The shoulder is a ball and socket joint with the greatest range of motion compared to any other joint in the body. The humeral head is the ball of the ball and socket joint, where the glenoid is the the socket. The socket is extremely small and shallow compared to the humeral head. This mismatch is essential to allow the amount and types of motion seen in this joint.
The scapulothoracic joint refers to the joint involving the shoulder blade, or scapula, and rib cage. This joint is extremely important in shoulder motion and functions. Elevation of the arm through the shoulder also involves rotation of shoulder blade in a specific ratio and pattern.
The labrum is a elastic cartilage structure the surrounds the shallow socket, effectively deepening the socket to help with stability of the joint. This structure is commonly torn in shoulder dislocations.
Ligaments and Capsule
The joint capsule and ligaments are structures that connect the ball to the socket. The different ligaments work in intricate patterns tightening and relaxing based on positions of the shoulders.
The rotator cuff is a group of 4 muscles that originate on the shoulder blade and become thick tendons as the attach to the lateral aspect of the humeral head. The muscles are important in raising the arm above the shoulder, but their true function is to pull the head into the socket. This action stabilizes the shoulder and allows the other large muscles about the shoulder to act on the arm to raise it.