Tooth decay: how it starts and what stops it

How decay starts

Plaque is a sticky film of bacteria that forms on the teeth every day. When food or drink containing sugars reaches the mouth, these bacteria turn the sugars into acid. The acid draws minerals out of the enamel, the hard outer layer of the tooth. This is called demineralisation, and each acid attack can last for up to about an hour.

Saliva works in the other direction. It neutralises the acid and returns minerals to the enamel, and fluoride helps it do so. Decay begins when the balance tips towards mineral loss: sugar taken often, plaque left on the teeth, or too little saliva, for example because of a medicine that dries the mouth.

How often sugar is taken matters more than how much. Sugar kept to mealtimes gives the teeth time to recover between attacks. Frequent snacks and sweet drinks sipped through the day keep them under attack for much longer.

How decay progresses

The earliest sign is a chalky white spot on the enamel. The surface is still whole, and at this stage decay can be stopped and partly reversed with fluoride, better cleaning and less frequent sugar.

If the attacks continue, the surface breaks down and a hole forms: a cavity. Under the enamel lies dentine, which is softer and contains tiny channels leading towards the nerve, so decay spreads faster there and the tooth may become sensitive. When decay reaches the pulp (the soft tissue with nerves and blood vessels at the centre of the tooth), it becomes inflamed and can die. Infection can then spread to the bone at the root tip and form an abscess.

Decay most often starts in the grooves on the chewing surfaces of the back teeth, between the teeth, along the gum line and around the edges of old fillings. Roots exposed by receding gums are covered by a softer tissue than enamel and decay more easily.

Signs to look out for

Early decay usually has no symptoms. As it progresses there may be:

• a twinge with sweet, hot or cold food and drink;
• toothache, or pain when biting;
• white, brown or black spots, or a visible hole;
• food catching between two teeth;
• swelling of the gum or face, or a bad taste, if an abscess has formed.

Regular check-ups find decay before it causes symptoms, and X-rays show decay between the back teeth that cannot be seen by eye.

How decay is usually treated

Treatment depends on how far the decay has gone. Current practice keeps as much healthy tooth as possible: the dentist removes the decayed tissue and makes the cavity no larger than it needs to be.

1. Early decay. Fluoride varnish, a higher-strength fluoride toothpaste or mouthwash, better cleaning and less frequent sugar. Sealants, thin coatings flowed into the grooves of the back teeth, can protect chewing surfaces.

2. A cavity. Under local anaesthetic, the decayed tissue is removed and the tooth is filled, usually with a tooth-coloured material. In very deep decay, a thin layer of softened dentine may be left over the pulp and sealed under the filling, so that the pulp is not exposed.

3. Extensive damage. When a large part of the tooth has been lost, an onlay or a crown may be needed to hold it together.

4. Pulp affected. If the pulp is inflamed beyond recovery or infected, root canal treatment removes it, and the tooth is then restored.

5. Tooth cannot be restored. The tooth is removed, and the gap can be left or filled with a bridge, a denture or an implant.

Limits of treatment

A filling repairs the damage but does not remove the cause. New decay can form around the edges of a filling, fillings wear and chip, and each time one is replaced the cavity tends to grow a little. Sensitivity after a filling is common and usually settles. A deep filling can irritate the pulp, which occasionally then needs root canal treatment. The habits that led to the decay matter as much as the filling itself.

Preventing decay

• Brush twice a day, last thing at night and at one other time, with a fluoride toothpaste. Spit out after brushing rather than rinsing.
• Clean between the teeth every day with floss or interdental brushes.
• Cut down how often sugary food and drink are taken, and keep them to mealtimes. Water and plain milk are kinder to the teeth between meals.
• Keep check-ups as often as the dentist recommends. Fluoride varnish and sealants may be advised for people at higher risk, including many children.
• Mention a dry mouth to the dentist or doctor, especially if it started with a new medicine.

The World Health Organization recommends that adults and children keep sugars added to food and drink, together with the sugars naturally present in honey, syrups and fruit juices, below 10% of their daily energy intake. It suggests that going below 5% brings further benefits, including for the teeth. Much of this sugar is hidden in soft drinks, sweetened coffee and tea, and processed foods.

When decay reaches the pulp, root canal treatment may keep the tooth. For the daily routine that protects teeth and gums, see cleaning your teeth properly.