2026-07-22 · 8 min
Dental caries: what it is, stages, types and treatment
Most cavities don't hurt when they start, and that's exactly the problem. By the time pain shows up, the cavity has usually crossed the enamel and moved into the tooth, where repairing it means sacrificing more healthy tissue. In the clinic, and in the operative dentistry I teach, decay is the problem we work with most, and the story repeats almost every day: a lesion that fluoride and a change of habits would have stopped in time, caught late, ends up needing a restoration. This article explains what a cavity actually is, how it advances in stages, how to recognize it, and how it's treated depending on how far along it is.
What a cavity is
A cavity, or dental caries, is the progressive destruction of a tooth caused by bacteria, and it's the result of a chemical process that happens daily in everyone's mouth. A film of bacteria, the plaque or biofilm, lives on the tooth surface. Some of them, above all Streptococcus mutans and lactobacilli, feed on dietary sugars and, in doing so, produce acids. Those acids dissolve the minerals in enamel, the tooth's outer layer and the hardest tissue in the body. When that mineral loss repeats over and over and outpaces the natural repair, the enamel weakens, breaks, and a cavity forms. That's tooth decay.
How it forms: the demineralization cycle
Enamel doesn't dissolve at just any pH. There's a threshold, around 5.5, below which the environment turns acidic enough for the mineral in enamel to start leaching out. Every time you eat something with sugar, the bacteria ferment it and the pH of the plaque drops below that threshold for roughly twenty to thirty minutes. During that window the tooth loses mineral: the demineralization phase. Then saliva steps in, neutralizing the acid, washing away debris, and returning calcium and phosphate to the surface: remineralization. As long as the two phases balance out, the tooth stays healthy. Trouble begins when the balance tips steadily toward loss.
Frequency matters more than amount here. Eating a sweet in one go produces one pH drop and one recovery. Nibbling that same sweet across a whole afternoon keeps the tooth in acid for hours, giving saliva no time to repair. That's why the pattern of consumption weighs as much as what you eat, something I go into in diet and dental health. Acidic drinks add a second front: their own acid attacks enamel directly, with no bacteria needed.
The stages of a cavity
A cavity advances in stages, and recognizing where along that path a lesion sits is what decides the treatment.
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White spot lesion (early or incipient caries): the first sign. Mineral has been lost below the surface, but the enamel hasn't broken yet. It looks like a white, opaque, chalky patch, often near the gum or in the grooves. This is the only reversible stage: with fluoride, hygiene, and fewer acid attacks, the enamel can remineralize and the lesion can stop.
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Enamel cavity: if the process continues, the surface breaks and a real hole appears in the enamel. There's no going back through remineralization now, because the lost tissue doesn't regenerate and a restoration is needed.
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Dentin caries: beneath the enamel lies dentin, softer and less mineralized. Once decay reaches it, it advances faster. This is the stage where symptoms tend to begin: sensitivity to cold, to sweets, or to biting.
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Pulp involvement: at the centre of the tooth is the pulp, with the nerve and vessels. If decay reaches it, inflammation sets in and, with it, intense or spontaneous pain. At this stage a simple restoration is no longer enough.
Types of cavities
Cavities are also classified by where they appear, and each location has its own difficulty:
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Occlusal (on the chewing surface): forms in the grooves and pits of molars and premolars, where the brush reaches worst. It's the most common and, when shallow, among the easiest to spot by eye.
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Interproximal (between two teeth): appears on the contact surface between two teeth, a spot you can't see directly and the brush can't reach. That's why floss matters so much there, and why these cavities usually turn up only on radiographs.
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Root caries: when the gum recedes and exposes the root, that surface, softer than enamel, decays easily. It's a typical adult and older-adult cavity, and one reason decay stays a problem well into adult life.
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Recurrent or secondary: the kind that appears at the edge of an old restoration, where the seal has leaked. It's the most common reason a filling or crown has to be redone.
Childhood caries, including bottle caries, follows a different pattern and belongs to paediatric dentistry; here I focus on the adult tooth.
How to know if you have a cavity
The question I get asked most is how to know if you have a cavity, and the uncomfortable answer is that at first you can't feel it. An early cavity doesn't hurt. There's no warning symptom while the lesion is in the enamel, which is exactly when treatment would be simplest. Pain is a late sign: by the time it appears, the cavity has usually reached the dentin or the pulp.
Even so, there are signs worth watching:
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A white, opaque patch, or a brown or black spot on the tooth surface.
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Sensitivity to cold, heat, or sweets that doesn't settle.
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A hole or rough area you feel with your tongue.
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Food that always traps in the same spot.
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Bad breath or a bad taste localized to one area.
The special case is interproximal cavities, the ones growing between two teeth. You won't see them in the mirror or catch them by eye until they're advanced, because they hide in the contact point. They're found with bitewing radiographs, where the cavity shows up as an area darker than sound enamel. That's why a routine check-up includes X-rays: it's the only way to see what the eye can't reach, and so it belongs in a complete exam.
How it's treated by stage
Treatment depends entirely on the stage, and the logic is always the same: the most conservative intervention that solves the problem.
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White spot lesion: no drill. It's treated by reversing the process, with fluoride applications, better hygiene, and control of how often sugars and acids hit the tooth. Fluoride hardens enamel and drives remineralization of lesions that are starting, and I explain what it's for and how it works. Handled well, an early lesion can stop without losing any tissue.
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Small or medium cavity: once the enamel has broken, the diseased tissue is cleaned out and filled with tooth-coloured composite (resin) in the same session. It's the most common restoration and keeps almost all the healthy tooth.
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Large cavity: if decay destroyed much of the tooth, a direct composite is left without enough support. That's where an inlay or onlay comes in, a custom-made restoration that restores shape and strength while removing far less tissue than a crown.
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Decay that reached the pulp: when the nerve is affected, a root canal is needed to remove the inflamed tissue, and then the tooth is restored. It's the scenario we try to avoid by treating earlier.
How it's prevented
Almost all of the above is avoidable, and preventing cavities is less about tricks than about consistency:
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Fluoride daily: brushing twice a day with fluoride toothpaste is the single best-supported individual measure. Fluoride prevents and, on top of that, helps remineralize lesions that are starting.
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Clean between the teeth: the brush doesn't reach the interproximal space, where many cavities begin. Floss or interdental brushes, daily.
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Sugar frequency matters more than amount: spacing out sugars and acids gives saliva time to repair. Grazing for hours is what does the most damage.
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Regular check-ups and cleanings: in the office we catch early lesions before they cause symptoms, and a professional dental cleaning removes the plaque and tartar the brush leaves behind.
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Sealants: a sealant closes off the deep, hard-to-clean grooves of the molars, a common entry point. They work in adults, just as in childhood.
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Saliva: a dry mouth is a real risk factor. Some medications, mouth breathing, and certain conditions reduce saliva and, with it, the natural defence against decay.
If one idea sticks, make it the time factor. A cavity caught as a white spot reverses without a drill; the same cavity a year later can need a composite, an inlay, or a root canal. None of this hurts until it's late, so don't wait for pain to get checked. A check-up with X-rays every six to twelve months, depending on your risk, is what separates a lesion that stops from one that takes out half a tooth. And if you notice a spot, sensitivity that won't settle, or food that always jams in the same place, book a visit before the problem decides for you.
