The shoulder is unlike any other joint in the body. No other joint can match the shoulder’s near-universal range of motion. But, unlike the hip, for example, which is a sturdy ball-and-socket joint and very stable, the shoulder joint is quite shallow – more like a golf ball on a tee.
Because of its structure, the shoulder relies on the surrounding soft tissues to maintain its stability, including the rotator cuff, the biceps tendon and the capsular ligaments, which are thickenings of the capsule that encloses the joint cavity. Inside the joint cavity, the labrum, a triangular shaped fibrocartilaginous structure, acts as a static stabilizer, holding the shoulder in place, while small fluid-filled sacs called bursa reduce the friction between tissue and bone.
The two main bones of the joint itself are the humerus (the upper arm bone) and the scapula (shoulder blade). Articular cartilage cushions the head of the humerus and the face of the glenoid (from the Greek, for form or shape), which is the cavity on the scapula that receives the head of the humerus. The scapula extends up and over the shoulder joint at the back to form a kind of bony overhang called the acromion at the front, the scapula extends over the joint in a bony projection called the coracoid process (so called because it resembles the shape of a crow’s beak).
The rotator cuff joins the scapula and a bony projection on the head of the humerus called the greater tuberosity. It consists of four muscles and their tendons, which combine to form a “cuff,” or sheath, over the upper end of the humerus. The cuff helps to lift and rotate the arm and keep the humerus from popping out, making it essential to shoulder stability.