The Shoulder Complex - Detailed Notes

1. Overview

1. Glenohumeral joint - the "true" shoulder joint (ball and socket)
2. Acromioclavicular (AC) joint - clavicle to acromion
3. Sternoclavicular (SC) joint - clavicle to sternum; the only true bony connection between the upper limb and the trunk
4. Scapulothoracic articulation - a physiological (not synovial) joint between the scapula and the thoracic cage

2. Bones of the Shoulder Complex

Clavicle

• S-shaped bone; medial two-thirds are convex anteriorly, lateral third is concave anteriorly
• Acts as a strut, transmitting forces from the upper limb to the axial skeleton
• Most commonly fractured bone at its middle third
• Articulates medially with the manubrium (SC joint) and laterally with the acromion (AC joint)

Scapula

• Flat, triangular bone lying on the posterior thorax (ribs 2-7)
• Key landmarks:
  • Spine - horizontal ridge separating the supraspinous and infraspinous fossae
  • Acromion - lateral projection of the spine; forms the AC joint
  • Coracoid process - hook-shaped projection anteriorly; attachment for muscles and ligaments
  • Glenoid cavity - shallow, pear-shaped fossa; articulates with the humeral head
  • Supraglenoid tubercle - attachment for long head of biceps
  • Infraglenoid tubercle - attachment for long head of triceps
  • Suprascapular notch - converted into a foramen by the superior transverse scapular ligament

Humerus

• Anatomical neck - between the head and tubercles; joint capsule attaches here
• Surgical neck - between the tubercles and shaft; site of fractures; axillary nerve closely related
• Greater tubercle - posterolateral; receives supraspinatus (superior facet), infraspinatus (middle facet), teres minor (inferior facet)
• Lesser tubercle - anterior; receives subscapularis
• Intertubercular sulcus (bicipital groove) - between the tubercles; houses the tendon of the long head of biceps brachii, held by the transverse humeral ligament
• Deltoid tuberosity - on the lateral shaft; insertion of deltoid muscle

3. Glenohumeral Joint

Articular Surfaces

• Humeral head covered in hyaline cartilage; much larger than the glenoid
• Glenoid cavity covered in hyaline cartilage; only ~25-30% of the humeral head contacts it at any time

Glenoid Labrum

• A fibrocartilaginous rim that attaches to the glenoid margin
• Deepens and expands the glenoid cavity
• Superiorly, it is continuous with the tendon of the long head of biceps brachii (which attaches to the supraglenoid tubercle)
• Increases joint stability; torn in anterior dislocations (Bankart lesion)

Joint Capsule

• Attaches to the glenoid cavity margin (outside the glenoid labrum) and to the anatomical neck of the humerus
• Medially, the attachment extends inferiorly onto the shaft (creating a redundant fold that accommodates abduction)
• Thickened in three locations to form the glenohumeral ligaments

• Attaches to articular margins, lines the fibrous capsule
• Loose inferiorly - this redundant portion accommodates abduction
• Protrudes through capsule apertures to form bursae (e.g., subtendinous bursa of subscapularis)
• Wraps around the long head of biceps tendon within the joint, then extends along it into the intertubercular sulcus

Ligaments of the Glenohumeral Joint

Coraco-Acromial Arch

• Formed by the coracoid process, coraco-acromial ligament, and acromion
• Forms a protective arch superior to the humeral head
• The subacromial space lies beneath this arch; contains supraspinatus tendon and subacromial bursa

Bursae

1. Subacromial (subdeltoid) bursa - between the acromion/deltoid and supraspinatus/capsule; does NOT directly communicate with the joint cavity in normal anatomy. Clinically the most important - becomes inflamed in impingement.
2. Subtendinous bursa of subscapularis - between subscapularis tendon and joint capsule; communicates with the articular cavity through an aperture
3. Subcoracoid bursa - between the coracoid process and joint capsule
4. Subcutaneous acromial bursa - between acromion and skin

Blood Supply

• Anterior and posterior circumflex humeral arteries (branches of axillary artery)
• Suprascapular artery (branch of subclavian/thyrocervical trunk)

Nerve Supply

• Axillary nerve (C5, C6)
• Suprascapular nerve (C5, C6)
• Lateral pectoral nerve (C5, C6, C7)

Movements

4. Rotator Cuff

5. Muscles of the Shoulder Region

Superficial Layer

• Origin: Superior nuchal line, external occipital protuberance, ligamentum nuchae, spinous processes C7-T12
• Insertion: Posterior border of lateral 1/3 of clavicle, acromion, superior edge of spine of scapula
• Nerve: Accessory nerve (CN XI) (motor); C3, C4 (proprioception)
• Action: Elevates scapula (upper fibers), retracts scapula (middle fibers), depresses scapula (lower fibers), rotates scapula during arm abduction above horizontal
• Clinical note: Test by asking patient to shrug against resistance

• Large, triangular muscle forming the characteristic contour of the shoulder
• Origin: Continuous U-shaped line - anterior border of lateral 1/3 clavicle, lateral margin of acromion, inferior edge of spine of scapula
• Insertion: Deltoid tuberosity of humerus
• Nerve: Axillary nerve (C5, C6) - enters via the quadrangular space with the posterior circumflex humeral artery
• Action: Major abductor of the arm; anterior fibers flex and medially rotate; posterior fibers extend and laterally rotate

Deep Layer - Posterior Scapular Region

• Origin: Transverse processes of C1-C4
• Insertion: Medial border of scapula (superior angle to spine)
• Nerve: Dorsal scapular nerve; C3, C4 directly
• Action: Elevates scapula

• Origin: Ligamentum nuchae + C7, T1 spines (minor); T2-T5 spines (major)
• Insertion: Medial border of scapula
• Nerve: Dorsal scapular nerve (branch of brachial plexus)
• Action: Retract and elevate the scapula

• Origin: Posterior surface of inferior angle of scapula
• Insertion: Medial lip of intertubercular sulcus
• Nerve: Inferior subscapular nerve (C6, C7)
• Action: Medial rotation and extension of the arm
• Note: NOT a rotator cuff muscle; does not blend with the joint capsule

• Origin: Lateral surfaces of ribs 1-8
• Insertion: Costal surface of medial border of scapula
• Nerve: Long thoracic nerve (C5, C6, C7) - very long course, vulnerable to injury
• Action: Protracts scapula; holds medial border against thoracic wall; rotates scapula (with trapezius) for arm elevation above horizontal
• Clinical: Injury causes "winging of the scapula" (medial border protrudes posteriorly)

6. Gateways: Spaces and Foramina

Suprascapular Foramen

• Formed by the suprascapular notch + superior transverse scapular ligament
• Suprascapular nerve passes through the foramen
• Suprascapular artery and vein pass superior to the ligament (above the foramen)
• Mnemonic: "The army goes over the bridge, the nerves go under" (artery over, nerve under the ligament)

Quadrangular Space

• Superior: Teres minor
• Inferior: Teres major
• Medial: Long head of triceps brachii
• Lateral: Surgical neck of humerus

Triangular Space

• Superior: Teres minor
• Inferior: Teres major
• Lateral: Long head of triceps brachii (medial margin)

Triangular Interval

• Superior: Inferior margin of teres major
• Medial: Long head of triceps brachii (lateral margin)
• Lateral: Shaft of humerus

7. Acromioclavicular (AC) Joint

• Synovial plane joint between the lateral end of clavicle and the acromion
• Has an incomplete fibrocartilaginous articular disc (meniscus)
• Allows gliding movements during scapular rotation

• AC ligament (fibrous capsule) - limits anterior-posterior translation
• Coracoclavicular ligament - the main suspensory ligament; has two parts:
  • Conoid ligament (medial, cone-shaped) - limits superior displacement of clavicle
  • Trapezoid ligament (lateral, quadrilateral) - limits lateral displacement

• Minor injury: tears AC ligament only (Grade I-II) - coracoclavicular ligaments intact
• Severe injury: tears both AC and coracoclavicular ligaments (Grade III+) - clavicle elevated on X-ray

8. Sternoclavicular (SC) Joint

• The only synovial joint connecting the upper limb to the axial skeleton
• Between the medial end of clavicle, manubrium sterni, and first costal cartilage
• Has a complete fibrocartilaginous articular disc that divides the joint into two cavities

• Anterior and posterior sternoclavicular ligaments
• Interclavicular ligament (between both clavicles across the jugular notch)
• Costoclavicular ligament (strongest; from first rib/costal cartilage to inferior clavicle)

• Anterior dislocation: more common
• Posterior dislocation: less common but dangerous - the clavicle can compress great vessels (subclavian/brachiocephalic), trachea, or esophagus in the root of the neck

9. Clinical Correlates

Glenohumeral Dislocation

• Most mobile joint = most commonly dislocated joint in the body
• Anterior (anteroinferior) dislocation (95%): Usually from a fall on an outstretched hand or forced external rotation. The humeral head moves inferiorly into the subcoracoid position.
  • Risk of axillary nerve injury (deltoid weakness, loss of sensation over the "regimental badge" area of lateral deltoid)
  • Bankart lesion: tear of the anteroinferior glenoid labrum (with or without bony fragment)
  • Hill-Sachs lesion: compression fracture of the posterolateral humeral head from impaction on the glenoid rim
  • Recurrent instability treated with capsular repair/Latarjet procedure (coracoid transfer)

• Posterior dislocation (<5%): Associated with epileptic seizures or electrocution (strong bilateral muscle contractions). Often missed on AP X-ray (requires axillary/Y-view).

Clavicle Fractures

• Most common site: middle third (80-85%)
• Mechanism: fall onto outstretched hand or direct blow
• Lateral third fractures are less common but may involve coracoclavicular ligaments

Rotator Cuff Pathology

• Supraspinatus is the most commonly affected tendon - passes through the subacromial space beneath the coraco-acromial arch (fixed dimensions)
• Impingement syndrome: Narrowing of the subacromial space causes repetitive mechanical irritation of the supraspinatus tendon during arm elevation. Causes include subacromial bone spurs, thickened AC ligament, subacromial bursitis, or muscle swelling.
• Tendinopathy: Poor blood supply to supraspinatus (critical zone ~1 cm from insertion) makes it susceptible to degenerative change; calcium can deposit (calcific tendinitis)
• Rotator cuff tear:
  • Partial-thickness or full-thickness
  • Most common in older patients (>40 years)
  • Full-thickness tears may be asymptomatic
  • MRI is gold standard for diagnosis

Subacromial (Subdeltoid) Bursitis

• Inflammation of the bursa between supraspinatus/deltoid and acromion
• Causes painful glenohumeral movements
• Treated with corticosteroid + local anesthetic injection under image guidance

Axillary Nerve Injury

• Most common with anterior glenohumeral dislocation or surgical neck of humerus fracture
• Results in: weakness of deltoid (abduction impaired), weakness of teres minor, loss of sensation over the lateral deltoid (upper lateral cutaneous nerve of arm)
• Test: Assess shoulder contour (loss of deltoid roundness in complete paralysis)

10. Summary: Key Nerve-Structure Relationships