How is the shoulder joint structured?

The shoulder joints form the connection between the trunk and the arms. On account of their complex structure they provide us with a large freedom of movement: we can spread our arms in three directions, bend them in two directions, rotate them and stretch upwards beyond the shoulder joint. It is these movements which essentially allow us to carry, lift and throw.

As a link between the upper arm and the trunk the shoulder joint comprises parts of the following bone structures:the head of the humerus:

When in a healthy condition, all parts of the joint are surrounded by joint cartilage which acts as a lubricant in order to reduce friction between the parts of the joint. Additionally it helps absorb shock; it distributes and reduces the forces which act upon the shoulder joint.

The fixed joint capsule forms an envelope around the shoulder joint to seal it off from the surrounding tissue. This capsule produces so-called synovial fluid which serves to ensure friction-free movement of the shoulder joint and also supplies the joint cartilage with essential nutrients.

The spherical head of the humerus is only held very loosely in the small, shallow glenoid cavity of the shoulder blade. That is why muscles, tendons and ligaments assume the important role of stabilising the shoulder joint and at the same time making it as mobile as possible. This is mainly handled by a complex set of muscles, the so-called rotator cuff. It consists of four muscles which surround the shoulder joint like a cuff, protect it and make it possible to raise and rotate the arm.