Tooth decay and cavities

Tooth decay is a process. A cavity is the hole it eventually makes. The distinction is the most useful thing to know about it, because the process is reversible for a surprisingly long time and the hole never is.

How decay actually works

Plaque — a soft film of bacteria — re-forms on clean teeth within hours. When free sugars reach it, the bacteria ferment them and produce acid. As the acid builds, the pH at the tooth surface falls, and below roughly 5.5 the enamel starts to give up mineral into the surrounding fluid. That is demineralisation.

Then saliva does the repair. It neutralises the acid, washes away what is left of the sugar, and carries dissolved calcium and phosphate back into the enamel surface. The whole recovery takes somewhere between twenty minutes and an hour after each exposure, depending on what was eaten and how much saliva there is.

This is why frequency matters more than quantity. One acid attack is comfortably handled. Six, spread across a day, keeps the surface losing mineral for much of it, with no time to recover between. The same amount of sugar eaten at one sitting does considerably less damage than the same amount grazed, sipped and nibbled from mid-morning to bedtime. A biscuit with lunch is not the problem; a biscuit every hour is.

One mechanism worth stating plainly, because it is often described the other way round: brushing does not neutralise the mouth. Saliva does that. What brushing does is remove the plaque that produces the acid in the first place, and deliver fluoride — which slows mineral loss, speeds repair, and leaves the repaired surface less soluble than the original. That is a different reason to brush, and a better one. Fluoride and teeth covers it.

The stages, and why pain comes so late

The white spot. Demineralised enamel looks chalky and dull, often along the gum margin or around a bracket. It does not hurt, and it is reversible: with fluoride, less frequent sugar and better cleaning, the surface can remineralise. A dentist who notices one and advises rather than drills is treating it correctly.

Cavitation. If demineralisation continues, the weakened surface collapses and a hole forms. From this point the tooth cannot repair itself and the lost structure has to be replaced.

Into the dentine. Dentine is softer, decays faster, and is sensitive. This is where the first symptoms usually appear — a sharp twinge with something sweet or cold, which stops when the stimulus is removed.

Into the pulp. When bacteria reach the nerve and blood supply at the centre, the result is severe toothache, often worse with heat and worse lying down. If the pulp then dies, the pain may stop for a while before returning as an abscess — infection spreading out through the end of the root into the bone.

The useful conclusion is that decay is painless for most of its course. By the time a tooth hurts, the cheap fix has usually been missed. Decay found at an examination is almost always found before it announces itself, which is the whole case for going when nothing is wrong.

Where it happens

Decay is not random. It starts where plaque sits undisturbed: in the pits and grooves on the biting surfaces of molars, on the surfaces where two teeth touch, at the gum margin, around the edges of existing fillings and crowns, and on root surface exposed by gum recession. Root surface has no enamel and decays faster, which is why decay in older people often appears at the gum line on teeth that have been sound for decades.

Who is at higher risk

Not everybody equally. The things that genuinely shift the odds:

• Having had decay before — the single best predictor of future decay.
• A dry mouth. Many common medicines reduce saliva, as do some conditions and some cancer treatments. Without saliva the repair side of the process barely functions, and decay can appear with startling speed.
• Frequent sugar, especially sipped drinks, and anything held in the mouth for a long time.
• Fixed orthodontic appliances, which make thorough cleaning much harder.
• Gum recession, exposing root surface.
• Needing help with daily care, whether from limited dexterity, illness or dependence on others.
• Smoking, and repeated exposure to stomach acid from reflux or frequent vomiting.

Anyone in several of these categories should say so at the dentist, because the preventive measures available — high-fluoride toothpaste on prescription, more frequent varnish, shorter recall intervals — are real and are usually offered on request rather than volunteered.

Erosion, which is not decay

These get confused constantly, and the advice differs.

Decay is acid produced by bacteria from sugar. Erosion is acid arriving in the mouth ready-made — from fizzy drinks, fruit juice, squash, sports and energy drinks, citrus fruit, vinegar and wine, or from stomach acid in reflux or vomiting. No bacteria and no sugar are required, which is why sugar-free fizzy drinks cause no decay at all and erode teeth just as effectively as the sugared kind. That is one of the most widely missed facts in dental health.

Erosion does not make holes. It thins and smooths the enamel generally: edges of front teeth become translucent and chip, the biting surfaces of molars develop shallow cups, existing fillings begin to stand proud of the tooth around them, and the teeth become sensitive across the board rather than in one spot. It is not reversible either.

What reduces it: acidic drinks with a meal rather than through the day, through a straw rather than swilled around the mouth, finished with water or plain milk, and no brushing for an hour afterwards, since softened enamel brushes away. Persistent reflux is worth treating for the teeth as well as for everything else.

What treatment involves

Early lesions are not automatically drilled. Fluoride varnish, high-fluoride toothpaste, a change in how often sugar is eaten and a review in a few months is appropriate treatment for a lesion that has not broken the surface, and it preserves tooth that drilling would remove. The first filling in a tooth starts a cycle: every restoration has a finite life and each replacement is larger than the last, so delaying the first one is worth something.

Fissure sealants — resin flowed into the grooves of permanent molars to leave nothing for plaque to sit in — are offered to children at higher risk, and are quick and painless.

Fillings. Tooth-coloured composite is now standard for most situations, bonded to the tooth and set with a light. Glass ionomer releases fluoride and is useful at the gum margin and in children. Amalgam is extremely durable and has been used for well over a century, but its use has been declining and is restricted for children and during pregnancy and breastfeeding, under an international agreement to phase down mercury.

In children's first teeth, treatment is often less invasive than people expect. A decayed back tooth can be sealed under a preformed metal crown fitted without drilling or an injection, which arrests the decay and keeps the tooth until it would have come out anyway. At the other end, decay left too long in a small child means extractions under general anaesthetic — which is where a great deal of avoidable childhood suffering actually happens.

Where decay has gone further, the options are a crown, root canal treatment or extraction.

Toothache

What the pain is doing is informative.

• A sharp twinge with cold or sweet that stops immediately usually means exposed dentine or a cavity. It needs an appointment, not an urgent one.
• Pain that lingers after the stimulus, throbs, is worse with heat, or wakes someone at night suggests the pulp is involved. That needs seeing promptly, and it will not settle on its own.
• Any facial swelling needs attention the same day — and urgent hospital attention if it is spreading, closing an eye, or making swallowing or opening the mouth difficult.

Paracetamol or ibuprofen taken by the label will hold most toothache for a while. Clove oil, whisky and aspirin held against the gum will not, and the last of those burns the tissue. Antibiotics do not treat toothache: they may damp down an infection temporarily while the cause goes untreated, and the cause needs a dentist rather than a GP. Where no practice is available, NHS 111 can direct people to an urgent dental service.

See also cleaning teeth and gums, gum disease, dental check-ups and dental care for babies.