How do braces and Invisalign make teeth move?

A question that I am often asked by my patients is, “How are braces able to move my teeth?” A very simple answer is that when gentle forces are placed on teeth, the bone around the teeth remodels, or “reshapes” to let the teeth move to a new position. In reality, the physiology of tooth movement is much more complicated, but also more interesting to those who are curious about science and the human body. So for those curious types, here is a more in depth (but still simplified!) explanation as to why teeth are able to be moved by braces.

pdlLet’s start with the players involved. The tooth itself is made of different materials. The part that can be seen in the mouth is covered by a material called enamel, while the root of the tooth (below the gums) is covered with a material called cementum. The cementum of the root is connected to the surrounding bony tooth socket by what is called the periodontal ligament (PDL). The periodontal ligament is made of many bundles of fibers, which in turn are made of a protein called collagen. Many types of cells also reside within the PDL, each with specific functions (more on that below). The main purpose of the PDL is to anchor the teeth to the bone socket (so your teeth do not fall out!), while also allowing slight movement, or “bounce”, to cushion the forces from chewing.

Normal chewing places forces on the teeth, PDL, and bone, and there is constant remodeling (tissue loss and rebuilding) taking place. Under normal conditions, this remodeling is usually in balance, and the teeth do not move. However, if additional forces (or pressure) are placed on teeth (such as with braces), something very interesting happens to the cells that reside within the periodontal ligament. On the side where the PDL is being compressed, or “squished”, cells called osteoclasts start removing, or “eating away” bone. Thus the tooth moves into this new space. On the back side, where the PDL is under tension, or is being “stretched”, cells called osteoblasts start producing bone. Thus bone is added where the tooth root once was. When the forces produced by braces or Invisalign are removed, the osteoclasts and osteoblasts return to their less active roles of maintaining the teeth when they are stationary.

Now that you know the basics of teeth movement, I’ll go over some other questions you might have.

Can I speed up tooth movement by wearing double the rubber bands?  Wearing more rubber bands than asked by Dr. Rejman will likely make your treatment take longer!  In fact, it is thought that the ideal force for orthodontic tooth movement is very light, slightly higher than capillary blood pressure. Excessive pressure can lead to areas around the tooth where cells are not present (hyalanized areas), which results in reduced tooth movement. The take home lesson: wear your rubber bands as directed!

When braces or Invisalign are removed, are the teeth permanently set in place?   No!!! Even though the bony socket has remodeled, and the body finds relative equilibrium (as described above), the PDL fibers are now “stretched”, and want to pull the tooth back towards its original position. Retainers must be worn to keep the teeth in place. Everyone is different, and Dr. Rejman will instruct each patient how long to wear their retainers each day, and which type of retainer will be best for each individual.

So, if placing a light force on a tooth can cause it to move, can I move my teeth with my tongue or my finger? Absolutely! Teeth do not know what is placing a force on it, all it knows is “How much force, and how long does it last?” This is why we are concerned with thumb sucking, which can dramatically tip teeth upwards and outwards. The pressure that the tongue places on a tooth is normally balanced nicely by the opposing pressure the cheeks and lips, and the tooth finds a happy space in the middle of these forces. However, I have seen patients with abnormally large tongues that place a larger outward force on the teeth, which moves them to a non-ideal “wide” position, often causing an open bite. They key is that the force must remain long enough to activate the cells that are responsible to remodel the tissues, as described above.