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SHOULDER JOINT (GLENOHUMERAL JOINT)

MBBS Anatomy Notes | 15 Marks | Based on Gray's Anatomy for Students & Vishram Singh

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1. INTRODUCTION

• Type: Synovial ball-and-socket joint (multiaxial)
• Articulation: Large spherical head of humerus + small shallow glenoid cavity of scapula
• Key feature: Most mobile joint in the body - wide range of movement at the cost of stability
• Stability is provided by surrounding soft tissues, NOT bony architecture

Mnemonic for stability: "RBLC" - Rotator cuff, Biceps long head, Ligaments, Coracoacromial arch

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2. ARTICULAR SURFACES

• The glenoid cavity is only 1/3rd the size of the humeral head
• Deepened by the glenoid labrum - a fibrocartilaginous rim attached to the margin of the glenoid fossa
• Superiorly, the labrum is continuous with the tendon of the long head of biceps brachii, which attaches to the supraglenoid tubercle

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DIAGRAM 1 - Articular Surfaces of Glenohumeral Joint

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3. JOINT CAPSULE

Fibrous Membrane:

• Attaches medially: to margin of glenoid cavity (outside attachment of glenoid labrum)
• Attaches laterally: to anatomical neck of humerus
• On the medial side of humerus, the attachment extends onto the shaft - forming a loose/redundant fold that accommodates abduction
• Inferiorly, the capsule is also loose - allows for full abduction

Openings in the fibrous membrane:

1. Anteriorly - for subtendinous bursa of subscapularis to communicate with joint cavity
2. In the intertubercular sulcus - for passage of tendon of long head of biceps (surrounded by synovial sheath)

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4. SYNOVIAL MEMBRANE

• Attaches to margins of articular cartilage
• Lines the fibrous capsule from inside
• Redundant inferiorly (accommodates abduction)
• Protrudes through apertures in the fibrous membrane to form bursae
• Folds around the long head of biceps tendon as it passes through the joint

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DIAGRAM 2 - Synovial Membrane and Joint Capsule

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5. LIGAMENTS

A. Intrinsic Ligaments (thickenings of fibrous capsule):

B. Extrinsic Ligament:

• Coracoacromial ligament: Coracoid process → acromion
• Forms the coracoacromial arch - protects the superior aspect of the joint
• Together with the acromion and coracoid, prevents superior dislocation of humeral head

High-yield: The inferior glenohumeral ligament is the most important static stabilizer against anterior-inferior dislocation (the most common type of shoulder dislocation)

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DIAGRAM 3 - Ligaments of Right Glenohumeral Joint (Anterior View)

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6. BURSAE

Bursae communicating with joint cavity:

1. Subtendinous bursa of subscapularis - between subscapularis muscle and fibrous membrane (most consistent)

Bursae NOT communicating with joint cavity:

1. Subacromial (subdeltoid) bursa - between acromion/deltoid and supraspinatus/joint capsule (clinically most important)
2. Between acromion and skin
3. Between coracoid process and joint capsule
4. Related to tendons of coracobrachialis, teres major, long head of triceps, latissimus dorsi

Clinical note: Inflammation of the subacromial bursa (subacromial bursitis) causes painful arc syndrome - pain during 60°-120° of abduction

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7. STABILITY OF THE JOINT

A. ROTATOR CUFF (Dynamic stabilizers - most important)

Mnemonic: "SITS" - Supraspinatus, Infraspinatus, Teres minor, Subscapularis

The cuff surrounds posterior, superior, and anterior aspects of the joint (NOT inferior - hence inferiorly is the weak point - "axillary recess")

B. Long Head of Biceps Brachii

• Passes superiorly through the joint cavity
• Restricts upward displacement of humeral head
• Acts as an additional stabilizer

C. Coracoacromial Arch

• Formed by coracoid process + coracoacromial ligament + acromion
• Prevents superior dislocation

D. Glenoid Labrum

• Deepens the socket and increases contact area

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DIAGRAM 4 - Rotator Cuff and Surrounding Muscles (Lateral/Coronal View)

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8. MOVEMENTS

Important: Full abduction (180°) requires scapular rotation (by trapezius and serratus anterior) in addition to glenohumeral movement. The glenohumeral:scapular rotation ratio = 2:1.

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9. BLOOD SUPPLY

• Anterior circumflex humeral artery (branch of axillary artery)
• Posterior circumflex humeral artery (branch of axillary artery)
• Suprascapular artery (branch of thyrocervical trunk)

The anterior circumflex humeral artery is the main supply to the humeral head - damage in fracture-dislocation causes avascular necrosis

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10. NERVE SUPPLY

Hilton's law: Muscles acting on a joint and the skin overlying those muscles receive innervation from the same nerve.

Axillary nerve injury: Most common nerve injury with shoulder dislocation/surgical neck fracture - causes loss of shoulder abduction (deltoid paralysis) + loss of sensation over the "regimental badge area" (lower deltoid)

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11. RELATIONS

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12. DISLOCATION OF SHOULDER JOINT

Most Common Direction: ANTERIOR (Subcoracoid) - 95%

• The inferior aspect of the capsule is weakest (no rotator cuff reinforcement)
• When the arm is forcefully abducted + externally rotated, the humeral head is levered anteroinferiorly
• The inferior glenohumeral ligament tears

• Loss of normal rounded contour of shoulder (flattening)
• Arm held in slight abduction and external rotation
• Hollow below acromion (empty glenoid)
• Axillary nerve injury - loss of sensation over regimental badge area

1. Axillary nerve injury (most common)
2. Recurrent dislocation (especially in young patients)
3. Bankart lesion - tear of anterior glenoid labrum
4. Hill-Sachs lesion - posterolateral compression fracture of humeral head
5. Rotator cuff tear (especially in elderly)
6. Axillary artery injury (rare)

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QUICK REVISION TABLE

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HIGH-YIELD EXAM POINTS (for 15-mark questions)

1. Glenoid labrum - fibrocartilaginous, continuous with long head of biceps superiorly; torn in Bankart lesion
2. Rotator cuff = SITS - surrounds anterior, superior, posterior; NOT inferior (hence inferiorly is the weak spot)
3. Subacromial bursa does NOT communicate with joint cavity (unlike subscapularis bursa)
4. Long head of biceps is intra-articular but extra-synovial (enclosed in synovial sheath)
5. Axillary nerve (C5,6) - passes through quadrangular space, supplies deltoid + teres minor; most commonly injured in shoulder dislocation
6. Inferior glenohumeral ligament = most important static stabilizer; forms "hammock" during abduction
7. Coracoacromial arch = secondary protection superiorly; prevents superior dislocation
8. Scapulohumeral rhythm - 2:1 ratio (glenohumeral : scapular rotation) for full 180° abduction
9. Transverse humeral ligament - retains biceps tendon in intertubercular sulcus
10. Anterior circumflex humeral artery - main blood supply to humeral head; damage = avascular necrosis

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