Tooth decay: its stages, symptoms and what is done at each stage

Plaque on the surface of the tooth is made up of bacteria that turn the sugars we eat and drink into acid. The World Health Organization (WHO) defines decay as plaque turning the free sugars in food and drink into acids that destroy the tooth over time; free sugars include sugars added by the manufacturer, cook or consumer, and sugars naturally present in honey, syrups and fruit juices. The WHO writes that, according to Global Burden of Disease 2021 data, untreated decay in permanent teeth is the most common health condition in the world.

Decay is not a one-way process but a balance. The US National Institute of Dental and Craniofacial Research (NIDCR) explains that enamel loses minerals under acid attack, and that this mineral loss can be repaired thanks to the minerals in saliva and the fluoride in toothpaste or applied by the dentist. If more mineral is lost than is regained, the enamel weakens, eventually collapses and a hole forms. That is why the frequency of sugar matters as much as the amount: every sugary thing snacked on throughout the day means a new acid attack.

Cleveland Clinic describes decay in five stages. The first is demineralisation: chalky white spots on the tooth. The second is enamel decay: the spots turn light brown. The third is dentine decay: as the decay reaches the softer layer under the enamel, full of tubules running to the nerve, sensitivity increases and the colour darkens. The fourth is nerve damage: pain starts, redness and swelling may be seen in the gum around the tooth, and the spots turn dark brown or black. The fifth is an abscess: pus collects at the root tip, and the pain can become severe and spread to the jaw and face. Cleveland Clinic writes that at a very early stage decay can be reversed, but that this is only possible before the decay reaches the dentine.

The symptoms vary with the stage, and the most important point is that early decay causes no symptoms. The NHS writes that early decay may have no signs at all, and that as it progresses there may be toothache, a sharp twinge when eating or drinking something hot, cold or sweet, white, brown or black spots on the tooth, and an abscess with a build-up of pus. Because the pain comes late, many people take a tooth to the dentist for the first time only after the nerve has become involved. Even in a tooth with no pain, an advanced cavity between two teeth may be found on an X-ray.

Decay does not progress at the same speed everywhere. Cleveland Clinic distinguishes three types: decay on smooth surfaces, which progresses slowly and can be reversed with good care; decay that starts in the grooves and pits on the chewing surface of the back teeth, usually appears in adolescence and progresses quickly; and root decay, which develops on the root surface exposed in adults with receding gums and is hard both to prevent and to treat. Because the root surface is covered not by enamel but by a softer tissue, it is affected by acid more quickly.

The risk is not the same for everyone. Cleveland Clinic lists dry mouth, sugary and starchy food and frequent snacking, a family history of decay, receding gums and past radiotherapy to the head and neck among the risk factors. Because saliva is the most important defence, neutralising acid and carrying minerals, decay can start in many teeth at once within a short time in people taking medicines that cause dry mouth. The edges of old fillings and crowns are also a common place for decay to start again.

The picture is a little different in children and adults. NIDCR highlights early childhood caries in young children, and decay on root surfaces exposed by gum recession in older adults. Baby teeth hold the place of the permanent teeth and serve chewing and speech; the pain and abscess of a decayed baby tooth can disrupt a child's eating and sleep. In adults, on the other hand, the problem is often underneath old fillings, at the edges of bridges and on root surfaces.

Where does untreated decay lead? When decay reaches the nerve, the tooth becomes inflamed, the nerve may die and an abscess can develop at the root tip. Cleveland Clinic writes that small untreated cavities turn into large ones over time and can lead to infection and tooth loss. How to fill the gap once a tooth has been lost is a separate decision; in mouths where many teeth are lost in the same period, this decision moves beyond individual teeth and becomes a plan for the mouth as a whole.

The exact stage is determined by an examination and an X-ray. The first list covers signs suggesting that the decay is still on the surface or at a level that a filling can resolve; the second list covers signs suggesting that the nerve or the bone is affected, which should not be left to wait.

The sequence starts by ruling out an emergency, then identifies where the decay is and how deep it goes, aiming for the longest life with the least treatment.

1. 1

   Emergency signs first

   If there is rapidly increasing swelling in the face or neck, or difficulty breathing or swallowing, the assessment is done in A&E. If there is swelling of the cheek along with a fever, you go to the dentist the same day.

2. 2

   Examination and X-ray

   The teeth are dried and examined; white spots, changes in colour and softened areas are looked for. Decay between two teeth and underneath old fillings is often visible only on an X-ray.

3. 3

   Fluoride at the white spot stage

   The NHS writes that early decay can be reversed with a fluoride mouthwash or varnish recommended by the dentist. At this stage the aim is not to drill the tooth but to stop the mineral loss.

4. 4

   A filling if a hole has formed

   The decayed tissue is cleaned out and the cavity is closed with filling material. NIDCR writes that cavities that have formed in teeth are most often treated with a filling. If the cavity is very large, an inlay, onlay or crown may be needed.

5. 5

   Root canal treatment if it has reached the nerve

   The NHS writes that root canal treatment may be needed for advanced decay. The inflamed nerve tissue is cleaned out and the canals are filled; afterwards the tooth is often protected by covering its cusps.

6. 6

   Extraction if it cannot be saved

   If the decay has gone deep into the root, or too little tooth tissue is left to support a restoration, the tooth is extracted. The space left by the extracted tooth is planned separately.

7. 7

   Preventing new decay

   If its cause has not been removed, treated decay starts again in other teeth or at the edge of the filling. How often you have sugar, dry mouth and your brushing habits should be discussed at the same appointment.

Which treatment is chosen for a given tooth is determined by the depth of the decay, the condition of the nerve and the amount of sound tooth tissue left. Single-tooth treatments are planned by your own dentist.

Below are wrong decisions often made in this area, and what they cost.

Whether treatment for decay lasts depends on removing the cause. The course below covers the usual course after treatment and advice on prevention.

1. The days after a filling

   There may be mild sensitivity to cold and biting, which is expected to ease within days. If the filled tooth touches first when you bite, ask for a check-up.

2. Daily care

   The WHO recommends brushing twice a day with toothpaste containing 1000–1500 ppm fluoride. The NHS recommends cleaning between the teeth every day too, and cutting down on sugary food and drinks.

3. In children

   The NHS recommends starting to brush as soon as the first baby tooth comes through, using only a smear of toothpaste with at least 1000 ppm fluoride up to the age of 3, and a pea-sized amount of 1000–1500 ppm toothpaste from 3 to 6; it recommends that fluoride varnish be applied at least twice a year from the age of 3.

4. Regular check-ups

   The NHS recommends regular check-ups even when there is no problem. Decide on the interval between check-ups with your dentist according to your risk of decay; if you have dry mouth or a history of frequent decay, the interval may be shorter.

Treatment for decay can vary greatly in scope depending on its stage; we do not give figures here. The main items that determine the scope are:

Stage of the decay

A white spot needs fluoride, a hole a filling, and decay that has reached the nerve root canal treatment and often a crown; each step increases the scope.

Number of decayed teeth

Decay in a single tooth and in many teeth at the same time require different planning and numbers of appointments.

Where the decay is

Decay underneath an old filling, at the edge of a bridge or on the root surface can be more complex.

The gap after extraction

Replacing a tooth that cannot be saved is a separate plan and budget item.