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, and humerus 

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.

Joints

In all there are four major joints associated with the Shoulder Complex.

Sternoclavicular Joint

Although referred to as the Scapulothoracic Joint, the articulation between the scapula and the thorax is does not have the characteristics of a fibrous, cartilaginous, or synovial joint.