Dental Caries: Types, Causes and Risk

How dental caries develops

Caries needs four things at once: a tooth surface, dental plaque (a sticky bacterial film, with Streptococcus mutans and Lactobacillus species prominent), fermentable carbohydrates (sugars and refined starches), and time.

Each time you eat or drink sugar, plaque bacteria make acid within minutes. Plaque pH falls below about 5.5, the level at which enamel starts to dissolve (demineralize). Saliva then buffers the acid over 20 to 40 minutes and returns calcium and phosphate to the tooth (remineralization). Fluoride speeds repair and makes the rebuilt enamel more acid-resistant.

If sugar exposures are frequent, or saliva is reduced, mineral loss outpaces repair. The sequence is:

1. White-spot lesion: chalky, opaque enamel. Still reversible.
2. Enamel cavity: the surface breaks down.
3. Dentin caries: decay spreads faster in the softer dentin, often undermining enamel.
4. Pulp involvement: inflammation of the nerve (pulpitis), then possible dental abscess.

For an everyday guide to signs and stages, see tooth decay.

Types of dental caries

Dentists also describe caries as active (soft, yellowish, progressing) or arrested (hard, dark, stable). Arrested lesions often only need monitoring.

Risk factors

Your caries risk is assessed at checkups, and it decides how often you need X-rays and extra prevention.

Diet

• How often you have sugar matters more than the total amount. Sipping a sweet drink over hours keeps plaque acidic.
• Sticky, starchy snacks and sugary drinks, including juice and sports drinks.

Saliva and medical conditions

• Dry mouth (xerostomia): from medicines such as antidepressants, antihistamines and blood pressure drugs, from radiation therapy, or from Sjögren's syndrome.
• Acid reflux and vomiting: GERD and eating disorders cause acid erosion that weakens enamel.
• Diabetes, and limited ability to brush from arthritis, disability or dementia.

Other factors

• Low fluoride exposure (non-fluoridated water, no fluoride toothpaste).
• Recent cavities: the strongest predictor of future cavities.
• Orthodontic braces, deep fissures, gum recession.
• In infants, transfer of decay-causing bacteria from a caregiver's saliva, and bedtime bottles.

How caries is diagnosed

• Visual and tactile exam: after drying the teeth, the dentist looks for white spots, shadowing and cavitation. Many use the ICDAS scoring scale from early enamel change to extensive cavity.
• Bitewing X-rays: show decay between back teeth, where it cannot be seen.
• Other aids: transillumination (shining light through the tooth) and laser fluorescence devices.
• Pulp tests: cold tests and tapping check whether the nerve is inflamed or dead.

Treatment

Modern dentistry favors minimal intervention: stop the disease first, then repair only what is needed.

Non-invasive

• Fluoride: toothpaste with 1,000 to 1,500 ppm fluoride for most people; high-fluoride prescription toothpaste and fluoride varnish for high-risk patients.
• Sealants: a plastic coating on molar grooves that prevents and can arrest early fissure caries.
• Silver diamine fluoride (SDF): a liquid painted on cavities that can arrest decay without drilling. It stains the decayed area black. Useful for young children and frail older adults.
• Resin infiltration: seals early lesions between teeth.

Restorative

• Fillings: tooth-colored composite, glass ionomer (releases fluoride, good for roots) or amalgam.
• Crowns: for extensively decayed teeth.
• Root canal treatment: when decay reaches the pulp.
• Extraction: when a tooth cannot be saved.

Fillings repair the hole but do not cure the disease. Without changes in diet, fluoride use and cleaning, new caries develops at the edges.

Prevention

• Brush twice daily with fluoride toothpaste; spit, do not rinse, so fluoride stays on the teeth.
• Clean between teeth daily.
• Keep sugary foods and drinks to mealtimes; limit snacks to a few a day.
• Drink fluoridated tap water where available.
• Chew sugar-free gum, ideally with xylitol, after meals to boost saliva.
• Children: first dental visit by age 1; no bottle in bed; sealants on permanent molars.

Frequently asked questions

What is the difference between dental caries and a cavity?

Dental caries is the disease process: acid from plaque bacteria dissolving the tooth over time. A cavity is the hole that forms when that process breaks through the surface. You can have caries (an early white-spot lesion) without having a cavity yet.

Can dental caries be reversed?

Early caries, seen as chalky white spots with an intact surface, can be reversed with fluoride, less frequent sugar and better plaque control. Once a cavity forms, the lost tooth structure cannot grow back, though the decay can be stopped and the tooth restored.

What bacteria cause dental caries?

Streptococcus mutans is the best-known species, with Lactobacillus species important in deeper decay. Caries is really a disease of the whole plaque community, which shifts toward acid-producing bacteria when sugar is frequent.

What is root caries?

Root caries is decay on the root surface of a tooth, exposed when gums recede. Root surfaces are softer than enamel and decay faster. It is common in older adults and people with dry mouth, and is prevented with high-fluoride toothpaste and good cleaning at the gumline.

Why do I keep getting cavities even though I brush?

Frequent sugar or acid exposure, dry mouth from medicines, acid reflux, gum recession and old fillings with gaps can all keep caries active despite brushing. Ask your dentist for a caries risk assessment, a high-fluoride toothpaste and a review of your diet and medicines.