Treating a Cavity: Filling, Onlay or Root Canal

What a Tooth Cavity Is and Why It Forms

A tooth cavity, also called dental caries or tooth decay, is an area of the tooth surface that has been permanently damaged by acid and has broken down into a small hole. It forms when bacteria in dental plaque feed on sugars, produce acid and draw mineral out of the enamel faster than saliva can put it back. Left untreated, a cavity can progress to pain, infection and loss of the tooth.

Several factors push that balance toward decay. Plaque that is not removed by brushing and flossing gives bacteria a place to stay on the tooth. Frequent snacking and sugary or acidic drinks keep the mouth acidic for long stretches of the day, so the enamel has little time to recover. A dry mouth adds to the risk, because saliva is the fluid that neutralises acid and carries minerals back to the tooth surface. Low saliva flow can follow certain medicines, smoking and alcohol use.

Untreated decay in permanent teeth remains a large problem at population level. Using Global Burden of Disease data, one analysis reported that the global age-standardised prevalence of untreated caries in permanent teeth fell only slightly, with the rate moving from 26593.58 to 25625.53 between 1990 and 2019 (PubMed 38871071). The same analysis found the global peak in prevalence in the 20 to 24 year age group and projected a significant upward trend for both sexes from 2020 to 2049 (PubMed 38871071). Decay is therefore a condition that follows people into adulthood, and prevention and early treatment matter at every age.

Early Signs and the Stages of Decay

The early signs of a cavity are sensitivity to hot or cold, a visible pit or hole, a change in the colour of the tooth and pain when eating sweet or hard foods. Many early lesions give no signal at all, and pain usually arrives only once decay has reached the deeper layers of the tooth.

Decay moves through recognisable stages, and each stage calls for a different response. It begins with initial demineralisation, a chalky white area where mineral has been lost from the enamel but the surface is still intact. At this stage the process can be reversed with fluoride and better plaque control. The next stage is enamel decay, in which the surface breaks down and a cavity forms. Because enamel has no living cells, it cannot rebuild a hole once it has opened.

When decay crosses into the dentine, the softer layer beneath the enamel, it spreads faster and sensitivity becomes more common. If it continues, it reaches the pulp, the nerve and blood supply at the centre of the tooth, where infection causes inflammation and often severe pain. The final stage is the formation of an abscess, a collection of pus at the root tip that can spread into the surrounding bone.

Recognising which stage a lesion has reached is what makes the treatment choice possible. An early lesion can often be managed without drilling, while deeper decay needs the tooth to be cleaned and rebuilt. This is also why regular check-ups matter: a lesion found at the white spot stage leaves more options open than one found when it already hurts.

How Dentists Find Cavities Early

Dentists find cavities by combining a visual and tactile examination with imaging, so that lesions can be picked up before they are large enough to be felt or seen by the patient. Each tool covers a part of the mouth that the others see less clearly.

The examination starts with a clean, dry tooth under good light, where early white spots, stained fissures and soft areas can be identified. Digital X-rays show lesions between the teeth, where two surfaces touch and a mirror cannot reach. They also show how close a deeper lesion has come to the pulp, which shapes the treatment plan. Laser fluorescence devices measure changes in the tooth structure on the chewing surfaces and help decide whether a stained groove is active decay. Intraoral cameras record images of the teeth, so the patient can see what the dentist sees and a lesion can be compared at the next visit.

Finding decay early reduces the need for invasive treatment. A lesion that is still confined to the enamel can be treated with fluoride or sealed, while one found later needs a filling or more.

Sealants are a preventive step in this approach. They are thin protective coatings applied to the chewing surfaces of the back teeth, where deep grooves trap food and bacteria that a toothbrush struggles to reach. Sealants are used widely in children once the permanent molars erupt, and they are also recommended for adults who are prone to cavities. They do not replace brushing and fluoride, but they close off a surface where decay often starts.

Tooth Cavity Treatment Options by Stage

Cavity treatment is chosen by how deep the decay has reached: fluoride for early demineralisation, a filling once the enamel has broken, root canal therapy when the pulp is infected, a crown when the remaining tooth is too weak and extraction when the tooth cannot be saved.

Fluoride treatments work at the earliest stage by helping mineral return to softened enamel. Once a cavity has formed, a filling is the standard option. The procedure is carried out under local anaesthesia. The dentist removes the decayed tooth material, cleans the cavity, fills it and then polishes the filling and checks the bite so that the new surface does not sit too high. Filling materials include tooth-coloured composite, amalgam and gold, and the choice depends on the size and position of the cavity.

Root canal therapy is needed when decay reaches the pulp and causes infection. The infected tissue is removed from inside the tooth, the canals are cleaned and sealed, and the tooth is then restored. When decay has destroyed a large part of the tooth, the remaining structure may not be strong enough to carry chewing forces on its own. In that case a crown covers and protects the tooth; crowns made of zirconia are one option. Extraction is reserved for teeth that are too badly damaged to be restored.

Treatment differs between children and adults mainly because of where the decay is found and how far it has gone. In children, the plan often relies on paediatric fillings, sealants and fluoride varnish. Adults more often need fillings, root canal therapy or crowns, depending on the damage.

Preventing Cavities at Home and in the Clinic

Cavities are largely preventable, and the preventive effect of fluoride has been measured in controlled trials. Daily habits at home and professional fluoride in the clinic work together.

The effect of professionally applied fluoride varnish has been measured in randomised trials. A Cochrane review of 22 trials with 12,455 randomised children and adolescents found that fluoride varnish reduced caries increment by 43 percent in permanent teeth and by 37 percent in primary teeth (PubMed 23846772). The review graded this evidence as moderate quality, with substantial heterogeneity between trials (PubMed 23846772).

At home, prevention rests on a few steady habits. Brushing twice a day with fluoride toothpaste removes plaque and brings fluoride to the tooth surface. Cleaning between the teeth once a day reaches the surfaces where the brush does not. Limiting sugary and acidic foods and drinks, and avoiding frequent snacking between meals, gives the enamel time to recover after each acid attack. Choosing water over sugary drinks helps, and chewing sugar-free gum after meals stimulates saliva. Calcium-rich foods such as cheese, milk and leafy greens can be part of a tooth-friendly diet.

Lifestyle also plays a part. Smoking and alcohol can reduce saliva flow and harm gum health, teeth grinding wears down enamel, and stress is linked to poor oral habits and a dry mouth. Natural remedies are often discussed but have limited support. Oil pulling is claimed to reduce bacteria without sufficient clinical evidence, clove oil can give temporary relief from toothache, and fluoride-free toothpaste may not give enough protection against cavities. Home remedies can ease symptoms for a short time, but they do not stop decay that has already formed a cavity, so it is sensible to talk to your dentist before relying on any of them.

Aftercare and When to See a Dentist

After a filling, mild sensitivity to cold or pressure for a few days is common and usually settles as the pulp calms down. Pain that lasts longer, pain on biting or swelling around the gums is a reason to return to the dentist.

On the day of treatment, it helps to wait until the numbness wears off before eating, so the cheek or tongue is not bitten by accident. Sticky and hard foods should be avoided for the first day around a new filling. Oral hygiene continues as normal, with gentle brushing and cleaning around the treated area. If the bite feels high or uneven once the anaesthetic has worn off, the filling may need a small adjustment; leaving it can make the tooth sore and put extra load on the restoration.

Pain that appears days after treatment can mean that the filling is sitting too high or that the pulp needs to be checked, especially after a deep cavity. Sensitivity that grows worse over time, instead of fading, deserves the same attention.

Outside of treatment, there are signs that call for a visit without waiting for the next check-up: persistent tooth pain, visible pits or dark areas on the teeth, and swelling or pus near the gums. Swelling or pus can point to an abscess, which needs prompt care because infection can spread into the surrounding bone. Regular check-ups, even when nothing hurts, are the simplest way to catch decay at the stage where treatment is smallest.