shoulder - Joints of the Shoulder, Major muscles, Rotator Cuff

Commonly used to refer to the rounded region at the top of the arm passing towards the neck and upper back; more specifically, in anatomy, the ball and socket joint between the humerus and the scapula. It is an extremely mobile joint, at the expense of some stability. When dislocation occurs, it tends to be a downward and forward movement of the humerus with respect to the scapula.

It is made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) as well as associated muscles, ligaments and tendons.

Joints of the Shoulder

There are three joints of the shoulder: The glenohumeral, acromioclavicular, and the sternoclavicular joints.

Glenohumeral Joint

The glenohumeral joint is the main joint of the shoulder and the generic term "shoulder joint" usually refers to it. It is formed by the articulation between the head of the humerus and the lateral scapula. The "ball" of the joint is the rounded, medial anterior surface of the humerus and the "socket" is formed by the glenoid fossa, the dish-shaped portion of the latter scapula.

The capsule is a soft tissue envelope that encircles the glenohumeral joint and attaches to the scapula, humerus, and head of the biceps. This capsule is strengthened by the coracohumeral ligament which attaches the coracoid process of the scapula to the greater tubercle of the humerus. There are also three other ligaments attaching the lesser tubercle of the humerus to lateral scapula and are collectively called the glenohumeral ligaments.

Acromioclavicular Joint

The acromioclavicular (AC) joint is located between the acromion process of the scapula (part of the scapula that forms the highest point of the shoulder) and the distal end of the clavicle.

The capsule of this joint is reinforced by the coracoclavicular ligament between the scapula and clavicle at the point of articulation.

Sternoclavicular Joint

The sternoclavicular occurs at the medial end of the clavicle with the manubrium or top most portion of the sternum. Just the scapula itself is capable of 6 types of movements: lateral and medial movement, elevation (upward motion), depression (downward motion), and rotation which simultaneously move the clavicle. The combination of the movement of the scapula with the arm muscles gives a wide range of motion to the arm.

Major muscles

The muscles that are responsible for movement in the shoulder attach to the scapula, humerus, and clavicle. The serratus anterior is a muscle that originates on the surface of the upper eight ribs at the side of the chest and inserts along the entire anterior length of the medial border of the scapula. It fixes the scapula into the thoracic wall and aids in rotation and abduction of the shoulders. The pectoralis minor arises from the third, fourth, and fifth ribs, near their cartilage and inserts into the medial border and upper surface of the coracoid process of the scapula. This muscle aids in respiration, medially rotates the scapula, protracts the scapula, and also draws the scapula inferiorly. The levator scapulae arises from the transverse processes of the first four cervical vertebrae and inserts into the vertebral border of the scapula. It is capable of rotating the scapula downward and elevating the scapula. They insert on the medial border of the scapula, from about the level of the scapular spine to the scapula's inferior angle. They are responsible for downward rotation of the scapula with the levator scapulae, as well as adduction of the scapula. It inserts on the lateral clavicle, the acromion process, and into the spine of the scapula.

Rotator Cuff

The rotator cuff is a structure composed of tendons that, with associated muscles (supraspinatus, infraspinatus, teres minor and subscapularis), holds the ball at the top of the humerus in the glenoid socket and provideoulder joint.