Anatomy and Kinesiology of the Shoulder Joint Complex

• Flexion and Extension
• Abduction and Adduction
• Medial (internal) and Lateral (external) rotation
• Circumduction (a combination of the above)

Exam Tip: While the GHJ is highly mobile, it is structurally weakest in the inferior direction.

Ligamentous Support and Bursae

Stability in the shoulder is provided by both intrinsic ligaments (part of the capsule) and extrinsic ligaments (strengthening the surrounding skeletal framework).

Ligaments

The GHJ is reinforced by several key ligaments:

1. Glenohumeral Ligaments: These include the superior, middle, and inferior ligaments, which strengthen the anterior aspect of the joint capsule.
2. Coracohumeral Ligament: An extrinsic ligament spanning from the coracoid process to the greater tubercle, providing superior strength.
3. Transverse Humeral Ligament: This ligament passes from the greater to the lesser tubercle over the bicipital groove, acting to hold the tendon of the long head of the biceps brachii in place.
4. Coracoacromial Ligament: An extrinsic ligament spanning between the coracoid and the acromion; it sits superior to the humeral head and prevents superior displacement of the humerus.

Bursae

Bursae are fluid-filled sacs that reduce friction between moving parts. The GHJ involves:

• Subscapular bursa: Located between the tendon of the subscapularis and the neck of the scapula.
• Subacromial and Subdeltoid bursae: These are located between the deltoid muscle, the supraspinatus tendon, and the glenohumeral capsule.

The Role of the Rotator Cuff

The tendons of the rotator cuff muscles are vital for joint stability. These four muscles form a "musculotendinous collar" that reinforces the joint anteriorly, posteriorly, and superiorly, eventually fusing with the fibrous joint capsule. This muscular support, combined with the skeletal arch formed by the acromion and coracoid process, helps prevent dislocation.

The Scapulothoracic (ST) Joint

Unlike the other articulations in the shoulder, the scapulothoracic "joint" is not a true skeletal articulation. Instead, it is a physiological joint.

In this arrangement, the scapula glides across the thorax. The contact occurs between the subscapularis muscle (on the scapula) and the serratus anterior muscle (on the thorax). This movement is essential for the full range of shoulder motion and follows a coordinated pattern known as scapulohumeral rhythm.

The ST joint allows for several movements:

• Elevation and Depression
• Protraction and Retraction
• Upward and Downward rotation

Clinical Correlations

Shoulder Dislocations

Because the GHJ is weakest inferiorly, most dislocations occur in that direction. While they are clinically categorized as "anterior" or "posterior" based on the position of the humeral head relative to the glenoid, almost all dislocations are technically inferior. An anterior dislocation is the most common type.

Adhesive Capsulitis ("Frozen Shoulder")

Adhesive capsulitis is characterized by inflammation of the shoulder joint. This inflammation leads to adhesive fibrosis (scarring) between the GHJ joint capsule, the rotator cuff, the subacromial bursa, and the deltoid. Patients typically experience a decreased active and passive range of motion, particularly during abduction. The condition often progresses through three stages: freezing, frozen, and thawing.

Clinical Note: When assessing a patient with a suspected clavicle fracture, look for severe pain during arm movement, a visible bump, and a "sagging" appearance of the shoulder.