Dental Caries: Formation, Stages, and Treatment

Dental caries, commonly called tooth decay or cavities, form when bacteria on your teeth produce acid that dissolves the hard mineral surface of enamel over time. The process is not sudden; it moves through recognizable stages, from invisible chemical changes to a visible white spot, then to a soft brown cavity in the enamel, deeper erosion into the underlying dentin, and eventually infection of the tooth’s inner pulp. Each stage changes what treatment looks like, and the earliest stages can actually be reversed without a drill.

How a Cavity Actually Forms

Your mouth is home to hundreds of species of bacteria, but one in particular gets most of the blame for cavities: Streptococcus mutans. This bacterium feeds on sugars left on your teeth, producing lactic and other organic acids as waste products. Those acids pull calcium and phosphate ions out of the enamel’s mineral structure, a process called demineralization. S. mutans is especially good at building sticky biofilms (what you feel as dental plaque) made of glucan polysaccharides, extracellular DNA, and proteins, and those biofilms trap acid right against the tooth surface where it does the most damage.¹ Studies isolating S. mutans from dental plaque have found that roughly a third of plaque-derived isolates are strong biofilm formers.²

A number you’ll hear often is a “critical pH” of about 5.5, below which enamel supposedly starts dissolving. That number is a useful shorthand, but it is not a fixed threshold. The actual point at which enamel begins to break down depends on the concentrations of calcium, phosphate, and fluoride in the fluid immediately surrounding the tooth. Someone with fluoride-rich saliva or enamel may tolerate a lower pH before mineral loss kicks in, and someone with lower mineral levels in their saliva may see demineralization start at a higher pH.³ Even the type of acid matters: laboratory work has shown that at the same pH and degree of undersaturation, lactic acid causes faster mineral loss than acetic or propionic acid, suggesting it interacts with enamel mineral in a more aggressive way.⁴

Why Saliva Is Your Best Natural Defense

Between acid attacks, your saliva works to repair what was lost. It carries calcium and phosphate that can redeposit into the enamel surface, and its bicarbonate content buffers the pH back toward neutral. This is why dry mouth (from medications, medical conditions, or simply not drinking enough water) is such a strong risk factor for cavities. Research comparing children without cavities to children with active decay has found that the cavity-free group had significantly higher salivary flow rates, higher pH, and stronger buffering capacity, while children with the most caries had thicker, slower-flowing saliva.⁵ Across the broader literature, the pattern holds: people with pathologically low saliva flow, weaker buffering, or high counts of S. mutans in their saliva consistently show more cavities.⁶

This also explains why chewing sugar-free gum after meals helps: it stimulates saliva production and speeds up the neutralization of acid. Anything that keeps your mouth moist and mineral-rich gives enamel a better shot at recovering between meals.

Sugar, Frequency, and Why Snacking Matters

You’ve heard that sugar causes cavities, but the relationship is a bit more specific than “eat less sugar.” A large study of U.S. adults found that the total amount of added sugar consumed was more consistently and strongly linked to dental caries than the frequency of consumption alone.⁷ Still, frequency is not irrelevant. In a study of UK children, those who consumed sugar-added foods four or more times a day were about half as likely to be caries-free compared to children with a sugar-free diet.⁸

The practical takeaway is that both how much sugar you eat and how often you eat it contribute, but constant snacking or sipping sweet drinks throughout the day is particularly damaging because it gives your saliva no time to recover and remineralize between acid exposures. A piece of cake with dinner is less harmful than slowly nursing a sugary coffee for three hours at your desk.

The Stages of Decay, From Invisible to Irreversible

Cavities don’t appear overnight. The progression moves through distinct phases, and recognizing them changes what you and your dentist can do about it.

White Spot Lesions

The first visible sign of enamel demineralization is a chalky white patch on the tooth surface, called a white spot lesion. The enamel is still intact at this point, but the subsurface has lost mineral and become porous. This is the one stage where the process is fully reversible: fluoride application, improved oral hygiene, and dietary changes can drive minerals back into the weakened area and restore it.⁹ White spot lesions are common after orthodontic braces are removed, because the brackets make cleaning difficult and plaque accumulates around them.

Enamel Cavitation

If demineralization continues unchecked, the weakened enamel eventually collapses and a physical cavity forms. At this point, the damage is no longer reversible by remineralization alone. The cavity creates a sheltered environment where bacteria and food debris accumulate, making it harder to clean and accelerating further breakdown. However, if the cavity is still confined to enamel and hasn’t yet reached the softer dentin underneath, treatment can sometimes be minimally invasive: resin infiltration, for example, fills the porous enamel with a liquid resin that seals it without drilling.¹⁰

Dentin Involvement

Once the decay passes through the enamel and reaches dentin, things speed up. Dentin is softer and more porous than enamel, and it contains tiny tubes (dentinal tubules) that lead toward the tooth’s nerve. Bacteria can travel along these tubes, and this is when you may start feeling sensitivity to hot, cold, or sweet stimuli. The tooth’s living tissue tries to fight back: specialized cells called odontoblasts lay down new dentin (called tertiary dentin) between the advancing decay and the pulp. If the cavity progresses slowly, this defensive dentin tends to be well-organized and tubular, resembling normal dentin. If the decay advances quickly, the defensive response is chaotic, producing atubular or irregular dentin, or in some cases no new dentin at all.¹¹ This is why rapidly progressing caries is more likely to lead to pulp damage and root canal treatment.

Pulp Infection and Beyond

When decay reaches the pulp, the soft tissue containing nerves and blood vessels inside the tooth, infection sets in. This usually brings significant pain, and the inflammation can become irreversible. Without treatment, the infection can spread past the root tips into the surrounding bone, forming an abscess. At this stage, the options narrow to root canal treatment (removing the infected pulp and sealing the canals) or extraction. A 25-year cohort study tracking root-canal-treated teeth found that the risk of eventually losing the tooth was significantly higher when there was a combined periodontal-endodontic lesion, when there were pre-existing complications, or when the root canal treatment itself failed clinically.¹²

How Dentists Detect and Stage Caries

For decades, the standard approach was a visual exam plus bitewing X-rays, which are still the backbone of caries detection. But the profession has shifted toward more granular staging systems. The International Caries Detection and Assessment System (ICDAS) scores lesions on a scale that captures everything from the earliest visible white spot through to extensive cavitation, and it also characterizes whether the lesion is active or arrested.¹³ Validation studies have shown the system is reasonably reproducible between examiners and has good diagnostic accuracy, particularly for dentin-level lesions.¹⁴

Newer technology is also entering the picture. Near-infrared light transillumination (NILT) shines light through the tooth and detects cavities based on how the light scatters in demineralized tissue. A systematic review and meta-analysis found that NILT has reasonably comparable accuracy to bitewing radiographs for detecting cavities between teeth, with the advantage of being radiation-free.¹⁵ It is not a replacement for X-rays yet, but it is increasingly used as a complement.¹⁶

Treatment Options by Stage

The range of treatments for caries has expanded well beyond “drill and fill,” though fillings remain the workhorse for cavitated lesions.

Fluoride and Silver Diamine Fluoride

For white spot lesions and very early decay, topical fluoride remains the first-line defense. Fluoride ions are smaller than the hydroxyl ions they replace in the enamel crystal, which makes the resulting mineral (fluorapatite) more tightly packed and more resistant to acid attack.¹⁷ Professional fluoride varnish, prescription-strength toothpaste, or fluoride gels can tip the balance back toward remineralization when a lesion is caught early.

For situations where drilling is impractical or undesirable, such as very young children or patients with limited access to dental care, silver diamine fluoride (SDF) has become an important tool. SDF works through a dual mechanism: the silver component kills cariogenic bacteria and inhibits biofilm growth, while the fluoride promotes remineralization. Studies have confirmed it is bactericidal to S. mutans, reduces mineral loss in both enamel and dentin, and protects the collagen structure of dentin from enzymatic breakdown.¹⁸ The catch is cosmetic: SDF permanently stains decayed tissue black. On a baby tooth that will fall out anyway, or on a back molar that nobody sees, this is a reasonable tradeoff. On a front tooth in an adult, it can be a harder sell.¹⁹

Fillings and Restorations

Once a cavity has progressed beyond what remineralization or infiltration can fix, restorative treatment is needed. The two main options for direct fillings are composite resin (tooth-colored) and glass ionomer cement. Randomized trials comparing the two in back teeth have found they perform similarly in terms of retention and secondary caries prevention over follow-up periods of two to four years.²⁰²¹ Even over ten years, both materials show comparable performance in retention, marginal adaptation, and resistance to secondary caries, though glass ionomer restorations tend to show more color change over time.²² Glass ionomer has the added advantage of releasing fluoride into the surrounding tooth structure, which may help prevent new decay at the margins. Composite resin looks better, blending in with the natural tooth color.

Root Canals and Crowns

When decay reaches the pulp, a filling alone is not enough. Root canal treatment removes the infected or inflamed pulp tissue, cleans and shapes the root canals, and seals them. The tooth then usually needs a crown to restore its strength, since a pulp-treated tooth is more brittle. Success rates for root canal treatment are generally high, but long-term tooth survival depends on several factors. The 25-year cohort study mentioned earlier found that teeth with a good operator-rated prognosis at the time of treatment had roughly half the risk of eventually being extracted compared to those with a poor prognosis.¹²

Early Childhood Caries

Young children are vulnerable to a particularly aggressive pattern of decay known as early childhood caries (ECC), sometimes called “baby bottle caries.” ECC often starts on the upper front teeth and can spread rapidly. The risk factors include cariogenic bacteria (often transferred from a parent to the child), frequent sugar exposure, and inadequate cleaning. Night-time bottle feeding is a major contributor: one study found the odds of ECC were about four and a half times higher in children who were bottle-fed at night, while breastfeeding for three to six months was protective compared to breastfeeding beyond twelve months.²³ Between one and twelve percent of children under six in developed countries are affected.²⁴

Because baby teeth have thinner enamel than adult teeth, decay can reach the pulp much faster. And while baby teeth are temporary, losing them prematurely can cause spacing problems for the permanent teeth coming in behind them, affect speech development, and lead to pain and infection that disrupts a child’s eating and sleep.

Root Caries in Older Adults

At the other end of the age spectrum, older adults face a distinct pattern of decay: root caries. As gums recede with age or periodontal disease, the root surfaces of teeth become exposed. Root surfaces are covered in cementum and dentin rather than enamel, and these materials are softer and more susceptible to acid attack. Add in reduced salivary flow from medications (many common drugs for blood pressure, depression, and allergies cause dry mouth), decreased manual dexterity that makes brushing harder, and dietary shifts, and the conditions are ripe for decay on surfaces that were previously protected.²⁵

Root caries can be controlled at a population level with daily fluoride toothpaste, and active root decay can be arrested with professional fluoride varnish applications or prescription-strength fluoride toothpaste. When an active lesion can’t be kept clean by the patient, minimally invasive restorative techniques are preferred.²⁶ Risk factors such as tobacco use and poor brushing technique have been shown to significantly influence root caries prevalence in the elderly, and men with long-term deleterious habits were more affected than women in at least one population-level study.²⁷

What the Oral Microbiome Tells Us About Prevention

An emerging line of research looks at cavities not just as a sugar-plus-bacteria problem, but as a microbiome problem. The mouth of a healthy person hosts a diverse community of bacteria that keeps itself in check. Cavities develop when that community shifts toward acid-producing species like S. mutans at the expense of protective species. Ancient DNA studies paint a dramatic picture of this shift over human history: analysis of calcified dental plaque from 34 early European skeletons showed that the transition from hunter-gatherer to farming diets pushed the oral microbiome toward a disease-associated composition. Remarkably, it stayed in that configuration through the medieval period, and cariogenic bacteria only became truly dominant during the Industrial Revolution, coinciding with the mass availability of refined sugar.²⁸

This ecological view of caries has prompted research into probiotics and targeted antimicrobials as prevention strategies. Probiotics derived from the dental plaque of healthy individuals have been shown in laboratory settings to sharply antagonize S. mutans, while targeted antimicrobials aim to selectively eliminate cariogenic species and allow a healthy microbial community to reestablish itself.²⁹ The idea of using prebiotics to selectively feed beneficial oral bacteria is also under investigation.³⁰ None of these approaches have replaced fluoride and brushing in clinical practice yet, but they represent a shift in thinking: instead of trying to sterilize the mouth, the goal is to nudge it back toward a healthier balance. Whether that approach will translate into practical products for everyday use remains an open question, but the science behind it is grounded in real microbiology rather than wishful thinking.