There are four main parts to any given joint. They are bones, ligaments, tendons, and cartilage. Ligaments attach one bone to another, tendons attach a bone to a muscle and cartilage (a white spongy tissue) aids in shock absorption and making sure the joint moves smoothly.

The three bones that compose the shoulder joint are the humerus (the upper arm bone), the scapula (the shoulder blade), and the clavicle (the collar bone). The roof of the shoulder joint is formed by a part of the scapula called the acromion. The junction between the acromion and the clavicle is referred to as the acromioclavicular (AC) joint. So, as you can see, there are actually 2 joints that make up the shoulder.

The ligaments and the fibrous joint capsule are the main stabalizing structures of the shoulder. These ligaments connect the humerus to the socket of the shoulder joint. Of the shoulder ligaments, the anterior inferior shoulder ligament is the main stabalizer during overhead throwing movements. There are other ligaments in the shoulder. Some ligaments attach the clavicle to the acromion, and two ligaments connect the clavicle to the scapula. As you can see, the shoulder joint is quite complex, but this complex configuration is what gives the shoulder the claim on being the joint with the greatest range of motion in the human body.

The tendons and muscles of the shoulder are what control our ability to raise the arm from the side of the body. The group of muscles and tendons in the shoulder are known collectively as the rotator cuff. One other muscle, the largest and strongest muscle of the shoulder contributes to movement of the arm. This muscle is called the deltoid muscle. The deltoid muscle takes over the lifting of the arm once the arm has been extended from the side of the body.

Another extremely important structure in the shoulder, and in many other parts of the body, is a bursa. Bursae exist wherever two body parts (muscles in this case) move against one another without the presence of a joint. A bursa is a small, enclosed space (like a sealed pocket) between two moving surfaces that contains a small amount of lubricating fluid inside. The function of a bursa is to reduce the friction between these moving surfaces and thus reduce wear and tear from excess rubbing.