IN-DEPTH LOOK AT SHOULDER GIRDLE AND JOINT

Introduction

The shoulder girdle consists of the clavicle (collarbone) and scapula (shoulder blade), which connect the upper limbs to the axial skeleton.

The shoulder joint, also known as the glenohumeral joint, is a highly mobile ball-and-socket joint formed by the head of the humerus (upper arm bone) and the shallow socket of the scapula. This joint allows a wide range of motion but is relatively unstable, making it susceptible to dislocation. Proper strengthening and stability exercises are important for maintaining shoulder health.

 

Shoulder Bone Features

Here are the principal distinguishing features of the scapula, clavicle, and humerus:

1. Scapula (Shoulder Blade):
   • Shape: The scapula is a flat, triangular bone located on the upper back, forming part of the shoulder girdle.
   • Borders: It has three prominent borders – medial (vertebral), lateral (axillary), and superior.
   • Processes: The scapula has several key processes, including the acromion process (which forms the top of the shoulder) and the coracoid process (which curves forward from the superior border).
   • Fossae: There are two significant fossae on the scapula – the supraspinous fossa (above the spine) and the infraspinous fossa (below the spine).
   • Glenoid Cavity: This is a shallow socket on the lateral part of the scapula where the humerus (arm bone) articulates, forming the shoulder joint.
2. Clavicle (Collarbone):
   • Shape: The clavicle is an S-shaped bone that runs horizontally between the sternum (breastbone) and the scapula.
   • Medial and Lateral Ends: The medial end articulates with the sternum, forming the sternoclavicular joint, and the lateral end connects to the acromion of the scapula, forming the acromioclavicular joint.
   • Function: The clavicle provides stability to the shoulder joint and acts as a strut, holding the arm away from the top of the ribcage, allowing for greater freedom of movement.
3. Humerus (Upper Arm Bone):
   • Proximal End: The head of the humerus is spherical and articulates with the glenoid cavity of the scapula, forming the shoulder joint.
   • Tubercles: The greater and lesser tubercles are bony projections near the head of the humerus, serving as attachment points for muscles and tendons.
   • Deltoid Tuberosity: A roughened area on the lateral shaft where the deltoid muscle attaches.
   • Trochlea and Capitulum: These are two distinct condyles on the distal end that articulate with the forearm bones (ulna and radius) to form the elbow joint.
   • Surgical Neck: This is a common fracture site on the humerus, located just below the greater tubercle.

These features play crucial roles in the movement and stability of the shoulder joint and upper limb.

 

Scapula Spaces & Contents

Rotator Cuff Muscles Explained

 

Axilla: Boundaries & Contents

The axilla, commonly known as the armpit, is a pyramidal-shaped region located between the upper arm and the lateral chest wall. It serves as a crucial passageway for blood vessels, nerves, and lymphatics traveling between the upper limb and the rest of the body. Here’s a detailed explanation of the boundaries and contents of the axilla:

Boundaries of the Axilla:

1. Anterior Wall: Formed by the pectoralis major and pectoralis minor muscles, which cover the front of the chest.
2. Posterior Wall: Comprised mainly of the subscapularis muscle, along with the teres major and latissimus dorsi muscles.
3. Medial Wall: The serratus anterior muscle, often referred to as the “boxer’s muscle,” contributes to this wall.
4. Lateral Wall: The bicipital groove (intertubercular groove) of the humerus, which houses the long head of the biceps brachii muscle, helps define the lateral border.

Contents of the Axilla:

1. Axillary Artery: A continuation of the subclavian artery, it gives rise to several branches that supply the upper limb and surrounding structures.
2. Axillary Vein: Drains deoxygenated blood from the upper limb, eventually joining with other veins to form the superior vena cava.
3. Brachial Plexus: A network of nerves formed by the lower four cervical nerves and the first thoracic nerve. It supplies motor and sensory innervation to the upper limb.
4. Axillary Lymph Nodes: These nodes are an essential part of the lymphatic system, filtering lymph and playing a crucial role in immune function.
5. Cords of the Brachial Plexus: The brachial plexus divides into lateral, medial, and posterior cords, which further give rise to the major nerves of the upper limb.
6. Subclavius Muscle: This small muscle runs beneath the clavicle and assists in stabilizing the shoulder.
7. Blood Vessels: Various arteries and veins supplying the muscles and skin of the chest and upper limb pass through the axilla.

The axilla is a significant anatomical region due to its role in the function of the upper limb and its importance in various medical procedures, such as lymph node examinations and surgical interventions in the region.

 

Shoulder Joint Components

The shoulder joint is a complex and highly mobile joint that allows a wide range of motion in the upper limb. It’s composed of several key components:

1. Glenoid Cavity (Glenoid Fossa): This is a shallow, cup-like depression on the lateral scapula (shoulder blade) where the head of the humerus (upper arm bone) articulates. It provides stability but also allows for significant mobility.
2. Humerus: The head of the humerus is the “ball” portion that fits into the glenoid cavity. It’s the long bone of the upper arm.
3. Labrum: This is a ring of fibrous cartilage that surrounds the glenoid, deepening the socket and providing additional stability to the joint.
4. Rotator Cuff: This is a group of four muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) and their tendons that surround the shoulder joint. The rotator cuff muscles help stabilize and control the movements of the joint.
5. Joint Capsule: A fibrous structure that encloses the shoulder joint, holding the bones together and containing synovial fluid, which lubricates the joint.
6. Synovial Membrane: This lines the joint capsule, secreting synovial fluid to further lubricate the joint, reducing friction during movement.
7. Ligaments: Several ligaments (glenohumeral ligaments, coracohumeral ligament, and coracoacromial ligament) provide stability to the joint, preventing excessive movement.
8. Bursae: These are small, fluid-filled sacs located between tendons, muscles, and bones to reduce friction during movement. The subacromial bursa is particularly important in the shoulder joint.
9. Acromion: Part of the scapula that forms the “roof” of the shoulder joint, providing protection to the underlying structures.
10. Clavicle: The collarbone, which helps to anchor the shoulder to the axial skeleton and provides stability.

These components work together to create the complex and dynamic shoulder joint, allowing a remarkable range of movements, including flexion, extension, abduction, adduction, rotation, and circumduction of the arm.

 

Shoulder Muscles

 

Shoulder Bursae

Bursae are small, fluid-filled sacs located near joints that help reduce friction between bones, tendons, and muscles, allowing smooth movement. In the context of the shoulder joint, several bursae play essential roles in maintaining the joint’s function and preventing irritation:

1. Subacromial Bursa: This is the most well-known bursa in the shoulder. It lies between the acromion (the bony projection of the scapula) and the rotator cuff tendons. Its primary function is to reduce friction and provide cushioning to the rotator cuff tendons as they pass beneath the acromion during shoulder movements.
2. Subscapular Bursa: Located beneath the subscapularis muscle, this bursa helps reduce friction between the muscle and the head of the humerus (upper arm bone). The subscapularis muscle is part of the rotator cuff and is responsible for internal rotation of the shoulder.
3. Supraspinatus Bursa: Found between the supraspinatus tendon and the superior part of the humeral head, this bursa assists in the smooth gliding of the supraspinatus tendon during abduction of the arm (raising it away from the body).
4. Infraspinatus Bursa: This bursa is located between the infraspinatus tendon (another rotator cuff tendon) and the humeral head’s posterior part. It helps reduce friction during external rotation of the shoulder.
5. Subcoracoid Bursa: Positioned beneath the coracoid process (a bony structure of the scapula), this bursa acts as a cushion between the coracoid process and the adjacent tendons, reducing friction during movements.
6. Subdeltoid Bursa: Situated between the deltoid muscle and the joint capsule of the shoulder, this bursa aids in reducing friction as the deltoid muscle moves over the joint.

These bursae are crucial for maintaining the integrity and proper function of the shoulder joint. However, overuse, trauma, or inflammation can lead to conditions such as bursitis, which involves the irritation and swelling of these bursae, causing discomfort and limiting the shoulder’s range of motion.

 

Shoulder Joint Supply & Innervation

The shoulder joint, also known as the glenohumeral joint, is supplied by several blood vessels and innervated by specific nerves. Here’s a brief overview:

1. Blood Supply:
   • The main artery supplying the shoulder joint is the suprascapular artery, which arises from the subclavian artery.
   • The circumflex scapular artery and the posterior circumflex humeral artery also contribute to the blood supply, providing collateral circulation to the joint.
2. Innervation:
   • The primary nerves providing innervation to the shoulder joint are the axillary nerve and the suprascapular nerve.
   • The axillary nerve supplies the shoulder joint from the posterior side, including the deltoid muscle and the skin over the shoulder.
   • The suprascapular nerve innervates the shoulder joint from the superior and posterior aspects, providing sensation to parts of the joint capsule.

These blood vessels and nerves play crucial roles in maintaining the function and sensation of the shoulder joint.