Movement of the pectoral girdle as a whole

Movements of the pectoral girdle serve to increase the range of movement of the shoulder joint, principally by changing the relative position of the glenoid fossa with respect to the chest wall. In all of these movements the clavicle, acting as a strut, holds the shoulder away from the trunk thereby securing greater freedom of movement of the upper limb. It should be remembered that movements of the pectoral girdle accompany virtually all movements of the shoulder joint.

In movements of the pectoral girdle, the glenoid fossa travels in an arc of a circle whose radius is the clavicle, while the medial border of the scapula, held against the chest wall, travels in a curve of shorter radius. Consequently, the relative positions of clavicle and shoulder blade must be capable of changing. This occurs at the acromioclavicular joint. It now becomes obvious that a rigid union between clavicle and shoulder blade would severely limit the mobility of the upper limb.

Lateral movement of the shoulder blade around the chest wall brings it to lie more in a sagittal plane so that the glenoid fossa faces more directly forwards(a,b). Medial movement towards the vertebral column brings the shoulder blade to lie more in the frontal plane, with the glenoid fossa facing more directly laterally. These two extreme positions of the scapula form a solid angle of 40° to 45°(a). Furthermore, the angle between the clavicle and shoulder blade decreases to approximately 60° on full lateral movement of the shoulder blade, and increases to approximately 70° on full medial movement(a). The total range of linear translation of the shoulder blade around the chest wall is about 15cm.

Elevation and depression of the shoulder blade has a linear range of some 10 to 12cm(c). However, it is usually accompanied by some degree of rotation, so that in elevation the glenoid fossa comes to face increasingly  upwards. In depression, the fossa points increasingly downwards. The rotation of the shoulder blade that occurs with respect to the chest wall does so about an axis perpendicular to the plane of the scapula, and is situated a little below the spine close to the superolateral angle. The total range of angular rotation of the scapula amounts to 60°(d). This involves a displacement of the inferior angle of the scapula of 10 to 12cm, and that of the superolateral angle of 5 to 6cm.

During abduction or flexion of the arm, the clavicle rotates about its long axis so that its anterior surface is increasingly directed upwards. Towards the end of the range of rotation of the scapula against the clavicle, the coracoclavicular ligament becomes taut and transmits the rotating force to the clavicle, whose rotation then accounts for the scapular rotation on the chest wall. Any impairment of the clavicle to rotate at either the sternoclavicular or the acromioclavicular joints will interfere with the free movement of the shoulder blade and upper limb as a whole.

Although movements of medial and lateral translation of the scapula, elevation and depression, and its rotation with respect to the chest wall have been discussed, it is important to remember that they do not occur as pure movements. All movements of the pectoral girdle will involve some degree of each of the above pure movements.

Biomechanics

Stresses on the clavicle

The clavicle is subjected to both compression and tension stresses, which under normal conditions are absorbed within it. They only become apparent when the integrity of the pectoral girdle is compromised, as by fracture, dislocation or muscular imbalance. Unlike the pelvic girdle, because the pectoral girdle is not a complete bony ring, the intrinsic stresses require the cooperation of muscles attached to it for there to be equilibrium.

A compressive stress along the length of the clavicle  directed towards the sternoclavicular joint is produced by the action of trapezius and pectoralisminor as they pull the clavicle towards the sternum. Forces transmitted medially from the upper limb to the glenoid fossa are transmitted from the scapula to the clavicle by trapezoid ligament, and from the clavicle to the first rib by the costoclavicular ligament. Consequently, falling on an outstretched hand or elbow puts no strain on either end of the clavicle at the joints. If the clavicle fractures as a result, it does so between these two ligaments. In such fractures, the two fragments tend to over-ride one another. These compressive stresses are increased by lying on one shoulder.

Tension stresses within the clavicle are produced, under the action of deltoid, when the upper limb is abducted. Hanging and swinging forwards by the arms increases these tension stresses. If sufficiently large they may lead to some discontinuity of the pectoral girdle; however,  a joint dislocation is much more likely to occur than a fracture.

A rotational force transmitted through the scapula to the clavicle tends to damage the acromioclavicular and/or sternoclavicular joints and their associated ligaments rather than causing a fracture of the bone. Downward forces applied to the lateral end of the clavicle create bending stresses within it, which, if sufficiently large or if the clavicle comes into contact with the coracoid process, may lead to fracture of the bone.