Bottom line: Cavities form when acid (mainly lactic acid) attacks tooth minerals (calcium and phosphate crystals) and, over time, dissolves them into saliva.
For a fuller understanding—and to know how to stop the process—we need to understand the composition and structure of the minerals that make up the tooth.
Remember the tug-of-war analogy from the previous article?
Here we use another analogy: tooth structure is like a honeycomb. The surface feels very smooth, but microscopically—like honeycomb—the tooth has many tiny pores surrounded by hard tissue. Unlike honeycomb, which is mostly airy with little hard material (so cells can fill with honey), our teeth are mostly densely packed mineral crystals in a matrix, with many small pores distributed throughout.
Those crystals are elongated, rod-like structures. When dissolution begins, it advances inward along those rod-like paths.
What are our teeth made of?
Tooth composition is about 86% minerals, 2% protein, and 12% water.
The main mineral crystal in the tooth matrix is hydroxyapatite (HA). Each HA crystal is built from 10 calcium atoms and 6 phosphate atoms arranged in a dense crystalline structure with oxygen and hydrogen (for chemistry enthusiasts: Ca10(PO4)6(OH)2). Other minerals can play roles in the crystal, including carbonate, sodium, and fluoride.
How do we “lose” minerals from our teeth?
Calcium and phosphate dissolve from teeth on acid exposure. Where does the acid come from? Two main sources: acidic food or drink (soft drinks, lemon, sour candies, and more) and “bad” oral bacteria that produce acid and are linked to caries. Whatever the source, the result is the same—dissolution of the crystals that form tooth structure.
How do cavities form?
To understand cavity formation, a brief detour into gum disease helps: bacteria like to settle and multiply in gingival “pockets” (the gum margin forms a pocket-like space where food debris and bacteria collect). The pocket structure lets them hide from toothbrush bristles that struggle to reach during routine brushing. If bacteria tried to harm us “in the open,” we would catch them more easily. Unsurprisingly, cavity-causing bacteria behave similarly.
Cavities develop beneath the surface
In the mouth, certain bacteria—especially Streptococcus mutans—drive mineral loss because they secrete lactic acid. Acid dissolves crystals and, over time, creates a cavity.
Those bacteria are not scattered randomly; they gather in large groups and, with food debris, form a sticky deposit called plaque. The more we routinely miss the same areas during cleaning, the more plaque that remains “ages.” Mature plaque full of acid-producing bacteria keeps acidity high under sticky layers, causing more damage. Acid also stays trapped under plaque, contacting the tooth longer. As minerals dissolve and leave the surface, the surface area grows and makes more room for bacteria. S. mutans advances through tiny surface pores like coal miners through tunnels. Those pores create rod-like structures toward the center of the tooth, as described above. Destruction therefore progresses beneath the surface under the outer enamel layer, and most of the process is invisible. At this stage saliva cannot reach the hidden damage, so it cannot remineralize the affected areas.
The key to stopping cavities
The bacterial “crew” tunneling under the surface still needs to eat. They keep a narrow passage (a kind of tunnel) to the surface for food supply. The work under the surface—decay of tooth tissue—may show nothing dramatic from outside. Sometimes a white, unimpressive mark appears: a “white spot lesion”—a red flag that balance is broken and cavity formation has begun. If demineralization wins the tug-of-war long enough, the honeycomb structure collapses into a true cavity.
But do not despair: even after a cavity starts, as long as it stays within the outer enamel layer, remineralization is still possible. The affected area will not look fully “healthy” again and the hole will not refill completely, but you can stop progression and strengthen remaining tissue so it better resists future acid attacks.
Stay tuned for Part 3 of the series, where we connect the puzzle pieces and explain how to stop cavity development and reverse the damage. Continue to Part 3.