Shoulder

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Introduction

The shoulder complex, composed of the clavicle, scapula, and humerus, is an intricately designed combination of three joints, the Glenohumeral Joint, the Acromioclavicular Joint and the Sternoclavicular Joint, that links the upper extremity to the axial skeleton at the thorax and work collectively with the Scapulothoracic Joint to achieve normal shoulder girdle movements.

Movements of the shoulder complex represent a complex dynamic relationship between muscles forces, ligament constraints and bony articulations. The articular structures of the shoulder complex, in particular the Glenohumeral Joint, are designed primarily for mobility, allowing us to move and position the hand through a wide range of space, allowing the greatest range of motion of any joint in the body. 

The freedom of movement has been developed at the expense of stability, and it is these competing mobility and stability demands combined with an intricate structural and functional design which make the shoulder complex highly susceptible to dysfunction and instability.

The shoulder demonstrates a unique functional balance between mobility and stability through active forces, known as Dynamic stabilisation, which is the reliance on active forces or dynamic muscular control rather than passive stabilisation through passive forces such as articular surface configuration, capsule, or ligaments. Thus in the shoulder it is muscle forces which serve as the primary mechanism for securing the shoulder girdle to the thorax and providing a stable base of support for upper extremity movements,

Osteology

The osseous segments of the shoulder complex comprise of the clavicle, scapula, humerus and sternum 

Clavicle

The Clavicle is located between the Sternum and the Scapula, and it connects to the body through the Humerus. The clavicle is the first bone in the human body to begin intramembranous ossification directly from mesenchyme during the fifth week of fetal life. Similar to all long bones, the clavicle has both a medial and lateral epiphysis. The growth plates of the medial and lateral clavicular epiphyses do not fuse until the age of 25 years. Peculiar among long bones is the clavicle’s S-shaped double curve, which is convex medially and concave laterally. This contouring allows the clavicle to serve as a strut for the upper extremity, while also protecting and allowing the passage of the Axillary Vessels and Brachial Plexus medially.

Sternum

The sternum is a flat bone, located at the midpoint of the anterior thorax, is composed of the manubrium, the body, and xiphoid process. The manubrium is the most superior portion of the sternum, which articulates with the clavicle forming the Sternoclavicular Joint. The body of the sternum serves as the anterior attachment for Ribs 2 through 7. The inferior tip of the sternum is called the xiphoid process, meaning “Sword Shaped.”

Joints

In all there are four major articulations associated with the Shoulder Complex involving the sternum, clavicle, ribs, scapula, and humerus, which work together to provide large ranges of motion to the upper extremity in all three plane of motion. 

Glenohumeral Joint

The Glenohumeral Joint (GH Joint) is a true synovial ball-and-socket style diarthroidal joint that is responsible for connecting the upper extremity to the trunk. This joint is formed from the combination of the humeral head and the glenoid fossa of the scapula. This joint is considered to be the most mobile and least stable joint in the body, and is the most commonly dislocated diarthoidal joint.

Acromioclavicular Joint

The Acromioclavicular Joint (AC Joint) is a gliding , or plane style synovial joint that is formed by the junction of the lateral clavicle and the acromion process of the scapula. It attaches the scapula to the clavicle and serves as the main articulation that suspends the upper extremity from the trunk.

Sternoclavicular Joint

The Sternoclavicular Joint (SC Joint) is formed from the articulation of the medial aspect of the Clavicle and the Manubrium of the Sternum. The SC Joint is generally classified as a plane style synovial joint, and has a fibrocartilage joint disk.  The ligamentous reinforcements of this joint are very strong, often resulting ain a fracture of the clavicle before a dislocation of the SC Join occurst.

Scapulothoracic Joint

Although referred to as the Scapulothoracic Joint, the articulation between the scapula and the thorax is not a "true" joint as it does not have the characteristics of a fibrous, cartilaginous, or synovial joint. It is an articulation of the anterior aspect of the scapula on the posterior thorax. It typically refers to the motion of the scapula relative to the posterior rib cage. The SC Joint and AC Joint are interdependent with the Scapulothoracic Joint as the scapula is attached through the acromion process to the lateral end of the clavicle and through the AC Joint; the clavicle, in turn, is attached to the axial skeleton at the manubrium of the sternum through the SC Joint. Any movement of the scapula on the thorax thus result in movement at either the AC Joint, the SC Joint, or both. Normal movement and posture of the Scapulothoracic Joint are essential to the normal function of the shoulder.