The shoulder is a complex combination of bones and joints where many muscles act to provide the widest range of motion of any part of the body. Numerous muscles help stabilize the three joints of the shoulder while giving it motion.
At the shoulder, three major bones meet and create a 90-degree angle:
- Clavicle: Also known as the collarbone, the clavicle extends across the front of the shoulder from the sternum to the scapula. It helps stabilize the shoulder’s movements.
- Scapula: More commonly known as the shoulder blade, the scapula is a flat triangular bone located in the upper back. It connects with the collarbone at the front of the body.
- Humerus: The largest bone of the arm, the humerus connects to the scapula and clavicle in the shoulder. The head of the bone has a ball-like knob to create a ball-and-socket joint with the scapula.
The junctions of these three bones and the sternum (chest bone) form three joints:
- Glenohumeral joint: This shallow ball-and-socket-style joint created by the humerus and scapula allows the arm to rotate circularly and to move up and out from the body. It is surrounded by soft tissue and strengthened by fibrous ligaments.
- Acromioclavicular (AC) joint: This joint forms the highest point of the shoulder and provides the ability to raise the arm above the head. The meeting of the scapula and clavicle forms it.
- Sternoclavicular joint: This joint is at the center of the chest where the clavicle meets the sternum. It allows the clavicles to move.
Each joint is surrounded by:
- cartilage, to pad the meetings of the bones;
- ligaments, to connect the bones;
- muscles; and
- tendons, to attach the muscles to the bones.
The collection of muscles and tendons in the shoulder is known as the rotator cuff. It stabilizes the shoulder and holds the head of the humerus in the glenoid, a shallow cavity in the scapula. The muscles of the rotator cuff include the suprasinatus, infraspinatus, teres minor, and subscapularis.
Because shoulders have such range of movement and are used often, they are commonly injured. Some injuries include:
- Dislocation of the glenohumeral joint
- Tear of the rotator cuff
- Fractured clavicle
- Fractured scapula