Anatomy of the Shoulder

The complexity of the human anatomy is probably best seen in the movements of the shoulder joint. The humerus-scapula connectivity via muscle mass and tissues, results in the formation of a group of structures that help in executing hand movements as well as generating the desired thrust to pick up and relocate objects in the periphery.

The shoulder joints facilitate movements such as, pushing and pulling, movements that are executed by everyone, throughout the day. They can, not only abduct and adduct, but can also rotate clockwise and anticlockwise, along the torso.

The shoulder joints and their movements are a result of the flexibility of the tendons, and articulations between the bones. The joint movements are facilitated by two kinds of cartilages. The articular cartilage is that, which extends along the ends of the bones. This white cartilage facilitates the gliding movement of bones on one another.

This constant friction in the region around the white cartilage causes it to wear out, initiating the onset of arthritis. Arthritis is a condition, in which, joints of the body; in this case the shoulders, become painful and stiff due to decreased lubrication between the conjoined bones.

The other cartilage in this region is the labrum. This fibrous cartilage is distinctly different from the articular cartilage. It is located only on the ends of the ball-and-socket joints.

The three main joints of shoulder are acromioclavicular, glenohumeral, and sternoclavicular. The glenohumeral joint or the 'ball-and-socket' joint enables the arm to rotate in a circular fashion, and to hinge out, away from the rest of the body alignment. This joint is the result of articulation between the lateral scapula and the head of the humerus bone.

It is characterized by the rounded anterior surface of the humerus bone and the dish shaped segment of the scapula, also referred to as the glenoid fossa. Its mobility is facilitated by the fossa's shallow design and relatively loose shoulder-body connection. However, this additional mobility does not come without a drawback.

The loose ball-and-socket joint is one that is most exposed to dislocation, compared to most other joints in the body. There is a soft tissue envelope that encompasses the glenohumeral joint, holding it in alignment with scapula, humerus, and biceps. The tissue is a synovial membrane that is strengthened by the coracohumeral and other glenohumeral ligaments.

The sternoclavicular shoulder joint is located at the end of the clavicle medial and the manubrium, the uppermost part of the sternum. This joint comprises a protective tissue and an intra-articular disc, both of which facilitate stability of the joint.

Most shoulder movements are limited by the costoclavicular ligament, a limitation that reduces muscle damage, and stabilizes the joint further. The range of its movement is increased due to the flexibility of a fibrocartilaginous disc. Though sternoclavicular subluxation is rare, it is triggered by direct trauma.

The serratus anterior shoulder muscle holds together the scapula and the thoracic wall, while the subclavius stabilizes the clavicle. The pectoralis minor muscle is responsible for the medial and protracting variations of the scapula.

The sternocleidomastoid muscle actions flex and rotate the head, aiding respiration. The levator scapulae, rhomboid major and rhomboid minor, trapezius, rotator cuff, and deltoid are other shoulder muscles that facilitate movement and functionality of the joint.