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The Shoulder Complex - Detailed Notes

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1. Overview

1. Glenohumeral (GH) joint - the true shoulder joint
2. Acromioclavicular (AC) joint
3. Sternoclavicular (SC) joint - the only true bony connection of the upper limb to the axial skeleton
4. Scapulothoracic (ST) joint - a "floating" or physiological joint (not a true synovial joint)

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2. Osteology (Bones)

Clavicle

• S-shaped bone connecting the sternum to the acromion
• Only bony connection between upper limb and axial skeleton
• Most commonly fractured at the middle third
• Medial end: rounded, articulates at SC joint
• Lateral end: flat, articulates at AC joint
• Provides attachment for: trapezius, deltoid, pectoralis major, sternocleidomastoid, subclavius

Scapula

• Triangular flat bone on the posterolateral thoracic wall (ribs 2-7)
• Key features:
  • Glenoid cavity - shallow, oval fossa at the lateral angle; articulates with the humeral head
  • Supraglenoid tubercle - attachment for long head of biceps brachii
  • Infraglenoid tubercle - attachment for long head of triceps brachii
  • Coracoid process - "crow's beak" projection; attachment for pectoralis minor, short head of biceps, coracobrachialis
  • Acromion - lateral extension of the spine; forms the AC joint
  • Spine of scapula - posterior ridge; attachment for trapezius and deltoid
  • Suprascapular notch - converted to foramen by superior transverse scapular ligament; transmits suprascapular nerve (artery passes above the ligament)

Humerus (Proximal)

• Head - large spherical; articulates with glenoid cavity; faces medially, superiorly, and posteriorly
• Anatomical neck - between head and tubercles; attachment for joint capsule
• Surgical neck - below the tubercles; common fracture site; axillary nerve at risk
• Greater tubercle - posterior and lateral; insertions: supraspinatus (superior facet), infraspinatus (middle facet), teres minor (inferior facet)
• Lesser tubercle - anterior; insertion: subscapularis
• Intertubercular sulcus (bicipital groove) - between tubercles; contains long head of biceps tendon held by transverse humeral ligament

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3. The Four Joints of the Shoulder Complex

3.1 Glenohumeral (GH) Joint

• Head of humerus (large) - covered by hyaline cartilage
• Glenoid cavity of scapula (small, shallow) - covered by hyaline cartilage
• The glenoid is only ~1/4 the size of the humeral head

• Fibrocartilaginous collar deepening and expanding the glenoid peripherally
• Attaches to the margin of the glenoid fossa
• Superiorly continuous with the tendon of the long head of biceps brachii (attaches at supraglenoid tubercle)
• Increases contact area and contributes to stability

• Fibrous membrane attaches to glenoid margin (outside labrum) and anatomical neck of humerus
• Medially, the capsule attaches lower - extending onto the shaft of humerus
• Loose inferiorly - this redundant area accommodates abduction of the arm
• The inferior "axillary recess" is a key feature; it is taut in adduction, lax in abduction

• Lines the fibrous capsule; attaches to margins of articular surfaces
• Loose inferiorly
• Protrudes through apertures to form bursae:
  • Subtendinous bursa of subscapularis (most consistent) - between subscapularis and fibrous membrane
• Folds around and extends along the biceps tendon into the intertubercular sulcus

• Subacromial (subdeltoid) bursa - between acromion/deltoid and supraspinatus/joint capsule - the most clinically significant
• Subcoracoid bursa - between coracoid process and joint capsule
• Bursa between acromion and skin
• Additional bursae near coracobrachialis, teres major, long head of triceps, and latissimus dorsi

• Formed by the coracoid process, coracoacromial ligament, and acromion
• Creates a protective arch superiorly over the GH joint
• The supraspinatus tendon + subacromial bursa pass beneath this arch → site of subacromial impingement

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3.2 Acromioclavicular (AC) Joint

• AC ligaments (capsular): Reinforce the joint capsule; resist anteroposterior translation
• Coracoclavicular ligament (CC): Primary vertical stabilizer of the AC joint
  • Conoid ligament - posterior and medial; cone-shaped; from coracoid base to conoid tubercle of clavicle
  • Trapezoid ligament - anterior and lateral; from coracoid to trapezoid ridge of clavicle

• Minor injury: tears AC capsule/ligaments → AC separation on X-ray
• Severe injury: disrupts CC ligaments (conoid + trapezoid) → elevation and upward subluxation of clavicle
• Classified by Rockwood classification (Types I-VI)

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3.3 Sternoclavicular (SC) Joint

• Anterior SC ligament - primary restraint against superior displacement
• Posterior SC ligament - strongest; primary restraint against anteroposterior (AP) instability
• Costoclavicular ligament - from first rib/cartilage to medial clavicle; limits elevation
• Interclavicular ligament - between medial ends of both clavicles across the jugular notch

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3.4 Scapulothoracic (ST) Joint

• The coordinated movement between GH and ST joints during arm elevation
• Classic ratio: 2:1 (for every 3° of total arm elevation, 2° occurs at GH and 1° at ST)
• In the first 30° of abduction: movement mostly at GH with ST "setting"
• Above 90°: increasing contribution of ST motion
• Disturbance of this rhythm (scapular dyskinesia) causes secondary subacromial impingement

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4. Muscles of the Shoulder Complex

4.1 Scapular Stabilizers (Extrinsic Muscles)

Key clinical note: Serratus anterior paralysis (long thoracic nerve injury) = winged scapula (medial border protrudes away from thorax)

4.2 The Rotator Cuff (SITS)

• Coracohumeral ligament
• Superior glenohumeral ligament
• Long head of biceps tendon

• Subacromial impingement: Supraspinatus tendon impinges under the coracoacromial arch; fixed dimensions of the space means any swelling (tendinopathy, fluid, bone spur) causes pain with abduction (particularly 60-120° - the "painful arc")
• Supraspinatus most commonly torn - passes under the narrowest part of the arch; watershed zone of poor vascularity near its insertion
• Tears: partial (articular surface, bursal surface, or intrasubstance) or full-thickness

4.3 Deltoid and Other GH Movers

4.4 Long Head of Biceps Brachii

• Origin: supraglenoid tubercle (intra-articular, extra-synovial)
• Passes through the joint cavity superior to the humeral head
• Exits through the intertubercular sulcus held by the transverse humeral ligament
• Role: Restricts upward migration of humeral head (weak GH depressor); acts as humeral head stabilizer
• Innervation: Musculocutaneous nerve (C5,C6)
• Clinical: SLAP (Superior Labrum Anterior to Posterior) tears involve the biceps anchor at the supraglenoid tubercle

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5. Neurovascular Supply

Brachial Plexus - Key Nerves

Quadrangular Space

• Formed by: teres minor (superior), teres major (inferior), long head of triceps (medial), surgical neck of humerus (lateral)
• Contains: Axillary nerve and posterior circumflex humeral artery

Triangular Space

• Formed by: long head of triceps (lateral), teres minor (superior), teres major (inferior)
• Contains: Circumflex scapular artery

Triangular Interval

• Formed by: long head of triceps (medial), humerus (lateral), teres major (superior)
• Contains: Radial nerve and profunda brachii artery

Blood Supply

• Axillary artery (continuation of subclavian at lateral border of 1st rib) gives:
  • Superior thoracic artery
  • Thoracoacromial artery (acromial, clavicular, deltoid, pectoral branches)
  • Lateral thoracic artery
  • Anterior circumflex humeral artery (smaller; supplies bicipital groove area, anterior GH capsule)
  • Posterior circumflex humeral artery (larger; passes through quadrangular space; supplies deltoid, posterior GH joint, greater tubercle)
  • Subscapular artery → thoracodorsal + circumflex scapular

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6. Scapulohumeral Rhythm in Detail

• 0-30°: mostly GH motion ("setting phase" of scapula)
• 30-180°: 2:1 ratio - GH contributes 2° for every 1° of scapular rotation on the thorax
• Full elevation (180°): ~120° at GH + ~60° at ST

• Trapezius (upper + lower) and serratus anterior - the "force couple" for scapular upward rotation
• Deltoid and supraspinatus - force couple for GH abduction

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7. Gateways and Clinical Spaces

Suprascapular Foramen

• Suprascapular notch + superior transverse scapular ligament
• Suprascapular nerve passes through the foramen
• Suprascapular artery and vein pass above the ligament (mnemonic: "army goes under the bridge; navy sails over")

Subcoracoid/Coracoacromial Arch

• Formed by: coracoid process + coracoacromial ligament + acromion
• The supraspinatus and subacromial bursa pass beneath this arch
• Site of subacromial impingement syndrome
• Acromion morphology types (Bigliani):
  • Type I: flat
  • Type II: curved
  • Type III: hooked (highest risk of impingement and rotator cuff tears)

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8. Clinical Correlations

Anterior GH Dislocation

• Most common direction (>95%)
• Head typically dislocates anteroinferiorly (subcoracoid position)
• Bankart lesion: tear of anteroinferior glenoid labrum (with/without bony fragment)
• Hill-Sachs lesion: compression fracture of posterolateral humeral head
• Axillary nerve is at risk (test deltoid function and sensation after reduction)

Posterior GH Dislocation

• Rare (~2%); often missed
• Associations: seizures, electrocution, direct blow
• "Lightbulb sign" on AP X-ray (humerus internally rotated)
• Reverse Hill-Sachs (McLaughlin lesion) on anteromedial humeral head

Rotator Cuff Disorders

• Painful arc: 60-120° of abduction (supraspinatus impingement)
• Supraspinatus tears: full-thickness tears → inability to initiate abduction or maintain against resistance
• Infraspinatus/teres minor: external rotation weakness
• Subscapularis tears: internal rotation weakness; positive lift-off test, belly press test

Frozen Shoulder (Adhesive Capsulitis)

• Contraction of GH joint capsule, particularly the rotator interval and IGHL axillary pouch
• Progressive loss of all GH movements, especially external rotation and abduction

AC Joint Injury (Shoulder Separation)

• Rockwood Types I-III most common (I: sprain, II: AC torn + CC intact, III: both torn)
• Types IV-VI are rare and require surgery

Clavicle Fractures

• Most common fracture in upper limb
• Middle third (80%): proximal fragment pulled up by SCM; distal fragment pulled down by weight of arm
• Lateral third: if CC ligament torn, displacement can occur

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9. Summary Table - The Four Joints at a Glance

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