It tears in type III–V acromioclavicular dislocations (Rockwood classification): an increased coracoclavicular distance on stress (weighted) radiographs indicates the injury.
- Origin
- Bend (posteromedial part) of the base of the coracoid process.
- Insertion
- Conoid tubercle of the clavicle, on the inferior surface of the lateral third.
- Function
- Suspends the scapula from the clavicle; limits anterior and superior translation of the clavicle (vertical stabilizer).
Lateral and anterior component of the coracoclavicular ligament, a nearly horizontal quadrilateral sheet. It works with the conoid: together they are the vertical stabilizers of the acromioclavicular joint.
- Origin
- Superior surface of the coracoid process.
- Insertion
- Trapezoid line, on the inferior surface of the lateral third of the clavicle.
- Function
- Limits compression of the acromioclavicular joint and vertical translation of the clavicle.
Triangular band that closes the coracoacromial arch, the “roof” of the subacromial space above the rotator cuff. In subacromial impingement it thickens and may be released during decompression.
- Origin
- Lateral border of the coracoid process.
- Insertion
- Tip and anterior border of the acromion.
- Function
- Protects the humeral head from above and limits its superior migration.
Thickening of the acromioclavicular joint capsule, strongest superiorly, where it blends with fibers of the trapezius and deltoid. It stabilizes the clavicle in the horizontal plane; in type I–II injuries the coracoclavicular ligaments remain intact.
- Origin
- Lateral end of the clavicle.
- Insertion
- Medial border of the acromion.
- Function
- Horizontal (anteroposterior) stabilizer of the acromioclavicular joint.
Reinforces the capsule of the sternoclavicular joint, the only true articulation of the shoulder girdle with the axial skeleton. Posterior dislocation, less common, can compress the trachea and mediastinal vessels.
- Origin
- Medial end of the clavicle (anterior surface).
- Insertion
- Anterior surface of the manubrium of the sternum.
- Function
- Prevents anterior translation of the medial end of the clavicle and limits its elevation.
Fibrous band above the humeral head, between the lesser and greater tubercles, covering the rotator interval together with the superior glenohumeral ligament. Its thickening is typical of adhesive capsulitis (an MR arthrography sign in the rotator interval).
- Origin
- Lateral base of the coracoid process.
- Insertion
- Lesser and greater tubercles of the humerus, above the intertubercular groove.
- Function
- Supports the humeral head against gravity and limits external rotation with the arm adducted.
Thin, in the rotator interval, in front of the long head of biceps tendon. Together with the coracohumeral ligament it limits inferior translation with the arm adducted.
- Origin
- Anterosuperior glenoid labrum (supraglenoid tubercle, 1 o’clock).
- Insertion
- Just above the lesser tubercle of the humerus.
- Function
- Stabilizer of the adducted arm: limits inferior translation and external rotation.
Highly variable in thickness (it may be absent); it lies under the subscapularis tendon. With the superior glenohumeral and coracohumeral ligaments it forms the “anterior capsular complex”; the foramen of Weitbrecht is the opening between the middle and superior ligaments.
- Origin
- Anterior glenoid labrum (3 o’clock).
- Insertion
- Anatomical neck, medial to the lesser tubercle.
- Function
- Limits external rotation at 45° of abduction and anterior translation.
The most important band of the inferior glenohumeral complex, forming a hammock with the posterior band and the axillary recess. It is the structure injured in anterior instability: detachment from the labrum (Bankart lesion) or injury at the humeral attachment (HAGL, humeral avulsion of the glenohumeral ligament).
- Origin
- Anteroinferior glenoid labrum (4–5 o’clock).
- Insertion
- Anatomical neck of the humerus, anteroinferiorly.
- Function
- Main stabilizer against anterior dislocation in abduction and external rotation (apprehension position).
Posterior band of the inferior hammock; thinner than the anterior band. In internal rotation contracture it thickens and limits internal rotation and flexion-adduction.
- Origin
- Posteroinferior glenoid labrum (7–8 o’clock).
- Insertion
- Anatomical neck of the humerus, posteroinferiorly.
- Function
- Limits posterior translation and internal rotation with the arm flexed and abducted.
Thin sheet that roofs the intertubercular groove and holds the long head of biceps tendon in the groove. Recent anatomical studies describe it as superficial fibers of the subscapularis, with contributions from the supraspinatus and coracohumeral ligament, rather than as a separate ligament (Gleason 2006; MacDonald 2007). Injury to this roof, often with a subscapularis tear, allows medial subluxation of the tendon, visible on dynamic ultrasound and MRI.
- Origin
- Lesser tubercle of the humerus (medial lip of the groove), continuous with the subscapularis fibers.
- Insertion
- Greater tubercle of the humerus (lateral lip of the groove).
- Function
- Keeps the long head of biceps tendon in the intertubercular groove and prevents its medial subluxation.
Rotator cuff 4
Passes under the coracoacromial arch and the acromioclavicular joint, in the subacromial space: it is the most common site of tendinopathy, impingement and cuff tear, especially in the critical zone about 1 cm from the insertion. Suprascapular nerve (C5–C6).
- Origin
- Supraspinous fossa of the scapula (medial two thirds) and supraspinous fascia.
- Insertion
- Superior facet of the greater tubercle of the humerus, with fibers blending into the capsule.
- Function
- Abduction of the arm (first 15°); centers and stabilizes the humeral head in the glenoid.
Partly covers the posterior capsule. Suprascapular nerve (C5–C6), which runs through the spinoglenoid notch: a paralabral cyst can compress it and cause isolated infraspinatus atrophy.
- Origin
- Infraspinous fossa (medial two thirds) and infraspinous fascia.
- Insertion
- Middle facet of the greater tubercle of the humerus.
- Function
- External rotation of the arm; posterior stabilizer of the joint.
Forms the upper border of the quadrangular space. Axillary nerve (C5–C6), which emerges from the quadrangular space and winds around the surgical neck of the humerus with the posterior circumflex humeral artery. Isolated atrophy is seen in quadrilateral space syndrome.
- Origin
- Upper two thirds of the lateral border of the scapula (dorsal surface).
- Insertion
- Inferior facet of the greater tubercle of the humerus and joint capsule.
- Function
- External rotation; weak adduction.
The largest cuff muscle. Its tendon passes under the coracoid process and in front of the capsule; the subscapular bursa communicates with the joint. Tears are typical of anterior dislocation and are associated with instability of the long head of biceps. Upper and lower subscapular nerves (C5–C7).
- Origin
- Subscapular fossa (costal surface of the scapula).
- Insertion
- Lesser tubercle of the humerus and anterior capsule.
- Function
- Internal rotation; anterior stabilizer of the joint.
Shoulder muscles 4
Anterior portion of the deltoid. Axillary nerve (C5–C6), which emerges from the quadrangular space and winds around the surgical neck of the humerus with the posterior circumflex humeral artery. The deltopectoral groove, containing the cephalic vein, separates it from the pectoralis major.
- Origin
- Lateral third of the anterior border and superior surface of the clavicle.
- Insertion
- Deltoid tuberosity of the humerus (common tendon of the three parts).
- Function
- Flexion, abduction and internal rotation of the arm.
Middle portion, multipennate. Beneath it lie the subacromial-subdeltoid bursa and the rotator cuff. Axillary nerve (C5–C6), which emerges from the quadrangular space and winds around the surgical neck of the humerus with the posterior circumflex humeral artery.
- Origin
- Lateral border and superior surface of the acromion.
- Insertion
- Deltoid tuberosity of the humerus.
- Function
- Abduction of the arm, especially beyond the first 15°.
Posterior portion, adjacent to the teres minor. Axillary nerve (C5–C6), which emerges from the quadrangular space and winds around the surgical neck of the humerus with the posterior circumflex humeral artery.
- Origin
- Inferior border of the spine of the scapula.
- Insertion
- Deltoid tuberosity of the humerus.
- Function
- Extension, abduction and external rotation of the arm.
Bounds the quadrangular space (teres minor above, surgical neck laterally, long head of triceps medially) and the triangular space. Lower subscapular nerve (C5–C6).
- Origin
- Inferior angle and lower third of the lateral border of the scapula (dorsal surface).
- Insertion
- Medial lip of the intertubercular groove of the humerus.
- Function
- Adduction, extension and internal rotation of the arm.
Chest and back muscles 13
The three parts converge into a U-shaped tendon that inserts with a twist: the lower fibers attach higher. Lateral and medial pectoral nerves (C5–T1).
- Origin
- Medial half of the clavicle.
- Insertion
- Lateral lip of the intertubercular groove of the humerus.
- Function
- Flexion, adduction and internal rotation of the arm.
Rupture, typical in weightlifting, involves the tendon at its humeral insertion; congenital absence is part of Poland syndrome. Lateral and medial pectoral nerves (C5–T1).
- Origin
- Anterior surface of the sternum and first six costal cartilages.
- Insertion
- Lateral lip of the intertubercular groove of the humerus.
- Function
- Adduction and internal rotation of the arm; lowers the raised arm.
Lower part of the pectoralis major; it forms the anterior axillary fold. Lateral and medial pectoral nerves (C5–T1).
- Origin
- Sheath of the rectus abdominis.
- Insertion
- Lateral lip of the intertubercular groove of the humerus.
- Function
- Adduction and internal rotation; lowers the raised arm.
The axillary neurovascular bundle passes beneath it: compression under its tendon is one cause of thoracic outlet syndrome. Medial pectoral nerve (C8–T1).
- Origin
- Outer surfaces of ribs 3–5, near the cartilages.
- Insertion
- Coracoid process (medial border and superior surface).
- Function
- Depresses and protracts the scapula, tilting it forward.
Small muscle between the clavicle and the first rib, in front of the neurovascular bundle. Nerve to subclavius (C5–C6).
- Origin
- Superior surface of the first rib, at its junction with the cartilage.
- Insertion
- Subclavian groove, on the inferior surface of the middle third of the clavicle.
- Function
- Depresses and stabilizes the clavicle at the sternoclavicular joint.
Upper fibers of the trapezius, which form the contour of the neck. Accessory nerve (CN XI) and cervical branches C3–C4; injury to the accessory nerve (for example during neck dissection) causes a drooping shoulder.
- Origin
- Superior nuchal line, external occipital protuberance and nuchal ligament.
- Insertion
- Lateral third of the clavicle.
- Function
- Elevates the scapula and rotates its lateral angle upward.
Middle, horizontal fibers of the trapezius. Accessory nerve (CN XI) and cervical branches C3–C4.
- Origin
- Spinous processes from C7 to T3.
- Insertion
- Medial border of the acromion and superior lip of the spine of the scapula.
- Function
- Adducts (retracts) the scapula.
Lower fibers of the trapezius; the insertion tendon glides over a smooth triangle at the root of the spine. Accessory nerve (CN XI) and cervical branches C3–C4. Only the upper segment is visible in the model.
- Origin
- Spinous processes from T4 to T12.
- Insertion
- Tubercle at the root of the spine of the scapula.
- Function
- Depresses the scapula and rotates its lateral angle upward.
Its tendon passes in front of the teres major with a twist and forms with it the posterior axillary fold. Thoracodorsal nerve (C6–C8). Only the upper part is visible in the model window.
- Origin
- Spinous processes from T7 to L5, thoracolumbar fascia, iliac crest, last 3–4 ribs and inferior angle of the scapula.
- Insertion
- Floor of the intertubercular groove of the humerus.
- Function
- Extension, adduction and internal rotation of the arm.
Beneath the trapezius, parallel to the rhomboid minor. Dorsal scapular nerve (C4–C5).
- Origin
- Spinous processes from T2 to T5.
- Insertion
- Medial border of the scapula, below the spine.
- Function
- Adducts the scapula and rotates its lateral angle downward.
Thin, above the rhomboid major. Dorsal scapular nerve (C4–C5).
- Origin
- Nuchal ligament and spinous processes of C7 and T1.
- Insertion
- Medial border of the scapula, at the root of the spine.
- Function
- Adducts the scapula and rotates its lateral angle downward.
Follows the neck, beneath the sternocleidomastoid and trapezius. Dorsal scapular nerve and cervical branches C3–C4.
- Origin
- Posterior tubercles of the transverse processes of C1–C4.
- Insertion
- Superior angle and medial border of the scapula, above the spine.
- Function
- Elevates the scapula and rotates its lateral angle downward.
Holds the scapula against the chest wall: palsy of the long thoracic nerve (C5–C7), for example after axillary lymph node dissection, causes scapular winging. Only the upper part is visible in the model.
- Origin
- Outer surfaces of the first eight or nine ribs, by digitations.
- Insertion
- Medial border of the scapula (costal surface), with the strongest portion at the inferior angle.
- Function
- Protracts the scapula and rotates its lateral angle upward; holds it against the chest wall.
Arm muscles 6
The tendon is intra-articular but extrasynovial: it passes over the humeral head and exits into the intertubercular groove under the transverse humeral ligament. Tenosynovitis, subluxation and rupture (Popeye sign) are common; a superior labral lesion (SLAP) involves the anchor. Musculocutaneous nerve (C5–C6).
- Origin
- Supraglenoid tubercle of the scapula and superior glenoid labrum.
- Insertion
- Radial tuberosity (common tendon with the short head).
- Function
- Elbow flexion and forearm supination; weak shoulder flexion; stabilizes the humeral head.
With the coracobrachialis it forms the conjoint tendon, which inserts on the tip of the coracoid process. Musculocutaneous nerve (C5–C6).
- Origin
- Tip of the coracoid process (common tendon with the coracobrachialis).
- Insertion
- Radial tuberosity and, through the bicipital aponeurosis, the antebrachial fascia.
- Function
- Elbow flexion and supination; flexes and adducts the shoulder.
The musculocutaneous nerve pierces it and it serves as a landmark for the nerve. The axillary artery runs medial to it. Musculocutaneous nerve (C5–C7).
- Origin
- Tip of the coracoid process (common tendon with the short head of biceps).
- Insertion
- Middle third of the medial surface of the humerus.
- Function
- Flexion and adduction of the arm.
Passes between the teres minor and teres major, separating the quadrangular space (laterally) from the triangular space (medially). Radial nerve (C6–C8).
- Origin
- Infraglenoid tubercle of the scapula.
- Insertion
- Olecranon, through the common tendon of the three heads.
- Function
- Extends the elbow; extends and adducts the arm.
Arises above the radial groove, which separates it from the medial head. Radial nerve (C6–C8).
- Origin
- Posterior surface of the humerus above the radial groove and lateral intermuscular septum.
- Insertion
- Olecranon, through the common tendon.
- Function
- Extends the elbow.
Deep, below the radial groove. Radial nerve (C6–C8).
- Origin
- Posterior surface of the humerus below the radial groove and intermuscular septa.
- Insertion
- Olecranon, through the common tendon.
- Function
- Extends the elbow.
Nerves 7
Arises from the anterior rami of C5–T1: three trunks (superior, middle, inferior) between the anterior and middle scalenes, above the first rib; behind the clavicle each divides into an anterior and a posterior division, which form the lateral, posterior and medial cords (named for their relation to the axillary artery). On MRI it is studied in oblique coronal and sagittal planes; it can be compressed in the thoracic outlet (thoracic outlet syndrome) and by lung apex tumors (Pancoast syndrome).
Mixed nerve (C5–C6) from the superior trunk. It passes through the suprascapular notch under the superior transverse scapular ligament (the artery stays above), supplies the supraspinatus, then curves around the lateral border of the spine through the spinoglenoid notch to supply the infraspinatus and give articular branches to the shoulder. A paralabral cyst can compress it and cause selective atrophy of the supraspinatus or infraspinatus, seen on MRI as muscle edema or atrophy.
Mixed nerve (C5–C6) from the posterior cord. It exits through the quadrangular space (teres minor above, with the subscapularis anteriorly, teres major below, long head of triceps medially, surgical neck laterally) with the posterior circumflex humeral artery; it supplies the deltoid and teres minor and gives off the superior lateral cutaneous nerve of arm (sensation over the lower deltoid). The quadrangular space is a site of compression syndrome; the nerve is at risk in anterior dislocations and surgical neck fractures.
Mixed nerve (C5–C7) from the lateral cord. It normally passes through the belly of the coracobrachialis, which it supplies, and descends between the biceps and brachialis (also supplied); it continues as the lateral cutaneous nerve of forearm (sensory to the radial side of the forearm). A high course of the nerve is a risk in anterior approaches to the shoulder.
Mixed nerve (C5–T1) from the posterior cord. It passes under the teres major with the deep artery of arm, winds posteriorly around the humerus in the radial groove, between the origins of the lateral and medial heads of triceps, and in the distal arm pierces the lateral intermuscular septum. It supplies the triceps and, further down, the extensor muscles; sensory to the back of the arm, forearm and radial hand. Humeral shaft fractures typically injure it (wrist drop).
Mixed nerve (C6–T1) from the lateral and medial cords, which join in front of the axillary artery. In the arm it descends with the brachial artery (first lateral, then medial to it) and gives no branches; motor to the forearm flexors and thenar muscles, sensory to the radial palm. Compression sites: at the wrist (carpal tunnel) and under the pronator teres.
Mixed nerve (C8–T1) from the medial cord. It descends medial to the artery; at mid-arm it pierces the medial intermuscular septum and passes behind the medial epicondyle. It gives no branches in the arm; motor to the flexor carpi ulnaris, part of the flexor digitorum profundus and the intrinsic hand muscles, sensory to the ulnar border of the hand. Compression site at the elbow (cubital tunnel).
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