The shoulders are the most mobile joints in your body. Their function seems almost limitless as they place our hands and arms in various positions for daily life, sports and other activities. The shoulder joint is designed to provide maximum range of motion and mobility. However, this large range of motion can lead to joint problems. When you think about all the different positions and ways we use our hands and arms every day, it is easy to understand how difficult life can be when to shoulder is not working correctly.
The humerus (the upper arm bone), the scapula (the shoulder blade) and the clavicle (the collarbone) all meet to form the shoulder joint. The acromion, a part of the scapula, forms the roof of the shoulder. The shoulder is actually made up of four joints. The main shoulder joint, called the glenohumeral joint, is the “ball and socket”, where the head of the humerus meets a shallow glenoid in the scapula. The acrominoclavicular joint (AC joint) is where the collarbone meets the shoulder blade. The connection of the arm and shoulder to the main skeleton in the front of the chest is through the sternoclavicular joint (SC joint). The scapulothoracic joint is formed where the shoulder blade glides against the back portion of the rib cage (or thorax). The scapulothoracic joint is important because all the muscular attachments of the scapula work to keep the socket lined up during shoulder movements.
Articular cartilage covers the ends of bones of any joint. It is a white, shiny material that is about a ¼-½ inch thick in most large joints. It allows surfaces to glide over one another without damaging either surface. Articular cartilage functions to absorb shock and provide a slippery smooth surface to facilitate motion. In the shoulder, articular cartilage covers both the end of the humerus and the socket of the glenoid.
Ligaments are strong bands of tissue that connect bone to bone. There are several important ligaments in the shoulder. In the shoulder, the joint capsule (a sac that surrounds the joint) is formed by a group of ligaments that connect the glenoid to the humerus. These ligaments are a major source of stability for the shoulder. They help maintain the shoulder in place during motion and keep it from dislocating.
A special type of ligament, the labrum, attaches around the edge of the glenoid.
The labrum is wedge-shaped and creates a deeper socket for the glenoid. Without the labrum, the glenoid socket is flat and shallow and the ball of the humerus does not fit tightly. The labrum creates a deeper cup in which the head of the humerus fits. The labrum is also where the biceps tendon attaches to the glenoid. The biceps tendon originates from the biceps muscle, travels in front of the shoulder to the glenoid, and then becomes part of the labrum. The area where the biceps attaches to the labrum can be a source of problems if the biceps tendon becomes damages or it detaches from the labrum.
The rotator cuff muscles lie just outside of the shoulder joint. The four muscles become four tendons that connect to the humerus. This group of muscles helps lift the arm and rotate the shoulder in many directions. The rotator cuff muscles and tendons also help with the stability of the shoulder by keeping the humeral head aligned with the glenoid socket.
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