Mad About Science: Dental cavities and how we fill them

By Brenden Bobby
Reader Columnist

Today’s subject is courtesy of Lyndsie Kiebert-Carey. Thanks, Lyndsie!

At some point in life, just about every one of us has suffered from a cavity in our teeth. Some are so small we’re totally unaware there’s an issue until we visit the dentist. Other times, there’s not enough lidocaine in the world to ease the throbbing pain and ceaseless suffering radiating from your mouth.

Ever wonder how they get there, and how dentists fix them?

It all begins with the bacteria in your mouth. Bacteria are a necessary part of the digestive tract from the beginning to the end, as they help us break down food for our bodies to use as fuel. The bacteria in our mouths love starches and sugars and they begin to form a sticky film over our teeth after we eat things that contain a lot of sugar or starch. This film is called plaque, and it’s effectively a protective barrier for the bacteria. As the bacteria consume trace amounts of food and convert it into energy, they release acid, which begins to erode the enamel of your teeth. So long as you’re regularly brushing and flossing, you can keep the level of bacteria in check and dramatically slow this process down. Hard-to-reach places like under your gums, between your teeth or concave portions of your molars make for great hiding places for bacteria, and this is likely where cavities begin to form.

The acid erodes the enamel first, then begins eroding the second layer of your teeth, called dentin. At this point, you may begin to feel some discomfort as tiny tubes in your dentin communicate with the nerves in your teeth. Once the bacteria has eroded through your dentin and begun to attack the pulp, where your nerves reside, you’re in for a world of hurt. The severe pain caused by a deep cavity or abscess is caused by the pulp swelling and squeezing the nerve.

A cavity that doesn’t require extraction is a fairly straightforward process for your dentist to fix. Getting a dental filling may feel like an eternity while you’re sitting in the chair, but it’s a relatively expedient process. First, your dentist will apply anesthetic, which blocks the receptors of your nerves around the injection site. This will make your mouth feel numb as it temporarily deadens your nerves. 

Next comes the part that everyone hates: the drill. Drills aren’t the only tools dentists use, but they are helpful for removing dead and dying parts of the tooth and providing healthy tissue for the filling to bind to. Your dentist will flush the area with water while doing this, washing out debris and bacteria in the process. So long as you’re properly numbed, you won’t feel anything but pressure during this part, so there’s no reason to fear the drill. Just think about Welsh corgi puppies galloping through a field of flowers.

Depending on a number of factors, the type of filling that goes into the filling at this point may vary. Amalgam fillings are made from an amalgamation of metals such as copper, tin and mercury. These last for a really long time, but they usually only go into teeth you can’t see like molars. 

Composite fillings are a mixture of glass and photopolymer resin. If you’ve followed Mad About Science for a while or are an avid 3D printing hobbyist, you know exactly how this stuff works. This resin will be deposited into the cavity, and your dentist will push it around to force out any air gaps, while also smoothing the surface to discourage new pockets for bacteria to grow. If your dentist puts on those cool orange sunglasses from your favorite crime procedural and shines a blue light in your mouth, this is when the magic happens. Ultraviolet light photons impact resin molecules and cause a rapid chemical reaction. This causes the resin to harden and cure, which will ensure it stays strong enough for you to chew with.

The final step is your dentist polishing the resin and the surrounding edges of the former cavity. This keeps bacteria from exploiting potential gaps and causing the cavity to form all over again. This is usually done with a tool that sounds like a drill, but looks like a tiny floor buffer like a custodian or a zamboni driver might use.

An alloy of copper and gold can also be used as a filling, and these last for up to 20 years.

Sometimes, the pulp of your tooth becomes severely infected while the surrounding tooth is relatively fine. When this happens, a dentist may recommend a dreaded root canal. This process is functionally similar to the one we described for filling cavities, but your dentist will actually remove the damaged pulp entirely and inject a rubbery material called gutta-percha, then they will seal the tooth and place a filling in it or a crown on it and it will be as good as new. A fully formed tooth can survive without the pulp, but not if bacteria are eating it from the inside out.

After all of this, you might be left wondering why we have bacteria in our mouths at all if they can wreak so much havoc. Unfortunately, they’re a necessary part of our survival and it will continue to be that way until we find some sort of scientific method to undo a billion years of evolution. Until then, it’s probably just easier to brush our teeth and pass on the sugary sodas. Water makes you feel better anyway.

Stay curious, 7B.