ACTION AND REACTION
The third law of Newton applies in almost all orthodontic movements; the law of Action and Reaction, which states that to every action there is always opposing and equal reaction. Sometimes the reaction to a movement results in an unwanted movement.
Another example of the third law of Newton would be when we want to align intruded or extruded teeth, where the same force produced to make these movements will be applied on the surrounding teeth provoking a movement in opposite direction to the one we want to produce. In order to determine the action of a bend, we must know where we are going to put it, this way we can identify what type of forces are going to be applied and the moments we can expect with the bends on a determined group of teeth.
The easiest way to determine the direction of a movement of a bend made on an arch wire is to put it in a passive manner in the slots of two braces, and we will observe where the arch is headed to, so that when we activate the wire, placing it in the slots, we can predict the direction of the movement to take place.
In these images we can observe the direction where the arch wire is going when it is placed in the slot of the brace of the central incisor in a passive way, and what movements we can expect once the arch is activated. In this case, as soon as we insert the arch in the slot of the brace on the lateral incisor, this toolh will be intruded and the cenlral incisor will extrude.
In the next set of images, the same case is presented but seen from another perspective; the arch wire is passive in the slot of the lateral incisor and the bend is over the brace of the central incisor; when activated, it is going to produce the same movements, extrusion of the central incisor and intrusion of the lateral incisor.
Both examples are useful in order to predict the direction of dental movements produced by the forces of the arch wire. Now we have to know where to place the bend so we can identify over which teeth we want to place the force.
If a bend is located far from the center of the arch wire, we will have two segments, a short one and a long one, that will produce equal forces in different directions. When the short segment of the bend is inserted in the tube, the long segment will indicate us the direction of the force produced over the LeeLh Lhat are going to receive this segment. In this case, what is produced is what we call force equilibrium, where equal and opposed forces with different moments take place, being the greater moment the side that receives the shorter segment of the bend.
On the contrary, if the bend is located in the center of the arch wire, between both teeth, the forces that are produced cancel one another when the wire is inserted in the slot of the braces. In this case the system is in equilibrium, because the associated forces are equal and opposed and cancel each other. This type of centered bend is only going to produce equal moments that are going to oppose each other.