The function of the deltoid muscle is to lift the arm further up once the arm is already away from the side of your body. The deltoid is the largest, strongest muscle of the shoulder. The large deltoid muscle is the outer layer of shoulder muscle. The rotator cuff muscles and tendons are extremely important in assisting glenohumeral joint stability because they help hold the humeral head in the shallow glenoid socket. They are extremely important muscles to the shoulder and are involved in many day-to-day activities. These muscles help raise the arm from the side and rotate the shoulder in the many directions. This group of muscles attaches from the shoulder blade to the humerus. Four rotator cuff tendons connect the deepest layer of muscles to the humerus. The tendons of the rotator cuff are the next layer in the shoulder joint. This connection can be a source of problems when the biceps tendon is damaged and pulls away from its attachment to the glenoid. At the very top of the glenoid, the biceps tendon attaches to the bone and actually becomes part of the labrum. The biceps tendon runs from the biceps muscle, up the front of the upper arm, to the glenoid. Muscles move the bones by pulling on the tendons. The labrum is also where the long biceps tendon from the upper arm attaches to the glenoid. This anatomical shape allows great mobility of the shoulder, but unfortunately does not provide much stability, hence the reason it dislocates much easier than some other joints. This is important because the glenoid socket is so flat and shallow that the ball of the humerus does not fit tightly. The shape and the way the labrum is attached create a deeper cup for the glenoid socket and helps to prevent dislocation by creating more stability. When viewed in cross section, the labrum is wedge-shaped. It is attached almost completely around the edge of the glenoid. The labrum is a special cartilaginous structure inside the shoulder. These ligaments are the main source of stability for the shoulder they help hold the shoulder in place and keep it from dislocating. In the shoulder, the joint capsule is formed by a group of ligaments that connect the humerus to the glenoid. A joint capsule is a watertight sac that surrounds a joint. Ligaments are soft tissue structures that connect bones to bones. There are several important ligaments in the shoulder. This joint is especially important because the muscles surrounding the shoulder blade work together to keep the socket lined up during shoulder movements. The scapulothoracic joint is formed where the shoulder blade glides against the thorax (the rib cage). The sternoclavicular (SC) joint supports the connection of the arms and shoulders to the main skeleton on the front of the chest. The acromioclavicular (AC) joint is where the clavicle meets the acromion. This shallow socket is called the glenoid. The main shoulder joint, called the glenohumeral joint, is formed where the ball of the humerus fits into a shallow socket on the scapula. There are actually four joints that make up the shoulder. The roof of the shoulder is formed by a part of the scapula called the acromion. The bones of the shoulder are the humerus (the upper arm bone), the scapula (the shoulder blade), and the clavicle (the collar bone). Understanding the anatomical structures discussed will help you understand why your shoulder dislocated. The shoulder has a unique and complex anatomy that allows range of motion and coordination needed for reaching, lifting, throwing, and many other movements.
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