eli5 Why does it take 2-3 years for braces to straighten your teeth but they can revert quickly if you don’t wear a retainer when the braces come off?
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If you want to reposition a tree branch, you have to slowly adjust it to another location to avoid breaking the branch (braces). Once you have moved it to the intended location, you still have to put in support (retainer) or else it will slowly revert to its original position due to gravity and tension. Only when a long time goes by can you remove the support as the new position is the new “normal.”
Path of least resistance. Braces mold to where your teeth aren’t naturally, while when they’re off they go back to their original position
Your teeth have a place they want to be. Braces put them where they should be but they still want to go home to where they were. Retainers help them obey.
Teeth are attached to your mouth by their roots (the part of the tooth inside your gums). Normally, the gum tissue that surrounds the root essentially holds the tooth in place. Gums are pretty hard, so there is very little “wiggle room” for a healthy adult tooth.
When you wear braces, they push the teeth (including the roots) into a new position. This slowly pushes into the gum tissue, creating new space for your teeth. (As you can imagine, the tooth cutting a path through gum tissue hurts the gums, so they do it very slowly to minimize pain and damage.)
However, the original space in your gums that the root was in doesn’t just close up. This means that when you first take your braces off, your gums aren’t holding your teeth in their new positions. (There’s more space around the root for the tooth to move around).
Now, here’s the problem: the extra space around the root is only on one side. The braces pushed the tooth against the gum tissue in one direction, so the gap is mostly on the other side. Once you take the braces off, normal things like chewing will easily push the teeth into that space – the space where your tooth’s root *used* to be. Wearing a retainer prevents this from happening.
As long as a person is young-ish when they get braces, their gums will “fill in” over time. Eventually, there will be probably be enough strong gum tissue to hold the teeth in their new positions, and a retainer won’t be necessary.
What about Smile Direct or Invisaline? How can they move teeth so quickly?
Your teeth move because they are connected to your jaw bone with tiny ligaments. When you push on the tooth one way there is tension on the ligaments which causes bone to disappear in the direction the tooth is heading and fill in behind it. This movement is inherently rather slow, and happens faster or slower depending on how hard you push, and it is much slower for big teeth with big roots like molars.
So, this is why it takes a couple years to line everything up, just that’s as fast as your body can safely remodel the bone to move your teeth around, especially if you’re moving the teeth with big roots very much. Just straightening front teeth can usually be done in a number of months, depending.
When braces are done, your bone and those ligaments are not completely done changing. If you just take the pressure off, the ligament is still kind of stretched out in the direction the tooth came from and compressed on the side it was headed. So it will push the tooth back a little bit. But this reverting is not nearly all the way back to where the teeth started, and it will mostly happen to the teeth that have smaller roots that move faster and easier – your front teeth. So this reverting happens just enough to make the end result not so great looking, and you end up with crowding or gaps in your front teeth.
Your teeth often shift as you age just because of natural changes and wear to your teeth and bone, so it’s a good idea to always wear a retainer if you don’t want your teeth to move.
It actually doesn’t take much time to move teeth. All my front teeth were moved WAY over and a huge empty space left for a a different tooth to be pulled down, then the front teeth moved back. It took less than 3 months to do it.
I didn’t wear a retainer after getting the braces off… badass 16 year old. Just one tooth is a bit crooked, but the rest are fine.
Ortho resident here. Think this topic has already been on askscience and eli5 a few times, but since no one else here has really answered it yet correctly or in detail, I’ll take a crack.
So first off, let’s talk about why teeth even relapse to begin with. Teeth tend to relapse after orthodontics for 2 reasons:
1. The first is that your teeth are surrounded by and attached to an array of microscopic protein fibers in the gums that are stretched taut during orthodontic treatment. They are gradually eaten away by the body and replaced in a process called “remodeling.” This process relieves the tension in these periodontal fibers but surprisingly happens much more slowly than the stimulated bone remodeling that occurs during tooth movement. So even for *years* after treatment has finished – even though the tooth sockets may have completely changed position and remain snug around the teeth – there will still remain some “pull” from the old periodontal fibers. The worst offenders are actually a subgroup called the supracrestal fibers, which are located near the surface of the gums and attach to the tooth close to where the crown meets the root rather than farther down the root. This is pretty convenient, because when the patient is willing, we can sever these fibers surgically with a scalpel or a laser in a procedure called a “circumferential supracrestal fiberotomy” to release these fibers. When they heal and reattach, they do so in a relaxed conformation. Research has shown this procedure to have a dramatic effect on relapse potential, especially for teeth that have undergone rotation about the long axis of the root!
2. The second reason for relapse is because your tooth positions are largely determined by what’s called the “functional matrix” of the mouth. They’re constantly at the center of a balance of push forces between the cheeks, lips, tongue, and even the jaws themselves. The strength, level of passive activation (“myotonicity”), and dimensions of these structures naturally pushes each of your teeth toward a position where all these tissue forces balance out, or in physics terms, where net force is zero. This is called “equilibrium theory,” and it explains why many syndromes and congenital disorders exhibit dental issues like anterior open bites and narrow archforms simply due to abnormalities in tongue size and posture. So it stands to reason that if you use orthodontics to force the teeth out of their “equilibrium positions” without changing the forces from the tongue, lips, and cheeks, those same tissues are going to exert imbalanced forces that will guide the teeth back to their equilibrium positions. In some cases, we can recommend oral myofunctional therapy to retrain the musculature to match the orthodontically treated tooth positions. When we can’t though, a retainer is the only way to fight off those tissue forces.
One thing to note about *both* types of relapse is that they happen in the same exact biological manner as normal tooth movement. When force is applied to the tooth – whether by braces; headgear; clear aligners; old, stretched-out periodontal fibers; or a tongue that is habitually positioned against the teeth – the root is compressed against bone on one side of the tooth socket, and the fibers on the other side of the tooth socket are stretched. The rate of relapse is still limited by the rate at which the bone can remodel. So that doesn’t explain why relapse happens faster than treatment.
The reason why you *may* notice relapse happen faster than treatment is that relapse forces are more direct. Old, taut periodontal fibers that have yet to remodel and equilibrium forces from the adjacent oral tissues pull and push (respectively) along force vectors that point directly from point B to point A. It’s a nonstop flight. Orthodontic treatment with braces on the other hand, has many logistical considerations that require making pitstops along the way. Teeth aren’t moved directly from point A to B. They must first be aligned with light, flexible wires that allow all the teeth to be linked without permanently deforming the wire. Gross alignment of the teeth along the archform happens here as well as “leveling” of any teeth that are higher or lower than the plane of occlusion (where the upper and lower teeth meet). Then, the flaring of the roots is addressed with stiffer rectangular wires that establish mechanical couples within the rectangular bracket slots. Finally, a rigid, rectangular stainless steel wire is placed to stabilize the root angulations for large movements that correct overbite/underbite tendencies or that open/close significant spaces. That’s a lot of layovers.
Hopefully that answers your question.
I’m confused just because every movie or TV show that showed a kid with a retainer were also wearing braces. I always thought retainers were part of the brace process.
So you’re telling me kids get braces, have them removed, and then still have to wear retainers?
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