The shoulder is located where the humerus (upper arm), clavicle (collarbone), and scapula (shoulder blade) meet. While many people think of the shoulder as a single joint, the shoulder actually has four joints:
- The sternoclavicular joint is located where the clavicle meets the sternum at the top of the chest.
- The acromioclavicular joint is located where the collarbone, or clavicle, glides along the scapula’s acromion. The acromion is at the highest point of the scapula, which is an irregular, somewhat flat, triangle-shaped bone. The acromioclavicular joint facilitates raising the arm over the head.
The glenohumeral joint is what most people think of as the shoulder joint. It’s the major joint in the shoulder, where the rounded top, or head, of the humerus, nestles into a rounded socket of the scapula, called the glenoid.
This ball-and-socket construction allows for circular movement of the arm. In the ball-and-socket construction of the hip, the femur head “ball” fits inside a socket of the pelvis. By contrast, the glenoid is too small and shallow for the humeral head to fit inside it. Rather, the rounded humeral head rests against the concave glenoid, like a golf ball is nestled on a tee.
- The scapulothoracic joint is sometimes considered a joint. It is located where the scapula glides against the thoracic rib cage at the back of the body. No ligaments connect the bones at this joint.
How the Shoulder Joint Works
The points below provide an in-depth description of the structures and functions of the shoulder joints.
- Where the bones meet they are covered with articular cartilage, an extremely slippery, strong, flexible material.
- Articular cartilage allows two bone surfaces to glide against each other.
- Articular cartilage also acts as a shock absorber, cushioning bones against impacting each other (e.g. while playing tennis or cutting wood).
- Articular cartilage in the shoulder tends to be thinner than articular cartilage in weight-bearing joints such as knees and hips. When articular cartilage is further thinned or damaged, its ability to protect against friction and impact is reduced.
- A strong piece of cartilage, called the labrum, rings the outer edge of the glenoid. The labrum deepens the socket joint, making the joint more stable, but its elasticity allows for flexibility.
- Rotator cuff muscles extend from the scapula to the humeral head, stabilizing the glenohumeral joint by hugging the humeral head to the glenoid.
- There is also a bursa around the glenohumeral joint. It is a small fluid-filled sac that helps the muscles and tendons slide freely as the shoulder moves.
- The shoulder joint has tendons that connect bone to muscle and ligaments that connect bones to bones. These tendons and ligaments essentially encase the movable parts of the joint, providing support for the integrity of the joint while allowing for full range of motion.
- The shoulder joints are encapsulated in the synovial membrane, which produces synovial fluid. This sticky, viscous fluid lubricates and circulates nutrients to the joint.
- When the shoulder is at rest, the synovial fluid is contained in the cartilage, much like water in a sponge.
- When the shoulder rotates or bears weight (e.g. lifting objects) the synovial fluid is squeezed out. Therefore, joint use is necessary to keep joints lubricated and healthy.
The shoulder joint is a remarkable combination of strong bones, flexible ligaments and tendons, and strong cartilage and muscles. It is designed to be incredibly flexible, enabling a wide range of motion. However, this intricate structure is also vulnerable to wear and tear over time, resulting in pain from arthritis and possible injury to any one of the structures.