“Cavity” is one of the most common words in dentistry — and one of the least understood. What actually is a cavity? How does a hole form in something as hard as a tooth? And what does a filling really do? Here is the whole story in plain language.
Your Tooth, in Layers
A tooth has three main layers. The enamel is the hard white outer shell — the hardest substance in your body. Under it is dentin, a softer, yellowish layer. At the center is the pulp, which holds the nerve and blood supply. A cavity is simply tooth decay that has eaten through one or more of these layers.
How a Cavity Forms, Step by Step
- Plaque builds up. Bacteria in your mouth form a sticky film on teeth, especially along the gumline and between teeth.
- Bacteria make acid. When you eat sugars or starches, plaque bacteria feast and produce acid as a byproduct.
- Acid dissolves minerals. The acid pulls minerals out of your enamel — a process called demineralization. Your saliva repairs minor damage (remineralization), but frequent sugar exposure overwhelms it.
- A hole forms. Once enough mineral is lost, the enamel surface collapses and a physical hole — the cavity — appears.
- Decay spreads inward. Dentin is softer than enamel, so decay moves faster once it gets past the shell. If it reaches the pulp, you feel pain — and you may need a root canal instead of a simple filling.
This whole process is silent at first. Small cavities don’t hurt, which is why checkups and X-rays catch them early, when the fix is smallest.
What a Filling Actually Does
A filling doesn’t “heal” the tooth — it removes the decayed portion and seals the hole so bacteria can’t get back in. The modern process:
- Numbing. The area is numbed so you feel pressure but not pain.
- Decay removal. The dentist removes all soft, decayed tooth structure, leaving only healthy tooth.
- Shaping. The cavity is shaped so the filling material locks in securely.
- Filling and bonding. Tooth-colored composite resin is placed in layers, hardened with a special light, then shaped and polished to match your bite.
Today’s composite fillings bond directly to the tooth, which means less healthy tooth needs to be removed than with old silver amalgam fillings. They are also color-matched, so the repair is invisible.
When a Filling Isn’t Enough
If decay is very large — or the tooth is cracked — a filling may not be strong enough, and a crown (a cap covering the whole tooth) is the safer choice. If decay reaches the nerve, a root canal cleans out the infected pulp before the tooth is sealed and crowned. Your dentist will show you on the X-ray exactly how deep the decay goes and why they recommend what they do.
Preventing the Next One
- Brush twice daily with fluoride toothpaste — fluoride remineralizes early damage.
- Floss daily — most cavities start between teeth where brushes can’t reach.
- Limit sugary snacks and sipping sweet drinks through the day — frequency matters more than quantity.
- Keep your six-month checkups — early cavities are tiny, cheap, and painless to fix.
Think you might have a cavity? Dr. Leila Nikpour is a prosthodontist in Roseville with more than 15 years of dentistry experience. Call Expert Dental Group at (916) 888-1646 or visit 5070 Foothills Blvd, Suite 2, Roseville, CA 95747 (Mon–Fri 9–6, Sat 9–1).
Frequently Asked Questions
Can a cavity go away on its own?
No. Once a physical hole has formed, it cannot remineralize closed. Only the very earliest stage (a white spot, before a hole forms) can sometimes be reversed with fluoride.
Do fillings hurt?
The area is numbed first, so you should feel pressure but not sharp pain. Mild sensitivity to cold for a few days afterward is normal and fades.
How long do tooth-colored fillings last?
Typically 7–10 years or more, depending on the size of the filling, your bite forces, and whether you grind your teeth. Larger fillings in heavy-chewing areas wear faster.
Why did I get a cavity if I brush every day?
Brushing misses the areas between teeth — that’s where floss matters. Frequent snacking, dry mouth, and deep tooth grooves can also cause decay despite good brushing.
Are silver (amalgam) fillings still used?
Much less than before. Tooth-colored composite is now standard for most fillings because it bonds to the tooth, preserves more structure, and looks natural.