Describe the shoulder joint under the following headings : I. Movement II. Blood Supply III. Ligaments

I. Movement

The shoulder joint is a ball-and-socket joint, formed by the articulation of the humeral head with the glenoid cavity of the scapula. This allows for a wide range of movements, including:

• Flexion: Movement of the arm forward in the sagittal plane (approximately 180 degrees).
• Extension: Movement of the arm backward in the sagittal plane (approximately 60 degrees).
• Abduction: Movement of the arm away from the midline of the body in the coronal plane (approximately 180 degrees).
• Adduction: Movement of the arm towards the midline of the body in the coronal plane (approximately 30 degrees).
• Internal Rotation: Rotation of the arm inwards, with the anterior surface facing medially (approximately 70 degrees).
• External Rotation: Rotation of the arm outwards, with the posterior surface facing medially (approximately 90 degrees).
• Circumduction: A combination of all the above movements, resulting in a conical motion of the arm.

These movements are achieved through the coordinated action of several muscles, including the deltoid, rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis), and other scapular stabilizers.

II. Blood Supply

The shoulder joint receives its blood supply from several arteries, primarily branches of the axillary artery. The key arteries involved are:

• Superior Thoracic Artery: Supplies the scapula and surrounding muscles.
• Thoracoacromial Artery: Provides branches to the deltoid and pectoral muscles.
• Lateral Thoracic Artery: Supplies the serratus anterior muscle.
• Subscapular Artery: A large branch of the axillary artery, supplying the subscapularis muscle and contributing to the arterial circle around the humeral head.
• Anterior and Posterior Circumflex Humeral Arteries: These arteries form an anastomosis around the humeral head, providing crucial blood supply to the joint capsule and humeral head itself. This anastomosis is vulnerable to disruption in fractures of the surgical neck of the humerus.

Venous drainage is primarily via the axillary vein and its tributaries.

III. Ligaments

The ligaments of the shoulder joint provide static stability, limiting excessive movement and preventing dislocation. The major ligaments include:

• Glenohumeral Ligaments: These are the primary stabilizers of the shoulder joint. They are divided into three parts:
  • Superior Glenohumeral Ligament (SGHL): Resists anterior dislocation with the arm in abduction and external rotation.
  • Middle Glenohumeral Ligament (MGHL): The most important restraint to anterior dislocation, particularly in the mid-range of abduction.
  • Inferior Glenohumeral Ligament Complex (IGHL): The most significant ligamentous restraint to posterior dislocation and inferior instability. It is comprised of the inferior glenohumeral ligament, anterior band of the IGHL, and posterior band of the IGHL.
• Coracohumeral Ligament: Helps to prevent superior translation of the humeral head.
• Coracoacromial Ligament: Forms the roof of the subacromial space and helps to prevent superior dislocation of the humeral head.
• Transverse Humeral Ligament: Holds the long head of the biceps tendon in the bicipital groove.

The glenoid labrum, a fibrocartilaginous rim surrounding the glenoid cavity, also contributes significantly to joint stability by deepening the socket and providing an attachment point for the glenohumeral ligaments.