Shoulders

When we talk about the shoulder joint, we usually mean the glenohumeral joint, the joint between the humerus and the shoulder blade. This joint is also extremely mobile, providing around two thirds of the total mobility of the arm. It is designed as a ball-and-socket joint, but in order to maintain its great mobility, the socket must not surround the ball, unlike in the hip. The socket only carries about a quarter of the joint surface compared to the ball. It is also not very deep. A bit like the golf ball on its base. This means that the bony guidance in the joint is very limited compared to the hip and that the joint has no bony stability. Stability must therefore come from various soft tissues. These include the static stabilisers, which include the joint capsule, the ligaments (reinforcements of the capsule) and the joint labrum. The latter serves to enlarge the joint surface and moulds itself to the humeral head in such a way that a vacuum effect is created. Very similar to the suction cup on a fridge door.

The muscles of the rotator cuff belong to the dynamic, i.e. mobile, stabilisers. These are four muscles and their tendons that reach from the shoulder blade to the head of the humerus and embrace it at the front, top and back in such a way that it looks like the open sleeve of a shirt. Hence the name (rotator) cuff. These four extremely important muscles serve to hold the humeral head in the joint during movement and thus enable mobility. The two muscles at the back also serve to turn the arm outwards (external rotation). If the large deltoid muscle, which extends from the acromion to the humerus, wants to move the arm, it pulls it upwards. The head of the humerus would then slide upwards and hit the acromion, which would be painful if we did not have the rotator cuff, which holds the head of the humerus in the joint and thus enables rotation in the joint without touching the acromion.

The acromioclavicular joint, also known as the AC joint, is located at the top of the acromion in the joint between the collarbone (clavicle) and the acromion. Just like the joint between the clavicle and sternum (sternoclavicular = SC joint), it is a little idiosyncratic because in many people it is not vertical but slanted. In addition, each of these joints has its own intervertebral discs. The collarbone serves as a support for the shoulder, especially when the arm is raised above the horizontal. This puts considerable pressure on the small AC joint. This is why it wears out in many people and leads to osteoarthritis. Surprisingly, although the SC joint is exposed to the same pressure loads, it hardly ever develops osteoarthritis. This may be because the rotational movements at the SC joint are much less than at the AC joint. At the latter, these act like a pepper mill on the small intervertebral disc in the joint.

Completely underestimated, but of considerable importance for arm mobility, is the mobility of the shoulder blade on the back. Seven muscles fix the shoulder blade to the rib cage and ensure its great mobility. When we raise our arm, we move it approximately to the horizontal in the shoulder joint, then the shoulder blade rotates upwards. But even when we move our arm forwards or backwards, the shoulder blade plays a major role in the movement. If this mobility is impaired, this often leads to muscle tension in the back and shoulder blade muscles, which can be very painful.

In addition to the skeleton and muscles, successful movement also requires a control system - the neurology. Impulses for movement are transmitted from the brain and cerebellum to the spinal cord. At the level of the lower cervical spine, the individual nerves emerge as roots and are found again in the brachial plexus. From there, the individual nerves make their way to the target muscle. They are responsible for controlling this delicate ballet of muscles that moves the shoulder and therefore the arm. If individual nerves fail or are injured, the corresponding muscles become paralysed and shoulder function is impaired. Depending on which muscle is affected, the impairment is more or less severe.