Shoulder Anatomy and Function Quiz

Shoulder Joint Overview

• The shoulder joint, also known as the glenohumeral joint, is a ball-and-socket synovial joint formed between the glenoid cavity of the scapula and the head of the humerus.
• This structure allows for a wide range of movements in multiple planes.
• Bone structure and movement range influence the joint's stability.
• Shoulder movement occurs at both the glenohumeral and scapulothoracic joints.

Joint Structure and Stability

• The joint is lined with synovium and enclosed in a fibrous capsule.
• The glenoid cavity is deepened by a glenoid labrum, a rim of fibrocartilage, but despite this, the glenohumeral joint remains relatively unstable.
• Instability arises due to the shallowness of the glenohumeral joint and the disproportion of the articular surfaces.
• Stability is maintained by the capsule, various ligaments, and the rotator cuff muscles.

Capsule Structure

• The capsule is attached to the glenoid labrum, the scapula, and the anatomical head of the humerus.
• It bridges the intertubercular groove of the humerus and extends medially to the surgical neck of the humerus.
• The capsule is tough but relatively lax to facilitate a broad range of movement.
• Thickening of the capsule occurs in regions where it attaches to three glenohumeral ligaments and a coracohumeral ligament.

Bursae of the Shoulder

• Subscapular bursa facilitates the movement of the subscapularis tendon over the scapula.
• A bursa is a fluid-filled sac that reduces friction between articulating surfaces, reducing friction between the tendon and the surrounding structures (e.g., scapula).
• The subscapular bursa connects to the synovium and the joint cavity, making movement smooth.
• Subacromial bursa facilitates the movement of the supraspinatus tendon under the coracoacromial arch and over the deltoid muscle.
• Inflammation of the bursa can result in subacromial bursitis, causing pain during arm abduction.

Ligaments of the Shoulder

• Intracapsular ligaments: Three fibrous bands between the glenoid labrum and the humerus. These support the anterior capsule, including the superior, middle, and inferior glenohumeral ligaments.
• Extracapsular ligaments:
  • Coracoacromial ligament links the acromion and coracoid process
  • Coracohumeral ligaments connect the coracoid process to the greater tubercle of the humerus.
  • Transverse humeral ligament holds the tendon of the long head of the biceps in place during movement.

Coraco Acromial Arch

• The coracoacromial arch is a strong osseoligamentous structure that provides a smooth undersurface over the acromion.
• The coracoacromial ligament and coracoid process form this arch.
• The arch overhangs the humeral head, preventing superior displacement of the humerus.
• Within this space, there's a subacromial bursa and other rotator cuff tendons.

Movements of the Shoulder

• Flexion: Primarily by the pectoralis major, anterior deltoid, coracobrachialis, and biceps brachii.
• Extension: Primarily by the latissimus dorsi and posterior deltoid.
• Abduction: Begins with the supraspinatus (15-20 degrees), then central deltoid (20-90 degrees), and finally scapula rotation by the trapezius and serratus anterior (above 90 degrees).

Neurovascular Supply

• Blood supply via the anterior and posterior circumflex humeral arteries and the suprascapular artery.
• Innervation by the suprascapular, axillary, and lateral pectoral nerves which originate from the brachial plexus (C5-C6).
• Injuries to the brachial plexus can affect shoulder movement.

Clinical Applications

• Axillary nerves and circumflex artery are vulnerable during intramuscular injections and shoulder dislocations.
• Rotator cuff problems include tendon rubbing under the coraco-acromial arch and inflammation or wear of tendons.
• Chronic inflammation can lead to problems like impingement syndrome, occurring between 70–120 degrees of abduction.

Shoulder Disorders

• Rotator cuff problems: Tendon rubbing under the coraco-acromial arch causes irritation and inflammation; this can lead to bursitis, supraspinatus tendonitis, rotator cuff injury, and degeneration or rupture of the tendons.
• Dislocation: The inferior aspect of the glenohumeral joint is the least protected, thus inferior displacement is common.
• Axillary nerve injury: Injured during dislocation or intramuscular injections. Symptoms include deltoid paralysis, loss of abduction, and sensory loss.
• Frozen shoulder: Adhesive fibrosis and scarring between the inflamed capsule, rotator cuff, subacromial bursa, and the deltoid. Often causes difficulty in abducting the arm.
• Glenoid labrum tears: Common in athletes or those with shoulder instability or partial dislocation (subluxation) marked by pain and popping/snapping sensations.
• Osteoarthritis: A less common cause due to degenerative changes in articular cartilage. Usually secondary to trauma, including previous dislocations, humeral head fractures, or large rotator cuff tendon tears.

Stability

• Tendons of the rotator cuff (anterior, posterior, superior) provide stability
• Glenohumeral intracapsular ligaments (anterior, inferior) provide stability
• Coracohumeral ligaments (superior)
• Coracoacromial arch (superior)
• Deepening of the glenoid cavity by the glenoid labrum
• Splinting effect of the long head of the biceps and triceps.