Do you know the 4 reactions of a tooth to force?

Orthodontic tooth movement is a process in which the application of a force induces bone resorption on the pressure side and bone apposition on the tension side. Thus, conventional tooth movement results from biological cascades of resorption and apposition caused by mechanical forces.

It's clear that braces don't move teeth, clear aligners don't move teeth,

but only force can move teeth.

When we apply a force on teeth, what kind of movement are we producing?

Orthodontic tooth movement is only possible if we apply a force on a tooth. Or if someone has bad periodontal disease, but that is another story.

What happens is, bone surrounding teeth can then be stimulated and induced into remodeling itself. When a light force is applied against a tooth to move it in a given direction, the bone will resorb in the direction that the tooth is moved, and new bone will be deposited on the opposite side where the tooth comes from.

But if you choose a tooth and apply a force, what kind of reaction will you see, or what type of movement?

4 different reactions

There are four (4) different things that can happen when a force is applied on teeth or any object:

The first thing you can see is ROTATION

There are 2 types of rotation: pure and hinge. A pure rotation happens when the center of resistance coincides with the center of rotation. This means that the forces applied to the tooth create a couple.

A couple is when two (2) equal and opposite forces separated by a perpendicular distance are applied to an object. It creates moments of couple that lead to pure rotation.

A hinge rotation is comparable to a door. Think of a tooth that would rotate where the mesial or the distal portion would stay at the same place. That point would then be the center of rotation.

The second is TRANSLATION

What is translation? It's when every point of an object will move at the same time in the same direction. This is what we call "pure translation" and for the mechanic's fan among you, this means that the force is applied directly in line with the center of resistance…

Like my friend, Dr. Gerry Samson would say "How does it feel to feel smart?"

Third: this one is a little tricky, and it is the one that is happening most of the time:

It's a COMBINATION of rotation and translation.

To understand the 4th reaction force can produce, imagine yourself pushing on an elephant. A big one. Really big. What would happen? Even if you push as hard as you can. Well, if the elephant doesn't notice... you are lucky. Otherwise, we can't promise the elephant's reaction would be good. Now, let's say the elephant doesn't feel it, what would happen? NOTHING!!!

Think of an ankylosed tooth or an implant. Even if you apply all the force you want, they will not move. Something might happen, but it would probably create permanent damage to the tooth, implant, surrounding tissues, or a combination of those things.

It’s all physics!

Every time we apply a force on a tooth or any object, the 3^rd law of Newton applies:

"For every action, there is an equal and opposite reaction".

This is not an opinion, this is a principle, a law. You can not argue that it is not true. This is why Isaac Newton is one of the only true guru in orthodontics.

When you plan your cases, when you think of the mechanics and the force that the clear aligners will apply on the teeth, always have in mind these 4 effects of the force. It will help you visualize the movements and plan where you need to add attachments.

Orthodontics is physics, it's mechanics… It's biomechanics! Working with clear aligners is nothing different than working with any other type of orthodontic appliance: it's a tool you use to move teeth. To apply a force and create movement, clear aligners answer the same laws and principles as any other appliance or instrument we use.

To have success with clear aligners, we need to learn how to use them to achieve the desired movements and final results. There is undoubtedly a learning curve. But the good news is: these things can be learned! And it's a good thing you found us in this corner of the internet because this is what we do 😉

To know more about clear aligner and tooth movement mechanics, we recommend you register for the Basic Tooth Movement Mechanics course here. The Attachments 101 course here may be of interest as well. 

Have fun Making the MOVE!