Bones of the Shoulder Girdle

The bones of the shoulder girdle, also known as the pectoral girdle, consist of the clavicle and scapula. These bones connect the upper limb to the axial skeleton and serve as attachment sites for muscles that move the shoulder and arm. The shoulder girdle provides both stability and remarkable range of motion, enabling complex movements of the upper limb.

Structure

The shoulder girdle comprises two paired bones on each side of the body:

• Clavicle (Collarbone): A long, S-shaped bone that connects the sternum to the scapula. It lies horizontally at the root of the neck, just above the first rib.
• Scapula (Shoulder blade): A flat, triangular bone that lies on the posterior thoracic wall between the second and seventh ribs.

Clavicle

The clavicle has two ends:

• Sternal (medial) end: Articulates with the manubrium of the sternum at the sternoclavicular joint.
• Acromial (lateral) end: Articulates with the acromion of the scapula at the acromioclavicular joint.

It has a double curve: convex medially and concave laterally, forming its characteristic S-shape. It is subcutaneous throughout its length and is the most commonly fractured bone in the body.

Scapula

The scapula has several key landmarks:

• Spine: A prominent ridge on the posterior surface, ending laterally as the acromion.
• Acromion: A bony process that articulates with the clavicle.
• Coracoid process: A hook-like projection on the anterior surface, serving as a muscle attachment site.
• Glenoid cavity: A shallow socket that articulates with the head of the humerus to form the shoulder joint.
• Subscapular, supraspinous, and infraspinous fossae: Depressions on the anterior and posterior surfaces serving as attachment areas for muscles.

Location

The shoulder girdle is located in the superior lateral thoracic region. The clavicle is positioned anteriorly and horizontally, extending from the manubrium of the sternum to the scapula. The scapula is positioned posterolaterally over the ribs of the upper back, between the second and seventh ribs, and lies over the posterior thoracic wall.

Function

• Structural support: The clavicle acts as a strut holding the scapula and arm laterally away from the thorax, providing a stable base for limb movement.
• Mobility: The shoulder girdle allows a wide range of upper limb motion through gliding movements at the sternoclavicular and acromioclavicular joints.
• Muscle attachment: Both bones provide broad surfaces for the origin and insertion of shoulder, back, chest, and arm muscles (e.g., deltoid, trapezius, pectoralis major).
• Joint formation: The glenoid cavity of the scapula forms the socket of the glenohumeral (shoulder) joint.

Development

The clavicle is the first bone in the body to begin ossification, with its primary ossification center appearing during the fifth to sixth weeks of fetal life. It ossifies through both intramembranous and endochondral processes. The secondary ossification center at the sternal end appears around puberty and fuses by age 25.

The scapula develops from several ossification centers, with the primary center appearing around the eighth week of fetal life. The acromion, coracoid process, inferior angle, and medial border ossify separately during childhood and fuse during adolescence.

Clinical Significance

• Clavicle fracture: Often due to falls on the shoulder or outstretched arm; common in children and athletes. Typically occurs at the junction of the medial two-thirds and lateral one-third.
• Shoulder dislocation: Involves displacement of the humeral head from the glenoid cavity. The shallow glenoid makes the joint prone to dislocation.
• Scapular winging: Occurs when the medial border of the scapula protrudes posteriorly due to weakness or paralysis of the serratus anterior muscle, often from long thoracic nerve injury.
• Acromioclavicular joint separation: Common in contact sports, involves disruption of the acromioclavicular and possibly the coracoclavicular ligaments.