The shoulder girdle is also called the pectoral girdle, and it is a bone ring, incomplete posteriorly, formed by two sets of bones, the scapulae, posteriorly, and the clavicles anteriorly and completed anteriorly by the manubrium of the sternum (part of the axial skeleton). Those bones are part of the appendicular skeleton- consisting bones of the limbs (upper and lower) and most of the bones forming the girdles (pectoral and pelvic).
The pectoral girdle connects the upper limb to the axial skeleton on the left and right side of the body. There is an anatomical connection between the pectoral girdle and the axial skeleton; that is, the sternoclavicular joints anteriorly .The pectoral girdle also consists of two other anatomical joints called the acromioclavicular and glenohumeral joints, both lying laterally. Posteriorly, there is a physiological joint – (scapulothoracic joint) or muscular connection between the shoulder girdle and the axial skeleton, which is formed by muscles including the trapezius, rhomboids and serratus anterior. This scapulothoracic joint is highly mobile and allows great movements of the pectoral girdle.
Note that the clavicle & scapula makes up the shoulder girdle, while the left & right pelvic bones (hip bones) as well as the sacrum and coccyx form the pelvic girdle. The pectoral girdle and bones of the free part of the upper limb form the superior appendicular skeleton, while the pelvic girdle and bones of the free part of the lower limb form the inferior appendicular skeleton.
The pectoral girdle is very mobile. However, the following movements of the girdle are mainly permitted by the physiological joint between the pectoral girdle and the rib cage, the scapulothoracic joint. The movements available for the pectoral girdle or the scapula in particular includes:
- Protraction: This is moving the shoulder blade (scapula) forwards, away from the rib cage and spine.
- Retraction: This movement pulls the scapula back towards the rib cage.
- Elevation: This movement allow the shoulder girdle to move upwards as in shrugging the shoulders.
- Depression: This brings a reverse of the elevation movement. The pectoral girdle and entire shoulder downwards.
- Downward rotation: This is rotating the lower scapula towards the rib cage as in moving the arm behind the back.
The shoulder girdle is formed by two pairs of bones, the scapulae and the clavicles.
The clavicle or collar bone is an S-shaped long bone lying superficial and is palpable along its entire length (figure 3). It connects the upper limb to the trunk.
The shaft (body) of the clavicle has a double curve in a horizontal plane. Its medial half is convex anteriorly and its sternal end is enlarged and triangular where it articulates with the manubrium of the sternum at the sternoclavicular joint. The medial two thirds of the shaft of clavicle are convex anteriorly, whereas the lateral two third is flattened and concave anteriorly. These curvatures increase the resilience of the clavicle and give it the appearance of an elongated capital S.
The superior surface of the clavicle, lying just deep to the skin and platysma muscle in the subcutaneous tissue is smooth. The inferior surface of the clavicle is rough and attachment to the 1st rib near its sternal end and suspends the scapula from its acromial end. The conoid tubercle near the acromial end of the clavicle, gives attachment to the conoid ligament, the medial part of the coracoclavicular ligament by which the remainder of the upper limb is passively suspended from the clavicle.
Other surface markings of the clavicle include the trapezoid line, which gives the trapezoid ligament attachment, the subclavian groove in the medial third of the shaft, which is a site of attachment of the subclavius muscle. More medially is the impression for the costoclavicular ligament, a tough, often depressed oval area that gives attachment to the ligament binding the 1st rib to the clavicle, limiting elevation of the shoulder.
The shoulder blade or scapula is a triangular flat bone (figure 4 below) that lies on the posterolateral aspect of the thorax, overlying the 2nd – 6th or 7th ribs.
The convex posterior surface of the scapula is unevenly divided by a thick projecting ridge of bone, the spine of the scapula, into a small supraspinous fossa and a much larger infraspinous fossa. The concave costal surface (ventral surface) of most of the scapula forms a large subscapular fossa.
The broad bony surfaces of the three fossa provide attachments for fleshy muscles. The triangular body of the scapular is thin and translucent superior and inferior to the scapula spine; although its borders especially the lateral border are somewhat thicker. The spine continues laterally as the flat expanded acromion which forms the subcutaneous point of the shoulder and articulates with the acromial end of the clavicle.
The spine and acromion serve as levers for the attached muscles particularly the trapezius muscle. The glenohumeral (shoulder) joint is almost directly inferior to the acromioclavicular joint, thus the scapula mass is balanced with that of the free limb, and the suspending structure (coracoclavicular ligament) lies between the two masses.
Superolaterally, the lateral surface of the scapula has a glenoid cavity (Greek word meaning “Socket”), which receives and articulates with the head of the humerus at the glenohumeral joint. The glenoid cavity is a shallow, concave, oval fossa directed anterolaterally and slightly superiorly, that is considerably smaller than the ball (head of the humerus) for which it serves as socket. The beak-like coracoid process is superior to the glenoid cavity and projects anterolaterally.
The scapula has medial, lateral and superior borders and superior, lateral and inferior angles. As indicated, the medial border is often called the vertebral border because that border runs parallel to and approximately 5cm lateral to the spinous processes of the thoracic vertebrae, when the scapula is in the anatomical position, the lateral border runs superolaterally toward the apex of the axilla; hence it is also called the axillary border.
The lateral border terminates in the truncated lateral angle of the scapula, the thickest part of the bone that bears the broadened head of the scapula - with the glenoid cavity being its primary feature. The shallow constriction between the head and the body defines the neck of the scapula. The superior border of the scapula is marked near the junction of its medial two-thirds and lateral third by the suprascapular notch, which is located where the superior border joins the base of the coracoid process. The superior border is the thinnest and shortest of the three borders.
As highlighted in the introduction, the true joints or anatomical joints of the shoulder girdle are the sternoclavicular joint, acromioclavicular joint and the glenohumeral (shoulder) joint.
The acromioclavicular joint (AC joint) is a plane type of synovial joint. It is located 2 to 3 cm from the point of the shoulder formed by the lateral part of the acromion. The AC joint is strengthened by several structures which also ensures its stability. Some of these structures include the acromioclavicular ligament, the sleevelike loose fibrous capsule of the joint, the nearly horizontal trapezoid ligament, the vertical conoid ligament as well as the coracoclavicular ligament. The AC joint is supplied by the suprascapular and thoracoacromial arteries and it is innervated by the suprascapular, lateral pectoral, and axillary nerves.
Sternoclavicular Joint (SC Joint)
This is a saddle type of synovial joint but functions as a “ball and socket joint”. The SC joint is divided into two compartments by an articular disc. The disc is firmly attached to the anterior and posterior sternoclavicular ligaments, thickenings of the fibrous layer of the joint capsule, as well as the interclavicular ligament. The great strength of the SC joint is a consequence of these attachments. Thus, although the articular disc serves as a shock absorber of forces transmitted along the clavicle from the upper limb, dislocation of the clavicle is rare, whereas fracture of the clavicle is common. The SC joint is the only articulation between the upper limb and the axial skeleton, and it can be readily palpated because the sternal end of the clavicle lies superior to the manubrium of the sternum.
The SC joint is made stable and strengthened by the several shoulder muscles, the joint capsule and ligaments including the anterior and posterior sternoclavicular ligaments which reinforce the joint capsule anteriorly and posteriorly, the interclavicular ligament and the costoclavicular ligament. Although the SC joint is extremely strong, it is significantly mobile allowing anterior and posterior movements of the shoulder girdle, elevation and depression as well as a special form of circumduction which is performed by moving the acromial end along a circular path.
The SC joint is nourished by the internal thoracic and the suprascapular arteries; and innervated by the branches of the medial suprascapular nerve and the nerve to the subclavius muscle.
The glenohumeral joint or shoulder joint is a ball and socket type of synovial joint that permits a wide range of movements including flexion, extension, abduction, adduction, rotation (medial and lateral rotation), and circumduction. However, its mobility makes the joint relatively unstable.
The glenohumeral joint is supported by its loose fibrous joint capsule, transverse humeral ligament, coracoacromial ligament and glenohumeral ligament. The internal surface of the joint capsule is well lined by synovial membrane and the joint is also rich in bursae (sac-like cavities) which contain capillary films of synovial fluid secreted by the synovial membrane. Those bursae include the subscapular bursa and the subacromial bursa. The glenohumeral joint is supplied by the anterior and posterior circumflex humeral arteries as well as branches of the suprascapular artery. The nerve supply include the axillary, suprascapular, and the lateral pectoral nerves.