Clavicle (Collarbone) : Structure, Attachments, Side Determination, Sex Determination, Functions & Clinical Anatomy
Overview
- The clavicle is a long bone that serves as a strut between the shoulder blade and the sternum or breastbone.
- There are two clavicles, one on the left and one on the right.
Structure
The bone has a shaft (cylindrical part), and two ends, lateral and medial.
Shaft
The shaft is divisible into the lateral one-third and the medial two-thirds.
- The lateral one-third of the shaft is flattened fromabove downwards. It has two borders, anterior and
posterior. The anterior border is concave forwards. The posterior border is convex backwards. This part of the bone has two surfaces, superior and inferior. The superior surface is subcutaneous and the inferior surface presents an elevation called the conoid tubercle and a ridge called the trapezoid ridge. - The medial two-thirds of the shaft is rounded and is said to have four surfaces. The anterior surface is
convex forwards. The posterior surface is smooth. The superior surface is rough in its medial part. The inferior surface has a rough oval impression at the medial end. The lateral half of this surface has a longitudinal subclavian groove. The nutrient foramen lies at the lateral end of the groove.
Ends (Lateral & Medial)
- The lateral or acromial end is flattened from above downwards. It bears a facet that articulates with the acromion process of the scapula to form the acromioclavicular joint.
- The medial or sternal end is quadrangular and articulates with the clavicular notch of the manu-brium sterni to form the sternoclavicular joint. The articular surface extends to the inferior aspect, for articulation with the first costal cartilage.
Side Determination
The side to which a clavicle belongs can be determined from the following characters:
- 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 two-thirds, and concave forwards in its lateral one-third.
- The inferior surface is grooved longitudinally in its middle one-third.
Peculiarities of the Clavicle
- It is the only long bone that lies horizontally.
- It is subcutaneous throughout.
- It is the first bone to start ossifying.
- It is the only long bone which ossifies in membrane.
- It is the only long bone which has two primary centres of ossification.
- It is generally said to have no medullary cavity, but this is not always true.
- It is occasionally pierced by the middle supraclavicular nerve.
Sex Determination
- In females, the clavicle is shorter, lighter, thinner, smoother, and less curved than in males.
- The midshaft circumference and the weight of the clavicle are reliable criteria for sexing the clavicle.
- In females, the lateral end of the clavicle is a little below the medial end; in males, the lateral end is
either at the same level or slightly higher than the medial end.
Attachments on Clavicle
At the lateral end the margin of the articular surface for the acromioclavicular joint gives attachment to the joint capsule.
At the medial end the margin of the articular surface for the sternum gives attachment to:
- the fibrous capsule all round;
- the articular disc posterosuperiorly; and
- the interclavicular ligament superiorly.
Lateral one-third of shaft
- The anterior border gives origin to the deltoid,
- The posterior border provides insertion to the trapezius,
- The conoid tubercle and trapezoid ridge give attachment to the conoid and trapezoid parts of the coracoclavicular ligament.
Medial two-thirds of the shaft
- The anterior surface gives origin to the pectoralis major,
- The rough superior surface gives origin to the clavicular head of the stemocleidomastoid,
- The oval impression on the inferior surface at the medial end gives attachment to the costoclavicular
ligament, - The subclavian groove gives insertion to the subclavius muscle. The margins of the groove give attachment to the clavipectoral fascia.
The nutrient foramen transmits a branch of the suprascapular artery.
Ossification
The clavicle is the first bone in the body to ossify. Except for its medial end, it ossifies in membrane. It ossifies from two primary centres and one secondary centre.
- The two primary centres appear in the shaft between the fifth and sixth weeks of intrauterine life, and fuse about the 45th day.
- The secondary centre for the medial end appears during 15-17 years, and fuses with the shaft during 21-22 years. Occasionally there may be a secondary centre for the acromial end.
Functions
- It supports the shoulder so that the arm can swing clearly away from the trunk.
- The clavicle transmits the weight of the limb to the sternum.
Clinical Anatomy
- Collarbone fracture : The clavicle is commonly fractured by falling on the outstretched hand (indirect violence). The most common site of fracture is the junction between the two curvatures of the bone, which is the weakest point.
- Acromioclavicular dislocation (“AC Separation”) : The lateral fragment is displaced downwards by the weight of the limb.
- Degeneration of the clavicle
- Osteolysis
- Sternoclavicular dislocations
- Cleidocranial dysostosis : The clavicles may be congenitally absent, or imperfectly developed in a disease called cleidocranial dysostosis. In this condition, the shoulders droop, and can be approximated anteriorly in front of the chest.
