Clavicle is a thinbone, wide medially at its sternal articulation and thinner at its lateral third. The clavicle assumes a gentle S-shape, the medial end convex forward and the lateral end concave forward. This shape has been likened to the musical symbol the clavicula, hence the name.

In the axial projection, the clavicle is also noted to have both medial and lateral flat expanses, linked by a thin, tubular middle. This central transitional area represents a weak link in the clavicular structure.

The midclavicle, therefore, is the most common site of fracture.

Normal Biomechanics of Clavicle

The clavicle 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 suffering medial collapse.

The clavicle increases the strength of shoulder girdle movements . Also, this strut function affords cosmesis and posture to the shoulder girdle.

In a displaced midshaft fracture, clavicular shortening may result, and the strut function of the clavicle is lost.

The shoulder girdle is stabilized against inferior displacement by two forces, 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 and the sturdy sternoclavicular ligaments that allows the clavicle to remain in position to withstand this downward load

Fracture Biomechanics

Midshaft fracture, the displacing forces are

• Stabilization of the medial segment by the sternoclavicular ligaments,
• Superior pull  on the medial segment through the sternocleidomastoid,
• Inferior and medial pull on the lateral segment through the pectoralis major and latissimus,
• Inferior pull on the lateral segment through the weight of the arm pulling through the coracoclavicular ligaments.
• The trapezius provides a stabilizing force against inferior displacement of the lateral segment.

For more lateral fractures, the displacing forces are

• Superior pull on the medial segment through the sternocleidomastoid and trapezius
• Medial pull on the distal segment through the pull of the pectoralis major, pectoralis minor, and latissimus dorsi;
• Inferior on the distal segment through the weight of the arm.

Mode of Injury

Unlike the conventional belief, the research has shown that direct injury to the shoulder is responsible for clavicular fractures rather than fall on outstrectched hand.

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Related posts:

1. Fractures of The Clavicle-Part II
2. Pseudarthrosis Of The Clavicle
3. Scapular Fractures
4. Acromioclavicular Joint Dislocation or Separated Shoulder
5. Proximal Humerus Fractures – Displaced Fractures of Tuberosity