Shoulder crane: a concept of suspension, stability, control and motion

Framework and suspensory cascade This novel model uses the structure and workings of the industrial crane as a simile to explain the function of the human shoulder. As a crane consists of a base, axial tower, boom and suspensory cascade that move and position loads in space, the base consists of the pelvic platform, with outriggers (legs) that provide stability in human body. The axial tower consists of an articulated spinal column and thoracic platform, which are stabilised by the core muscles. The clavicular boom articulates with the anterior thoracic platform and is elevated by the trapezius from the posterior tower. The ‘suspensory cascade' extends from the skull and cervical spine to the trapezius and on to the clavicle, coracoclavicular ligaments, coracoid process, coracohumeral ligament and humeral head. Motion The rotator cuff muscles take origin from the scapula and coalesce with each other to form a multilayered rotator cuff tendon and cable, which cups to closely contain the humeral head. The four muscles insert into the common tendon and together share the load to stabilise and mobilise the arm in space. The coracoid is a pulley that allows the scapula to swivel on the coracoclavicular ligaments to enable adjustment of the angle of force transmission delivered by the rotator cuff to the humeral head. Stability and control The inferior glenoid and labrum are a fixed organ of compression, which coalesces with the hammock formed by the static inferior glenohumeral ligaments. The rotator cuff and deltoid compress the humeral head onto this static structure. The biceps tendon passes adjacent to the condensations of the coracohumeral ligament to insert into the mobile superior labrum and glenoid. Contraction of the biceps pulls the mobile superior labrum onto the humeral head and tightens the glenohumeral ligaments that wrap around the humeral head at the extremes of motion. The coracohumeral ligament is a sensory organ that interfaces with these structures and is well positioned to work as a servomechanism to redirect the rotator cuff in providing stability, control and motion. Level of evidence Level V .