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,

Structure

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.

Scapula

The Scapula, commonly referred to as the Shoulder Blade, is a highly mobile, thin, flat triangular-shaped bone placed on the postero-lateral aspect of thoracic cage . It has two surfaces, three borders, three angles and three processes.

• The slightly concave anterior aspect of the bone is called the subscapular fossa, which allows the scapula to glide smoothly along the convex posterior rib cage.
• The glenoid fossa is the slightly concave, oval-shaped surface that accepts the head of the humerus, composing the glenohumeral joint.
• The superior and inferior glenoid tubercles border the superior and inferior aspects of the glenoid fossa and serve as proximal attachments for the long head of the biceps and the long head of the triceps, respectively.
• The scapular spine divides the posterior aspect of the scapula into the supraspinatous fossa (above) and the infraspinatous fossa (below).
• The acromion process is a wide, flattened projection of bone from the most superior-lateral aspect of the scapula. The acromion forms a functional “roof” over the humeral head to help protect the delicate structures within that area.
• The coracoid process is the finger-like projection of bone from the anterior surface of the scapula, palpable about 1 inch below the most concave portion of the distal clavicle. The coracoid process is the site of attachment for several muscles and ligaments of the shoulder complex.
• The medial and lateral borders of the scapula meet at the inferior angle, or tip, of the scapula. Clinically, the inferior angle is important in helping track scapular motion.

Humerus

The humerus is the both the largest bone in the arm and the only bone in the upper arm. It is located between the shoulder and the elbow joint . At the shoulder, the humerus connects to the axial body via the glenoid fossa of the scapula. At the elbow, it connects primarily to the ulna, as the forearm's radial bone connects to the wrist. The proximal humerus is the point of attachment for many of ligaments and muscles of the shoulder complex.

• The Humeral Head is nearly one half of a full sphere that articulates with the glenoid fossa forming the Glenohumeral Joint.
• The Lesser Tubercle is a sharp, anterior projection of bone just below the humeral head.
• The Greater Tubercle, forms a more rounded lateral projection of bone.
• The Intertubercular Groove, often referred to as the Bicipital Groove because it houses the tendon of the long head of the biceps, divides the greater and lesser tubercles.
• The Deltoid Tuberosity, which is the distal insertion for all three heads of the deltoid muscle, lies more distally, on the lateral aspect of the upper one third of shaft of humerus.
• The Radial or Spiral Groove, which helps define the distal attachment for the lateral and medial heads of the triceps, runs obliquely across the posterior surface of the humerus. The radial nerve follows this groove.

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 first rib of both sides, the upper part of the second costal cartilage and clavicle forming the Sternoclavicular Joint. The manubrium is quadrangular and lies at the level of the 3rd and 4th thoracic vertebrae. The jugular notch is the thickest part of the bone and is convex along anterior and concave along the posterior side.
• The body of the sternum is longer and thinner. It has margins that articulates with the first cartilages of Ribs 2 through 7. 
• The xiphoid process, meaning “Sword Shaped", is the inferior tip of the sternum and also the smallest part. It doesn’t articulate with the ribs. The xiphoid process anchors several important muscles like the rectus abdominus muscle and the transversus thoracis muscle, including the abdominal diaphragm, a muscle that is necessary for normal breathing.

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. Recall that the glenoid fossa is relatively flat, while the head of the humerus is a large, rounded hemisphere. This bony conformation, in conjunction with the highly mobile scapula, allows for abundant motion in all three planes but does not promote a high degree of stability. It is interesting to note that the ligaments and capsule of the GH Joint are relatively thin and provide only secondary stability to the joint, while the primary stabilizing force of this joint is gained from the surrounding musculature, in particular the rotator cuff muscles. 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. In essence, this joint links the motion of the scapula (and attached humerus) to the lateral end of the clavicle. Because strong forces are frequently transferred across the AC joint, several important stabilizing structures are required to maintain its structural integrity.

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. This joint provides the only direct bony attachment of the upper extremity to the axial skeleton, accordingly, the joint needs to be both stable while also allowing extensive mobility. The SC Joint is generally classified as a plane style synovial joint, and allows range of motion in all three planes. It has a fibrocartilage articular disk with a network of strong, thick ligamentous reinforcements, often resulting in fracture of the clavicle before a dislocation of the SC Joint occurs.

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.

Soft Tissue

Static Stabilisers

Dynamic Stabilisers

Resources

References