We studied what's in your retainer. Here's what we found.

Chewing simulation chambers used in the study. Photo from the study white paper.

If you've ever had braces or aligners, you probably went home with a clear plastic retainer and a simple instruction: wear it every night for life or your teeth will shift.

Many clear retainers are made from PETG, a close cousin of the plastic used in water bottles. If you've worn one for a while, you've probably watched it get scratched, cloudy or cracked. Some people even chew right through theirs. That got us wondering: where does all that plastic go? If you bite down on a piece of plastic every night for years, does it shed tiny pieces of itself? And if so, how many?

So we tested it.

What we did

We made four pairs of partial PETG retainer specimens, each covering three back molars, and fitted them onto model teeth. Then we took them to the Materials Testing Facility at the University at Buffalo School of Dental Medicine and put them in a chewing simulator, a machine built to mimic the motion and force of a human bite.

The machine chewed 58,240 times with about 11 pounds of force, in water kept at body temperature. We used that as a model of roughly six months of nightly wear. We also ran the machine with no retainer in it, so we could tell the difference between particles coming from the retainer and any background contamination.

Afterward, we weighed the retainers and sent the water to independent labs for analysis. One used an electron microscope to look at the particles up close. The other used an infrared technique that counts each particle and confirms what it's made of. Particle counts were available for three tested pairs.

What we found

Every retainer showed visible wear.

On average, each tested pair shed about 59,000 detectable plastic particles over six months of simulated wear. The roughly 118,000-per-year figure is a linear extrapolation under the same test conditions.

And that's likely an undercount. The method we used can only detect particles 15 microns and larger. Anything smaller wasn't counted at all.

Why this matters

For context, one widely cited study estimated annual exposure of 74,000 to 113,000 microplastic particles from the foods it studied and inhalation; it analyzed drinking water separately. Our numbers aren't a direct comparison, since we measured particles released into water in a lab, not particles a person actually swallows. But these numbers suggest that a retainer could be a real source of microplastics that nobody has been counting.

And unlike a water bottle, a retainer isn't something you use for a few minutes. It sits in your mouth for hours, every night, often for years.

What we don't know yet

We want to be clear about the limits of this study. It was a lab simulation, not a study of real people. We used purified water instead of saliva, and the test didn't include food, brushing, cleaning or taking the retainer in and out. Everyone's bite is different, and results varied quite a bit even between our samples. This was also a small study, and we plan to expand it.

Most importantly, we don't yet know how many of these particles are swallowed or what they do in the body. Research on the health effects of microplastics is still early.

Please don't stop wearing your retainer. Retainers keep your teeth from shifting back, and that matters. If you have questions or your retainer looks scuffed or cracked, talk to your orthodontist.

The bigger picture

Retention is standard after orthodontic treatment, and clear plastic retainers are a common choice. We think the materials we put in our mouths every night deserve the same scrutiny we've started giving to the plastics in our food and water.

Next, we're testing more retainers, looking for ways to detect particles smaller than 15 microns, and asking a question that matters just as much as how many particles are released: what happens to them afterward? That question is a big part of why we're developing a retainer made from PHA, a plastic produced by microbes, and we'll share what we learn as we test it.