The shoulder joint is of enormous importance for the function of the arms: it allows the greatest freedom of movement of all arm joints and is therefore a basic requirement for targeted grasping, transporting objects, shaking hands and all those functions that distinguish us as humans. All the more annoying are the various pain and wear syndromes known in today’s medicine, which can restrict the movement of the shoulder all too badly in some sufferers.
What is the shoulder joint?
Schematic diagram showing the anatomy of the shoulder. Click to enlarge. The shoulder joint, also known as the acromioclavicular joint in the strict sense, refers to the movable connection between the scapula and humerus bones. In medicine, these bones are called the scapula and humerus, so the joint’s anatomically correct name is the articulatio humeri. In a broader sense, the shoulder must also include the shoulder girdle, which, in addition to the articulatio humeri, also includes the joint between the scapula and clavicle, sternum and clavicle, and the muscles around the scapula and humerus. All of these are essential to the function of the shoulder as a whole.
Anatomy and structure
So, anatomically, the shoulder joint is composed of two bones: The scapula provides the glenoid cavity, which is somewhat hollow on the inside and forms the counterpart to the joint body on the humerus, the head of the humerus. This corresponds in principle to the structure of many other ball joints in the human body, but in the case of the shoulder it has the special feature that the glenoid cavity and the joint body are not pressed into each other for a precise fit, but rather slide loosely past each other. This allows greater freedom of movement of the arm, which has probably suited humans quite well in the course of evolution in developing the “tool function” of the upper extremity. At the same time, however, it also increases the risk of dislocations, which are not uncommon in the shoulder and are usually very painful. The soft tissue girdle, i.e. ligaments and above all the muscular framework, is therefore all the more important in the shoulder joint. The so-called “rotator cuff” attaches directly to the shoulder joint and, together with the chest and back muscles (most important: pectoralis and latissimus dorsi muscles), firmly “lashes” the upper head and neck in all directions to the scapula and clavicle as well as to the ribs and spine. Another important muscle of the shoulder joint is the deltoid muscle (deltoid muscle), which lies over the shoulder joint on the very outside under the skin like a shoulder pad and thus not only protects it, but is also responsible for the main movements, especially the abduction of the arm.
Function and tasks
The shoulder joint is a ball-and-socket joint in which three main axes can be distinguished: To the side, the arm can be spread to almost 180 degrees and brought back in (abduction and adduction); to the front and back, anteversion (to almost 180 degrees) and retroversion (to about 40 degrees) are possible; about the axis of rotation, the arm can also be rotated externally and internally (about 90 degrees each). However, it must be said about the degrees that they only come about in the interaction of the acromioclavicular joint with the clavicle joints of the shoulder girdle. You can see this if you hold the scapula and clavicle by pressure from above: the corner joint alone, with the scapula firmly fixed, only manages an abduction and anteversion of about 90 degrees each – after which the scapula moves with it. For most manual occupations, these degrees of movement are indispensable. In particular, overhead work and carrying and passing heavy loads is not possible without the free function of the shoulder joints.
Diseases and complaints
There are a handful of injuries and pain syndromes that can seriously spoil the enjoyment of one’s shoulder for those affected. In young people, this is particularly common, the fracture of the clavicle in a fall on the outstretched arm, which severely limits the overall function of the shoulder girdle (but does not, strictly speaking, affect the acromioclavicular joint itself). Dislocation, i.e. dislocation of the shoulder, usually forward, is also very painful – some people have an excessively flaccid soft tissue girdle, so that the shoulder is repeatedly dislocated even during slight movements, and the soft tissue girdle subsequently becomes even more flaccid as a result.A typical accident for a (first) dislocation is the hanging of the ski pole during downhill skiing, which causes the arm to be jerked backwards and upwards over the loop. In older age groups, the wear and tear of the shoulder joint is then more in the foreground: There is osteoarthritis of the acromioclavicular joint, medically called “omarthrosis” and other syndromes with a stiffened shoulder (“frozen shoulder“), for example, irritation of the tendon insertions or the “impingement syndrome“, which is also quite common in middle age, in which the tendon of a rotator cuff muscle is constricted in the bony structures of the shoulder blade and rubs along it with every movement (especially when splayed between 90 and 120 degrees, the so-called “painful arc”). Bursitis over the shoulder joint can also be very painful and prolonged.
