The system that keeps the skull in position is autonomous and involuntary. It is called the craniocervical mandibular system and works as a functional biomechanical unit. The cornerstones of this system are represented by the temporomandibular joint, the atlanto-occipital joint and by the “hanger system” of the hyoid bone. Other integral and fundamental parts of this system are: an adequate dental height, the swallowing mechanism, and all the muscles that activate these joints.
We can say that remaining upright is a natural process of the human body. The equilibrium reaction is a reflexive process and is involuntary. It is not possible to eliminate it, by trying to assume postures that are extremely straight, as it is stimulated in this case by a lack of dental height.
But what does the jaw do to support the skull?
The jaw and the stomatognathic system: a solid foundation whereon to find a foothold and use a lever.
The jaw bone (the mandible) is a bone of the splanchnocranium, and is a single, median bone. It is the only bone in the face provided with a moveable joint, the temporomandibular joint.
Being the only bone with a moveable joint, the jaw bone is suspended between the skull and the hyoid bone. It becomes a stable and load-bearing structure only at the moment of occlusal contact. The greatest stability is created more precisely in the act of swallowing. In addition, its mobility makes it difficult to understand that it can be a solid fulcrum and of fundamental importance in the maintenance of the skull. It is just this dual nature that makes the jaw, with the teeth and the stomatognathic apparatus, the base on which the skull rests, within this closed-circuit system.
Consequentially, what makes it difficult to understand the nature of the jaw is the fact that it does not offer constant support to the skull. Precisely for this reason this double function is not recognized by everyone today, and the important role of the jaw is not sufficiently appreciated.
In addition it is necessary to say that occlusal contact takes place either involuntarily (part. during swallowing) or voluntarily (part. when clenching the teeth). The support of the skull by means of occlusal contact becomes the most stable during a stronger closure of the jaw.
In fact, during the act of swallowing the muscles of the stomatognathic system go into action, and it is precisely at that moment that the jaw becomes a solid structure, just like the mast of sailboat supported by sheets.
During this process, the levator, as well as the depressor, muscles of the jaw contract together. When these muscles contract together, from opposite sides, they become ties that stabilize the jaw.
It is precisely during the act of swallowing that the forces generated by the contact of the teeth are transmitted to the underlying structures. Swallowing, like our heartbeat, accompanies us throughout our entire life.
With every swallow, the tongue thrusts with a force of about 1 kg per square inch. Considering that within 24 hours there are 1500-2000 swallows, we can say that in a day 2000 kg of force are transmitted to the underlying structures.
The muscles that allow this complex mechanism of the jaw are the masticatory muscles (temporal, masseter, external and internal pterigoid), the postural coordination muscles (suprahyoid and infrahyoid muscles), the muscles at the base of the skull (atlas trapezius muscles) and dental occlusion. This complex relationship between jaw, skull and hyoid bone connects, by means of the infrahyoid muscles, the masticatory apparatus with the sternum and the shoulder blades.
The integration of the synergistic activity of these groups of muscles, and in particular the hyoid muscles, allows during swallowing for the elevation of the hyoid bone and the lowering of the jaw.
This means that, during chewing, the jaw can make large movements, while maintaining the hyoid bone in a relatively stable position. Otherwise it would not be possible to speak, chew and swallow without losing the ability to remain mobile.
All this is possible because the jaw has a dual nature.
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