The biomechanics of the joints of the shoulder are as follows:

Sterno-clavicular joint

The clavicle acts as a strut, keeping the upper limb away from the chest. The sterno-clavicular joint relies on powerful ligaments to prevent displacement which is therefore relatively unusual.

The image shows the sterno-clavicular joint with the patient’s right side injured.

Acromio-clavicular joint

The clavicle has a complex relationship with the scapula. This allows scapular rotation in full abduction, while assisting in maintaining the position of the upper limb. It achieves this by the strong conoid and trapezoid ligaments which anchor the clavicle to the coracoid process of the scapula.

Learning bite

The upper limb is effectively suspended from the clavicle.

Scapulo-thoracic joint

In full abduction the gleno-humeral joint can only achieve around 90o of abduction at which point the scapula rotates on the chest wall to achieve the remainder of the arc.

Learning bite

The articulation of the scapula on the chest wall is the means by which full abduction is achieved.