Anatomy and Biomechanic of the Shoulder

The shoulder is one of the most complex joints of the human body. Consequently, they are susceptible to injury and degeneration. Mechanical shoulder pathology typically results when overuse, extremes of motion, or excessive forces overwhelm intrinsic material properties of the shoulder complex resulting in tears of the rotator cuff, capsule, and labrum. The fundamental central component of the shoulder complex is the glenohumeral joint. It has a ball-and-socket configuration with a surface area ratio of the humeral head to glenoid fossa of about 3:1 with an appearance similar to a golf ball on a tee. Overall, there is minimal bony covering and limited contact areas that allow extensive translational and rotational ability in all three planes. The glenohumeral joint has 2 groups of stabilizers, which are static (passive) and dynamic (active) restrains. Static stabilizers include the concavity of the glenoid fossa, glenoid fossa retroversion and superior angulation, glenoid labrum, the joint capsule, and glenohumeral ligaments, and a vacuum effect from negative intra-articular pressure. Dynamic stabilization is merely the coordinated contraction of the rotator cuff muscles that create forces that compress the articular surfaces of the humeral head into the concave surface of the glenoid fossa. During upper extremity movement, the effects of static stabilizers are minimized and dynamic or active stabilizers become the dominant forces responsible for glenohumeral stability The simple act of arm elevation is a complex task that occurs via the combination of glenohumeral and scapulothoracic motion, together known as scapulohumeral rhythm. In the first 120(0), glenohumeral arm abduction, the supraspinatus and deltoid work together and create a force couple that promotes stability, while raising the arm (deltoid contraction). In addition, the humerus must undergo 45(0) external rotation to not only clear the greater tuberosity posteriorly but also loosen the inferior glenohumeral ligament (IGHL) to allow maximum elevation. There are several anatomical updates regarding the rotator cuff and capsular footprint. The footprint of the supraspinatus on the greater tuberosity is much smaller than previously believed, and this area of the greater tuberosity is actually occupied by a substantial amount of the infraspinatus. The superior-most insertion of the subscapularis tendon extends a thin tendinous slip, which attaches to the fovea capitis of the humerus. The teres minor muscle inserts to the lowest impression of the greater tuberosity of the humerus and additionally inserts to the posterior side of the surgical neck of the humerus.