The shoulder is made up of three bones: the scapula (shoulder blade), clavicle (collarbone) and humerus (upper arm bone). Two joints in the shoulder allow it to move: the acromioclavicular joint, where the highest point of the scapula (acromion) meets the clavicle, and the glenohumeral joint.

The glenohumeral joint is the one most people think of as the shoulder joint. It is formed where a ball (head) at the top of the humerus fits into a shallow cuplike socket (glenoid) in the scapula, allowing a wide range of movement. The surfaces of the bones where the ball and socket meet are covered with smooth, elastic cartilage that absorbs shock and allows the joint to move easily.

Because the socket of the shoulder is shallow, the joint relies on surrounding soft tissues to support it and hold the components in place. Many of these soft tissues surround the joint to form a protective capsule, which is lined with a thin membrane called the synovium. The synovium produces a fluid (synovial fluid) to cushion and lubricate the joint.

Other key structures of the shoulder include:

  • labrum, a fibrous ring of cartilage that surrounds the glenoid, or shoulder socket, to create a deeper socket for the ball to stabilize the joint
  • rotator cuff, a network of muscles and tendons that cover the top of the humerus, or upper arm bone, to hold it place and enable the arm to rotate 
  • deltoid, the largest and strongest muscle of the shoulder, the deltoid muscle provides the strength to lift the arm.
  • biceps tendon, originating at the top of the shoulder socket, this tendon attaches to the biceps muscle, which allows the elbow to bend and the forearm to rotate.