A body is considered to be in static equilibrium when, initially at rest, it persists in this state of rest.
As physics teaches us, a body at rest needs the sum of the forces that work on it to be equal to 0. In our case, the pressures that arise at the moment of occlusion transfer, by means of the teeth, an equal and opposing force that generates muscle tension. Thus the force that the teeth oppose is equal to the force of gravity and the muscular forces that come to meet it. In the face of this parity of forces the skull remains in a correct position. Obviously, this does not happen if the teeth are not properly extruded. In fact, teeth that are not properly extruded are not capable of creating adequate pressure to oppose the forces generated by the masseter muscles and the weight of the skull.
At this point a quick glance at picture 29 would permit us to intuit easily how decadence of the skull takes place: in the face of the thrust of the forces fm (muscular force) and fcgc (force of the center of gravity of the skull) represented by an acting force (Fi), there is an reacting force, or juxtaposing (or, opposing) force (Fj). The opposing force is the force created by the impact of the teeth during occlusion. If this force lacks its principal instrument, dental height, it is not able to impart an equal and opposing force to counterbalance Fi (fm + fcgc). This leads to a sinking of the skull until it finds a new occlusal contact.
There is a lack of the action-reaction principle, which is Newton’s Third Law of Motion:
“When one body exerts a force Fi -> j on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction FJ -> i to that of the first body.” (Isaac Newton)
In our case the action is exerted by the upper teeth operated by the forces fm and fcgc that in turn do not find in lower dental height their opposing force.
We can say that:
Fi -> j =F (fm+fcgc) of i (upper dental height) → j (lower dental height)
FJ -> i =F (fm+fcgc) of j (lower dental height) → i (upper dental height)
We can say again that in the case of FJ -> i = – Fi -> j the skull will be in a state of equilibrium on the jaw.
It is to be noted that if the body i exerts a force on j, the force that j exerts on i is “simultaneous”.
At this point we should ask ourselves: why ever did we mention fcgc, which is a force that constantly comes from above and which pulls the skull downward, in both equations?
How is it possible that the force of gravity which pulls the skull downward is used to drive the skull upward?
The human body is an extraordinary organism and for that reason has found in an organic manner the way to remain erect with as little expenditure of energy as possible.
To do this it has reused part of the weight of the skull imposed by gravity. In other words it has used a very simple mechanism: it has made use of a lever, whose role I will explain in the following pages.
