Clavicle bone - (General Features & Peculiarities of the Clavicle)
The clavicle is a long bone. 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. The bone has a cylindrical part called the shaft, and two ends, lateral and medial.
The shaft is divisible into the lateral one-third and the medial two-thirds.
The lateral one-third of the shaft is flattened from above 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 are 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.
Lateral and Medial Ends:
1. 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.
2. The medial or sterna end is quadrangular and articulates with the clavicular notch of the manubrium sterna to form the sternoclavicular joint. The particular surface extends to the inferior aspect, for articulation with the first costal cartilage.
The side to which a clavicle belongs can be determined from the following characters.
1. The lateral end is flat, and the medial end is large and quadrilateral.
2. The shaft is slightly curved, so that it is convex forwards in its medial two-thirds, and concave forwards in its lateral one-third.
3. The inferior surface is grooved longitudinally in its middle one-third.
Peculiarities of the Clavicle :
1. It is the only long bone that lies horizontally.
2. It is subcutaneous throughout.
3. It is the first bone to start ossifying.
4. It is the only long bone which ossifies in membrane.
5. It is the only long bone which has two primary centres of ossification.
6. It is generally said to have no medullary cavity, but this is not always true.
7. It is occasionally pierced by the middle supraclavicular nerve. It receives weight of upper limb via lateral one- third through coracoclavicular ligament and transmits weight of upper limb to the axial skeletal via medial two-thirds part.
Sex Determination :
1. In females, the clavicle is shorter, lighter, thinner, smoother and less curved than in males.
2. The midshaft circumference and the weight of the clavicle are reliable criteria for sex determination of the clavicle.
3. 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.
Particular Features :
1. At the lateral end, the margin of the articular surface for its acromioclavicular joint gives attachment to the joint capsule.
2. At the medial end, the margin of the articular surface for the sternum gives attachment to:
(a) Fibrous capsule of sternoclavicular joint all around.
(b) Articular disc posterosuperiorly.
(c) Interclavicular ligament superiorly.
3. Lateral one-third of shaft
(a) The anterior border gives origin to the deltoid.
(b) The posterior border provides insertion to the trapezius.
(c) The conoid tubercle and trapezoid ridge give attachment to the conoid and trapezoid parts of the coracoclavicular ligament.
4. Medial two-thirds of the shaft
(a) Most of the anterior surface gives origin to the pectoralis major.
(b) Half of the rough superior surface gives origin to the clavicular head of the sternocleido-mastoid.
(c) The oval impression on the inferior surface at the medial end gives attachment to the costoclavicular ligament.
(d) The subclavian groove gives insertion to the subclavius muscle. The margins of the groove give attachment to the clavipectoral fascia.
(e) The posterior surface close to medial end gives origin to sternohyoid muscle. The subclavian artery and cords of brachial plexus pass behind the inferior surface of the clavicle.
The nutrient foramen transmits a branch of the suprascapular artery.
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.