The shoulder is one of the most complex joints in the body and also one of the most easily injured. It is a ball-and-socket joint, much like the hip, and allows for the greatest range of motion of all the joints in the human body. The shoulder is comprised of three major bones including: the humerus (arm bone); scapula (shoulder blade); and the clavicle (collarbone). The top of the humerus is the ball of this ball-and-socket joint. It is covered in a layer of cartilage that cushions its movement within the glenoid cavity, the socket of the joint.
The clavicle attaches the shoulder to the rib cage and secures its position away from the body. The scapula is the larger, triangular bone on the posterior side of the joint.
These bones are held together by an assortment of muscles, tendons and ligaments called the rotator cuff. These muscles and tendons connect the bones as well as hold the ball of the upper arm securely in the shoulder socket.