The clavicle elongated, S-shaped bone that lies horizontally between the sternum and the acromial end of the scapula. It is an important connection between the axial skeleton and the pectoral girdle.
The pectoral girdle or the shoulder girdle is the term used for bones in the appendicular skeleton which connects to the arm on each side. The clavicle and scapula constitute the pectoral girdle in humans.
Clavicle is a slender bone, wider medially at its sternal articulation and noticeably thinner at its lateral third. The clavicle assumes a gentle S-shape, the medial end convex forward and the lateral end concave forward.
The shape resembles the musical symbol clavicula. It is also known as the collarbone.
The female collarbone is shorter, lighter, thinner, smoother, and less curved than in males. The lateral end is a little below the medial end in females whereas, in males, the lateral end is either at the same level or slightly higher than the medial end.
Structure of Clavicle
The bone has following parts
- Medial end
- Lateral ends
- A cylindrical shaft in between
The lateral end is also called acromial end and is flat from above downwards. It articulates with the acromion process of the scapula through a facet. The articular surface for the acromioclavicular joint gives attachment to the joint capsule.
The medial end is called sternal end, is quadrangular in shape. This part articulates with the clavicular notch of the manubrium sterni to form the sternoclavicular joint. It also articulates with first costal cartilage via an extension of the articular surface.
The shaft of the collarbone can be divided into the lateral one third and the medial two thirds.
- The lateral one-third
- Flattened from above downwards.
- It has a concave anterior border and convex posterior border and two surfaces- the superior and inferior.
- The inferior surface has elevation called the conoid tubercle and a ridge called the trapezoid ridge.
- The medial two-thirds
- Rounded and has four surfaces- Convex anterior, thick posterior, superior and inferior which harbors a longitudinal groove called subclavian groove in its lateral half.
Determination of Side of Collarbone
- The lateral end is flat, and the medial end is large and quadrilateral.
- The shaft is slightly curved, so that it is convex forwards in its medial 2/3, and concave forwards in its lateral 1/3.
- The inferior surface is grooved longitudinally in its middle 1/3.
Attachments on Collarbone
The bulbous medial clavicular end contributes to forming the sternoclavicular joint. Several ligaments support this joint.
Capsular ligaments are thickenings of the sternoclavicular joint capsule are referred to as the capsular ligaments on anterosuperior and posterior aspects of the capsule. These are responsible for limiting superior displacement of the medial clavicular or, through the clavicular moment arm, inferior displacement of the lateral end of an intact clavicle.
The posterior part of the capsule resists both anterior and posterior translation at the sternoclavicular joint.
This ligaments has strong bands that span the medial clavicle to the superior sternum to the contralateral clavicle. The ligament loosens with shoulder elevation and prevents downward displacement of the lateral end of the clavicle.
These strong ligaments run from the upper aspect of the first rib and adjacent aspects of the sternum to the inferior clavicle. The costoclavicular ligament stabilizes the medial clavicle against both upward and downward rotation, respectively.
From the base of the coracoid process of the scapula to the inferior aspect of the lateral clavicle are two ligaments, called trapezoid and conoid ligaments. Trapezoid ligament is lateral and attaches on a specific osseous ridge, whereas the more medial conoid inserts at the conoid tubercle. These ligaments serve the important function of suspension of the shoulder girdle from the clavicle.
The capsule of the acromioclavicular joint forms the acromioclavicular ligaments. Posterosuperiorly, the ligament serves to resist anteroposterior displacement of the distal clavicle.
- Medially, the collarbone serves as an origin of the pectoralis major and sternohyoid muscles.
- Superomedial part serves as an origin of the sternocleidomastoid.
- In a midshaft collarbone fracture, the sternocleidomastoid becomes a medial clavicle elevator.
- The undersurface of the middle collarbone serves as an insertion point for the subclavius muscle.
- Laterally, the anterior clavicle is the site of origin for the anterior deltoid with the posterosuperior clavicle serving as an accessory insertion for the trapezius.
Functions of Collarbone
The collarbone is responsible for bracing the shoulder girdle and propping it away from the sternum and thoracic cage. This allows the shoulder to reach into cross-body and internal rotation positions without medial collapse. The clavicle increases the strength of shoulder girdle movements
The shoulder girdle is stabilized against inferior displacement by two mechanisms, one dynamic and one static. Posteriorly, the trapezius acts as a dynamic scapular elevator. Anteriorly, the shoulder girdle hangs from the clavicle by the coracoclavicular ligaments, similar to a sign hanging from a signpost.