Applied Anatomy of the Shoulder Joint

Dislocation of the Shoulder Joint

The most commonly dislocated joint in the body.

Why it dislocates:

• Lax capsule, especially inferiorly (no rotator cuff reinforcement)
• Large humeral head in a small, shallow glenoid cavity
• Dislocation usually occurs with the arm abducted — head presses against the unsupported inferior capsule

Primary direction: Subglenoid (inferior). Clinically described as anterior or posterior depending on where the head finally settles.

Common cause: Excessive extension and lateral rotation of the humerus.

Clinical features:

• Loss of the normal rounded contour of the shoulder
• Prominence of the shoulder tip (acromion becomes prominent)
• Hollow below the acromion
• Arm held in slight abduction and lateral rotation

Complication: Axillary nerve injury (closely related to inferior capsule) — causes:

• Paralysis of deltoid → loss of abduction up to 90°
• Sensory loss over lower half of deltoid (regimental badge area)

Axillary Nerve Injury

Causes:

• Dislocation of shoulder joint
• Fracture of surgical neck of humerus

Effects:

• Loss of rounded contour of shoulder (greater tubercle becomes prominent)
• Deltoid paralysis → inability to abduct arm 0°–90°
• Sensory loss in regimental badge area (lower half of deltoid) — from injury to upper lateral cutaneous nerve of the arm

Frozen Shoulder (Adhesive Capsulitis)

Definition: Shrinkage and adherence of the two synovial layers, causing pain and uniform restriction of all shoulder movements.

Typical patient: 40–60 years of age.

Features:

• Progressively increasing shoulder pain
• Stiffness and restriction of all movements — especially external rotation, abduction, and medial rotation
• Altered scapulohumeral rhythm (excessive scapular shrugging)
• Disuse atrophy of surrounding muscles
• No radiological changes

Course: Self-limiting; spontaneous recovery in approximately 2 years; faster with physiotherapy.

Exclusion of shoulder joint disease: If the patient can raise both arms above the head and bring the palms together, the shoulder joint and deltoid/axillary nerve are likely intact.

Rotator Cuff Disorders

• Most common cause of shoulder pain
• Typically in males after 50 years or with repetitive overhead activity
• Calcium deposition in supraspinatus tendon → irritates subacromial bursa → subacromial bursitis
• Painful arc syndrome: Pain during 60°–120° of abduction (inflamed bursa caught under acromion)
• Dawbarn’s sign: Pain on pressing below acromion with arm adducted; pain absent with arm abducted (bursa disappears under acromion)

Optimum Position of the Shoulder

In diseases requiring immobilisation, the arm is held in 45°–90° of abduction to prevent ankylosis in a non-functional position.

Aspiration of the Shoulder Joint

Needle may be introduced:

• Anteriorly through the deltopectoral triangle (closer to deltoid side)
• Laterally just below the acromion

Shoulder Tip Pain (Referred Pain)

Irritation of the diaphragmatic peritoneum (gallbladder, stomach, duodenum) causes referred pain at the shoulder tip.

Basis: Phrenic nerve (C3, C4) carries impulses from diaphragmatic peritoneum; supraclavicular nerves (C3, C4) supply the skin of the shoulder tip — shared spinal segments cause referral.