A cavity filling restores a tooth by removing decayed material and replacing it with a durable substitute that seals the opening and prevents further damage. The process typically takes 20 to 60 minutes per tooth, and modern materials bond directly to your tooth structure at a microscopic level. Here’s what actually happens at each stage, from numbing to the final polish.
Removing the Decay
Your dentist starts by injecting a local anesthetic to numb the area around the affected tooth. Once the numbness sets in, a high-speed dental handpiece (the drill) or, in some offices, a laser removes the decayed portion of the tooth. The goal is to eliminate every bit of soft, damaged tooth structure while preserving as much healthy material as possible.
After drilling, an acid gel is applied to cleanse the prepared cavity and strip away remaining bacteria and debris. This step also begins the critical process of preparing the tooth surface for bonding, which is how the filling will actually stay in place.
How the Filling Bonds to Your Tooth
If you’re getting a tooth-colored composite filling, the bonding step is what makes everything work. Your dentist applies a mild phosphoric acid solution (around 37% concentration) to the enamel and the inner layer of your tooth called dentin. This “etching” creates thousands of microscopic pores and roughened surfaces in the tooth, like tiny channels for the filling material to grip.
Next comes a liquid bonding agent, a resin that seeps into those microscopic pores and into the exposed network of collagen fibers in the dentin. When this layer hardens, it forms what’s called a hybrid layer: a zone where tooth structure and resin are physically intertwined at the molecular level. This micromechanical bond is remarkably strong, reaching adhesion forces of 20 to 25 megapascals on enamel. Think of it like tree roots growing into soil. The filling doesn’t just sit on the tooth; it locks into it.
Older silver amalgam fillings work differently. They don’t chemically bond to the tooth at all. Instead, the dentist shapes the cavity with slightly undercut walls so the metal is mechanically wedged in place once it hardens.
Placing and Hardening the Material
With composite fillings, the resin is applied in layers. Each layer is a paste made of tiny glass or ceramic particles suspended in a resin matrix. Your dentist shapes each layer, then aims a blue light at it for several seconds. This light triggers a photochemical chain reaction inside the material: special molecules absorb the blue light photons and become energized enough to generate free radicals. Those free radicals start linking the resin’s carbon-carbon double bonds into single bonds, chaining the molecules together into a rigid, cross-linked three-dimensional network.
This transformation from soft paste to hard solid happens fast. Within seconds, the material enters a glassy state and becomes stiff. Interestingly, the chemical reaction continues briefly even after the light is turned off, but only in the areas the light actually reached. That’s why your dentist is careful to cure each layer thoroughly before adding the next one. Incomplete curing is one of the leading causes of post-filling sensitivity and premature failure.
One trade-off of this hardening process is polymerization shrinkage. As the molecules link together, they pack more tightly, and the material contracts slightly. Layering the composite in thin increments rather than filling the entire cavity at once minimizes this shrinkage and reduces the risk of gaps forming between the filling and the tooth wall.
Protecting the Nerve in Deep Cavities
When decay reaches deep into a tooth, the remaining wall of healthy tooth between the cavity floor and the nerve (the pulp) can be dangerously thin. If less than half a millimeter of tooth remains above the nerve, your dentist will place a calcium hydroxide liner at the deepest point before filling. This material is alkaline, which helps kill bacteria and encourages the tooth’s living cells to survive and even form a protective layer of new dentin.
For moderately deep cavities where between 0.5 and 1.5 millimeters of tooth remain, a glass ionomer liner often replaces the lost inner tooth structure. Glass ionomer chemically bonds to the tooth, releases fluoride over time, and acts as a cushioning base beneath the composite. In the deepest cases, both materials are used together: calcium hydroxide directly over the near-exposed nerve, then glass ionomer on top as an insoluble barrier against any bacteria that might leak in later.
Bite Adjustment and Polishing
Once the cavity is filled and fully cured, your dentist checks your bite using thin colored paper that marks high spots. Even a fraction of a millimeter of excess material can feel like a boulder when you close your teeth. The filling is trimmed and adjusted until your bite feels natural, then polished smooth. A well-polished surface resists staining, reduces plaque buildup, and feels more like a natural tooth.
Composite vs. Amalgam Fillings
Composite resin fillings are tooth-colored, bond directly to tooth structure, and allow more conservative cavity preparation since less healthy tooth needs to be removed. They typically last 5 to 10 years. Their main drawbacks are technique sensitivity (moisture contamination during placement can weaken the bond) and the polymerization shrinkage mentioned earlier.
Silver amalgam fillings are metal alloys that have been used for over 150 years. They’re durable, lasting 10 to 15 years on average, and less sensitive to placement technique. However, they require more tooth removal to create the mechanical retention needed, and they don’t match tooth color. Concerns about their mercury content have led to restrictions or outright bans in several countries.
The FDA states that available evidence does not show mercury exposure from dental amalgam causes adverse health effects in the general population. However, the agency strongly encourages non-amalgam alternatives for pregnant women, nursing mothers, children under six, people with kidney impairment, and those with neurological conditions. If you already have amalgam fillings that are in good condition with no decay underneath, the FDA does not recommend removing them preventively.
Ceramic fillings (porcelain inlays or onlays) offer a third option, combining the durability of amalgam (10 to 15 years) with a natural appearance. They’re fabricated outside the mouth and cemented in, making them more expensive but highly resistant to staining and wear.
Sensitivity After a Filling
Some degree of sensitivity after a composite filling is common, occurring in roughly 10 to 15% of posterior (back tooth) restorations. You might notice a sharp twinge when eating hot or cold foods, or a dull ache when biting down. This usually fades within a few days to a couple of weeks as the tooth settles.
How long sensitivity lasts depends on how deep the cavity was, how close the filling sits to the nerve, and your individual biology. Shallow fillings may cause no sensitivity at all, while deep restorations near the pulp can take longer to calm down. Sensitivity that intensifies over time rather than gradually improving, or sharp pain that wakes you at night, suggests something beyond normal healing and warrants a call to your dentist.
How Fillings Fail Over Time
Fillings don’t last forever. The constant pressure of chewing, grinding, and clenching gradually wears them down. Over years, composite fillings can chip or crack, and the seal between the filling edge and the surrounding enamel can break down. Once that seal is compromised, bacteria and food particles work their way underneath, and new decay can develop beneath the old filling. This is called recurrent or secondary decay, and it’s one of the most common reasons fillings eventually need replacement.
You can extend a filling’s life by keeping up with regular brushing and flossing around restored teeth, avoiding using your teeth as tools, and wearing a night guard if you grind your teeth. Your dentist checks your fillings at routine visits for early signs of wear, cracking, or margin breakdown, catching problems before they grow into something that needs a crown instead of a simple replacement filling.