The Pectoral Girdle Consists Of The

The pectoral girdle, a critical component of the human skeletal system, serves as the connection between the upper limb and the axial skeleton. Its unique structure and function allow for a wide range of movements and provide support for the upper extremities. Understanding the pectoral girdle's anatomy is essential for comprehending the biomechanics of the shoulder and upper limb.

Anatomy of the Pectoral Girdle

The pectoral girdle, also known as the shoulder girdle, is composed of two primary bones:

• Clavicle (Collarbone): A long, slender bone that articulates with the sternum (breastbone) medially and the scapula (shoulder blade) laterally.
• Scapula (Shoulder Blade): A flat, triangular bone located on the posterior aspect of the thorax, articulating with the clavicle and the humerus (upper arm bone).

Detailed Overview of the Clavicle

The clavicle is an S-shaped bone that plays a vital role in:

• Supporting the upper limb: Acts as a strut, keeping the upper limb away from the thorax, allowing for greater range of motion.
• Transmitting forces: Transfers forces from the upper limb to the axial skeleton.
• Protecting underlying structures: Shields the subclavian artery and vein, as well as the brachial plexus.

Anatomical Features of the Clavicle:

• Sternal End: The medial end of the clavicle, which articulates with the manubrium of the sternum at the sternoclavicular joint. This joint is the only bony attachment of the pectoral girdle to the axial skeleton.
• Acromial End: The lateral end of the clavicle, which articulates with the acromion process of the scapula at the acromioclavicular joint.
• Shaft: The main body of the clavicle, which is curved in two planes. The medial two-thirds are convex forward, while the lateral third is concave forward.
• Conoid Tubercle: A small, cone-shaped projection on the inferior surface of the lateral end of the clavicle, serving as an attachment site for the conoid ligament (part of the coracoclavicular ligament).
• Trapezoid Line: A ridge on the inferior surface of the lateral end of the clavicle, serving as an attachment site for the trapezoid ligament (part of the coracoclavicular ligament).
• Subclavian Groove: A groove on the inferior surface of the medial end of the clavicle, serving as an attachment site for the subclavius muscle.

Detailed Overview of the Scapula

The scapula is a flat, triangular bone that lies on the posterior aspect of the thorax, overlying ribs 2-7. It is highly mobile, gliding across the ribcage during upper limb movements. Its primary functions include:

• Providing attachment sites for muscles: Serves as an origin or insertion point for numerous muscles that control shoulder and upper limb movement.
• Articulating with the humerus: Forms the glenohumeral joint (shoulder joint), allowing for a wide range of arm movements.
• Contributing to shoulder stability: The scapula's position and movements influence the stability of the shoulder joint.

Anatomical Features of the Scapula:

• Body: The main, flat portion of the scapula.
• Spine: A prominent ridge on the posterior surface of the scapula.
• Acromion: A flattened, expanded process at the lateral end of the spine, which articulates with the clavicle.
• Coracoid Process: A hook-like projection on the anterior aspect of the scapula, providing attachment sites for muscles and ligaments.
• Glenoid Cavity: A shallow, pear-shaped depression on the lateral angle of the scapula, which articulates with the head of the humerus.
• Superior Angle: The superior-most point of the scapula.
• Inferior Angle: The inferior-most point of the scapula.
• Lateral Border (Axillary Border): The lateral edge of the scapula.
• Medial Border (Vertebral Border): The medial edge of the scapula.
• Superior Border: The superior edge of the scapula.
• Supraspinous Fossa: A depression on the posterior surface of the scapula, superior to the spine.
• Infraspinous Fossa: A depression on the posterior surface of the scapula, inferior to the spine.
• Subscapular Fossa: A large, concave depression on the anterior surface of the scapula.

Joints of the Pectoral Girdle

Several joints are associated with the pectoral girdle, facilitating movement and stability:

• Sternoclavicular Joint (SC Joint): The articulation between the sternal end of the clavicle and the manubrium of the sternum. It is a synovial joint with an articular disc that increases stability and shock absorption. It allows for elevation, depression, protraction, retraction, and rotation of the clavicle and scapula.
• Acromioclavicular Joint (AC Joint): The articulation between the acromial end of the clavicle and the acromion process of the scapula. It is a synovial joint with a thin articular disc. It allows for gliding and rotational movements of the scapula on the clavicle.
• Glenohumeral Joint (Shoulder Joint): The articulation between the glenoid cavity of the scapula and the head of the humerus. It is a ball-and-socket synovial joint, allowing for a wide range of motion, including flexion, extension, abduction, adduction, internal rotation, external rotation, and circumduction. The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) provide stability to this joint.
• Scapulothoracic Joint: This is not a true anatomical joint, but rather a physiological joint formed by the articulation of the anterior surface of the scapula with the posterior thoracic wall. It allows for the scapula to glide, rotate, elevate, and depress along the ribcage, contributing significantly to upper limb movement.

Muscles Acting on the Pectoral Girdle

Numerous muscles attach to the pectoral girdle, controlling its position and movement. These muscles can be categorized based on their primary actions:

Muscles that Move the Scapula

These muscles control the position and movement of the scapula on the thorax:

• Trapezius: A large, superficial muscle that extends from the occipital bone and cervical vertebrae to the thoracic vertebrae and scapula. It elevates, depresses, retracts, and rotates the scapula.
• Rhomboids (Rhomboid Major and Rhomboid Minor): Located deep to the trapezius, these muscles originate from the thoracic vertebrae and insert on the medial border of the scapula. They retract and elevate the scapula.
• Levator Scapulae: Originates from the cervical vertebrae and inserts on the superior angle of the scapula. It elevates the scapula and rotates it downward.
• Serratus Anterior: Originates from the ribs and inserts on the medial border of the scapula. It protracts the scapula and rotates it upward, allowing for overhead movements.
• Pectoralis Minor: Originates from the ribs and inserts on the coracoid process of the scapula. It protracts and depresses the scapula.

Muscles that Move the Humerus and also Affect the Pectoral Girdle

While these muscles primarily act on the humerus, their attachment to the scapula means they also influence the position and stability of the pectoral girdle:

• Deltoid: A large, triangular muscle that covers the shoulder joint. It originates from the clavicle, acromion, and spine of the scapula and inserts on the humerus. It abducts, flexes, and extends the arm.
• Pectoralis Major: A large, fan-shaped muscle that originates from the clavicle, sternum, and ribs and inserts on the humerus. It adducts, flexes, and internally rotates the arm.
• Latissimus Dorsi: A broad, flat muscle that originates from the thoracic and lumbar vertebrae, ribs, and iliac crest and inserts on the humerus. It adducts, extends, and internally rotates the arm.
• Rotator Cuff Muscles (Supraspinatus, Infraspinatus, Teres Minor, Subscapularis): These muscles originate from the scapula and insert on the humerus. They provide stability to the glenohumeral joint and control its rotation.

Functions of the Pectoral Girdle

The pectoral girdle performs several crucial functions:

• Connects the Upper Limb to the Axial Skeleton: This connection allows for the transfer of forces between the upper limb and the trunk.
• Provides Mobility to the Upper Limb: The scapula's ability to glide and rotate on the thorax, combined with the wide range of motion at the glenohumeral joint, enables a remarkable degree of upper limb movement.
• Supports the Upper Limb: The clavicle acts as a strut, keeping the upper limb away from the thorax and preventing it from collapsing inward.
• Provides Attachment Sites for Muscles: The scapula and clavicle serve as origins and insertions for numerous muscles that control shoulder, arm, and forearm movement.
• Protects Underlying Structures: The clavicle protects the subclavian artery and vein, as well as the brachial plexus, from injury.

Clinical Significance

Understanding the anatomy of the pectoral girdle is essential for diagnosing and treating various clinical conditions:

• Clavicle Fractures: The clavicle is one of the most frequently fractured bones in the body, often resulting from falls onto an outstretched arm or direct blows to the shoulder.
• Scapular Fractures: Scapular fractures are less common than clavicle fractures due to the scapula's protected position. They are often associated with high-energy trauma.
• Shoulder Dislocations: The glenohumeral joint is the most commonly dislocated major joint in the body, due to its inherent instability.
• Acromioclavicular Joint Injuries (Shoulder Separations): These injuries involve damage to the ligaments surrounding the AC joint, resulting in separation of the clavicle from the acromion.
• Rotator Cuff Tears: Tears of the rotator cuff muscles are a common cause of shoulder pain and dysfunction, often resulting from overuse or trauma.
• Scapular Dyskinesis: An alteration in the normal movement pattern of the scapula, which can contribute to shoulder pain and instability.
• Thoracic Outlet Syndrome: A condition in which the nerves and blood vessels in the space between the clavicle and the first rib are compressed, leading to pain, numbness, and tingling in the arm and hand.

Development of the Pectoral Girdle

The pectoral girdle develops from mesoderm during embryonic development. The clavicle undergoes intramembranous ossification, while the scapula undergoes endochondral ossification. These processes begin during the fifth week of gestation and continue throughout childhood and adolescence. Understanding the developmental origins of the pectoral girdle is important for understanding congenital anomalies that can affect this region.

Common Injuries and Conditions

The pectoral girdle, due to its role in mobility and weight-bearing, is prone to several injuries and conditions:

• Impingement Syndrome: This occurs when the tendons of the rotator cuff muscles are compressed as they pass beneath the acromion, leading to pain and inflammation.
• Bursitis: Inflammation of the bursae (fluid-filled sacs that cushion joints), often occurring in the shoulder due to repetitive movements or overuse.
• Osteoarthritis: Degeneration of the cartilage in the glenohumeral joint, leading to pain, stiffness, and decreased range of motion.
• Frozen Shoulder (Adhesive Capsulitis): A condition characterized by stiffness and pain in the shoulder joint, often resulting from inflammation and thickening of the joint capsule.

Exercise and Rehabilitation

Strengthening and rehabilitating the muscles that act on the pectoral girdle is essential for maintaining shoulder health and preventing injuries. Exercises such as:

• Rows: Strengthen the rhomboids and trapezius muscles, improving scapular retraction.
• Push-ups: Strengthen the pectoralis major, serratus anterior, and deltoid muscles, improving scapular protraction and stability.
• Lateral Raises: Strengthen the deltoid muscle, improving shoulder abduction.
• Internal and External Rotation Exercises: Strengthen the rotator cuff muscles, improving shoulder stability and rotation.
• Scapular Squeezes: Improve scapular retraction and posture.

These exercises, when performed correctly, can help to improve shoulder strength, stability, and range of motion.

Advanced Imaging Techniques

Advanced imaging techniques are often used to diagnose and evaluate conditions affecting the pectoral girdle:

• X-rays: Used to visualize fractures, dislocations, and other bony abnormalities.
• MRI (Magnetic Resonance Imaging): Provides detailed images of soft tissues, such as muscles, tendons, and ligaments, allowing for the diagnosis of rotator cuff tears, labral tears, and other soft tissue injuries.
• CT (Computed Tomography) Scans: Provide cross-sectional images of the bones and soft tissues, useful for evaluating complex fractures and dislocations.
• Ultrasound: Used to visualize tendons and muscles, useful for diagnosing rotator cuff tears and other soft tissue injuries.

Frequently Asked Questions (FAQs)

• What is the primary function of the pectoral girdle?

  The primary function of the pectoral girdle is to connect the upper limb to the axial skeleton, allowing for a wide range of movements and providing support.

• What bones make up the pectoral girdle?

  The pectoral girdle is composed of the clavicle (collarbone) and the scapula (shoulder blade).

• What is the importance of the rotator cuff muscles?

  The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) provide stability to the glenohumeral joint (shoulder joint) and control its rotation.

• What are common injuries affecting the pectoral girdle?

  Common injuries include clavicle fractures, scapular fractures, shoulder dislocations, acromioclavicular joint injuries, and rotator cuff tears.

• How can I improve the strength and stability of my pectoral girdle?

  Regular exercise, including exercises that target the muscles that move the scapula and the rotator cuff muscles, can help to improve strength and stability. Examples include rows, push-ups, lateral raises, and internal/external rotation exercises.

• What is scapular dyskinesis?

  Scapular dyskinesis is an alteration in the normal movement pattern of the scapula, which can contribute to shoulder pain and instability.

• What is thoracic outlet syndrome?

  Thoracic outlet syndrome is a condition in which the nerves and blood vessels in the space between the clavicle and the first rib are compressed, leading to pain, numbness, and tingling in the arm and hand.

• Why is the shoulder joint so prone to dislocation?

  The glenohumeral joint (shoulder joint) is prone to dislocation because it is a ball-and-socket joint with a shallow socket (glenoid cavity), which allows for a wide range of motion but also makes it less stable.

Conclusion

The pectoral girdle, comprising the clavicle and scapula, is a complex and crucial structure that connects the upper limb to the axial skeleton. Its unique anatomy allows for a wide range of movements, provides support for the upper extremities, and protects underlying structures. Understanding the anatomy, function, and clinical significance of the pectoral girdle is essential for healthcare professionals, athletes, and anyone interested in maintaining shoulder health and preventing injuries. By engaging in regular exercise, maintaining good posture, and seeking prompt medical attention for any shoulder pain or dysfunction, individuals can optimize the function and longevity of their pectoral girdle.