In human anatomy, the shoulder joint comprises the part of the body where the humerus attaches to the scapula. The shoulder refers to the group of structures in the region of the joint.

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. The articulations between the bones of the shoulder make up the shoulder joints. The shoulder must be flexible for the wide range of motion required in the arms and hands and also strong enough to allow for actions such as lifting, pushing and pulling. The compromise between these two functions results in a large number of shoulder problems not faced by other joints such as the hip.

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 a ball and socket joint that allows the arm to rotate in a circular fashion or to hinge out and up away from the body. 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 lateral scapula. The shallowness of the fossa and relatively loose connections between the shoulder and the rest of the body allows the arm to have tremendous mobility, at the expense of being much easier to dislocate than most other joints in the body.

The capsule is a soft tissue envelope that encircles the glenohumeral joint and attaches to the scapula, humerus, and head of the biceps. It is lined by a thin, smooth synovial membrane. 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.

There is also a ligament called semicirculare humeri which is a transversal band between the posterior sides of the tuberculum minus and majus of the humerus. This band is one of the most important strengthening ligaments of the joint capsule.

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.