The pectoral girdle (or shoulder girdle) is a pair of structures, each consisting of a clavicle and scapula, that connects the upper limbs to the axial skeleton and forms articulations, or joints, with the upper limbs. The human body has a left and right pectoral girdle, each containing two bones, the scapula and clavicle. The scapulae articulate medially (toward the midline of the body) with the vertebral column and laterally (away from the midline of the body) with the clavicle and the humerus bone of the upper arm. The clavicles articulate medially with the sternum or breastbone and laterally with the scapulae. Whether it is elevating the shoulders or bringing the arm to the body, these upper limb body actions cannot be performed without help from the pectoral girdle.The pectoral girdle has other functions outside of providing a bridge of connection between the upper limbs and axial skeleton. The pectoral girdle positions the shoulders and provides a base for arm movement. There must be a degree of mobility at the shoulder region in order for the arms to move. The pectoral girdle serves as a location of attachment for many muscles of the back, chest, and neck that help the upper limbs move. One unique characteristic of the pectoral girdle is that it can move in many directions. This is due to the loose attachment of the scapula to the axial skeleton, which allows the scapula to move back and forth against the thorax (rib cage) with the help of muscular activity. There is no articulation point between the pectoral girdles and the vertebral column. This enables the arm to move with a wide range of motion. That is, people can move their arm away from, toward, or even above their head. Another anatomical feature of the pectoral girdle that contributes to this mobility is the connection between the humerus and scapula. This bone connection is shallow, which means it permits a wide range of upper limb mobility.
Anatomy of the Scapula
The scapula, or shoulder blade, is a triangular flat bone that forms the posterior region of the pectoral girdle. The primary function of the scapula is to serve as a connection point between the humerus (upper arm bone) and the clavicle (collarbone). The scapula can be palpated, or felt at the surface of the skin, on the upper region of the back near the shoulder. Its loose attachment to the axial skeleton (specifically the rib cage) allows the scapula to slide in all directions over the thorax. Movements can occur independently or in response to movements of the arm.
The major regions on the scapula include three sides and three angles because of its triangular shape. The sides are referred to as borders and are named superior, medial, and lateral, and the angles are superior, lateral, and inferior. The superior border of the scapula is the horizontal edge at the top of the scapula, described as being thin, sharp, and short. It joins with the medial border at the superior angle of the scapula. The medial (or vertebral) border is the edge close to the vertebrae and is described as running parallel to the vertebral column. The lateral (or axillary) border is a thick border that is near the axillary (underarm) region. The lateral border connects to the superior border at the lateral angle and to the medial border at the inferior angle.
The lateral angle contains a large protrusion that originates from the posterior (back) side called the acromion and a fingerlike projection on the anterior (front) side called the coracoid process. The acromion region of the scapula articulates with the clavicle at the acromioclavicular joint. Within the lateral angle is the glenoid cavity, a shallow cavity formed in the scapula where the humerus articulates with the pectoral girdle. The glenohumeral joint, or shoulder joint, is formed when the scapula at the glenoid cavity articulates with the humerus of the upper limb. It resembles a socket or cuplike opening in which the head of the humerus fits. The edges of the glenoid cavity serve as attachment sites for shoulder and arm muscles. The posterior side of the scapula has a prominent ridge that runs almost horizontally from the acromion to the medial border.
Anatomy of the Clavicle
The clavicle, or collarbone, is an S-shaped long bone that runs horizontally between the shoulder blade (scapula) and the sternum (breastbone). The clavicle has a round end and a flattened end. The rounded end is called the medial or sternal end, which is the portion of the clavicle that articulates with the top region of the sternum (manubrium). This articulation is referred to as the sternoclavicular joint. The lateral or acromial end is the broad, flat side of the clavicle that articulates with the acromial process of the scapula to form a shoulder joint. The surface of the clavicle is smooth on the superior (upper) surface while the inferior (lower) surface has grooves and ridges. These bone markings serve as attachment sites for muscles and ligaments.
The entire length of the clavicle can be palpated, or felt, at the base of the neck. Besides connecting the upper arms to the body, it keeps the shoulders in place by serving as an anchor that supports the scapula. In doing so, the shoulder joint is placed in a specific position that ensures the upper limb is able to exhibit a wide range of motion. When the clavicle is fractured, the shoulder folds forward and inward. The clavicle is one of the most commonly fractured bones in the body. Clavicle fragility is age dependent. Babies have very fragile clavicles because the paired bones are brittle and at greater risk of being broken. The same applies to young children, as the bone remains slender in size. Adults are less susceptible to a broken clavicle because it becomes thicker with age.