The functional shoulder

The shoulder girdle has evolved from weight bearing to being able to elevate and perform independent tasks. This chapter combines the evolutionary, anatomic and functional aspects from this book and presents a functional concept of the shoulder. Mechanically, the shoulder is a crane that elevates the arm. The base and articulated spinal tower provide a platform for the thorax, on which is mounted the shoulder girdle. The core muscles provide the primary stability of the spine and thorax. The boom of the crane is the clavicle, which hinges on the sterno-clavicular joint and is elevated by the trapezius. From the lateral clavicle, there is a "suspensory cascade", which extends to the humeral head. A pulley is a wheel designed to support movement and change its direction. The scapula is a pulley that swivels on the coraco-clavicular ligaments to change the rotator cuff alignment and optimise shoulder stability and function. The scapula is stabilised and mobilised by a triangle: (a) the thoracic cage, (b) the hinged but static clavicle and (c) peri-scapular muscles control the scapula as it traverses the "scapula track" on the thoracic cage. The rotator cuff tendons coalesce with the coraco-humeral ligament and "rotator cuff cable", to form a common tendon, which provides dynamic stability and allows humeral rotation in any position of shoulder elevation. The humeral head is suspended into the glenoid cavity by the coraco-humeral ligament, like a "ball on a string". The superior labrum is mobile and off the glenoid face. It works in tension with the SGHL and biceps tendon. The inferior labrum is on the glenoid face and works in compression. Evolution has extended shoulder function, which has led to further evolution of the entire upper limb.