Teeth whitening

People tend to picture whitening as cleaning a stain off a shirt — scrubbing the surface layer clean. In fact enamel is close to translucent; the colour you see is mostly the dentine underneath showing through it.

Hydrogen peroxide passes through the enamel into the dentine and breaks the pigment molecules there into smaller fragments that reflect less light, so the tooth looks lighter. Nothing is abraded away in the process — which is why properly performed whitening does not thin the enamel.

One consequence follows directly: anything that is not tooth structure does not whiten. Composite fillings, crowns and veneers keep their original shade. If you have restorations in your smile line, whiten first and then replace them to match the new shade — doing it the other way round leaves a permanent mismatch.

Dentine contains thousands of microscopic tubules running straight to the pulp. During whitening those tubules temporarily widen and the pulp reacts — which is felt as sharp twinges, usually in the first 24 to 48 hours, and then it stops. This is a reversible response, not an injury.

What makes it worse: gel concentration too high, gel left on too long, untreated cervical wear or enamel cracks, and older lamps that generate heat in the pulp.

The clinical way to control it is to split treatment into several short cycles and ask the patient how they are between them, rather than leaving gel on continuously for the full time; to use a lamp that produces no heat; and to apply a remineralising fluoride or potassium nitrate gel immediately afterwards.

Yellowing from food, coffee, tea and tobacco responds best — this is the group that gets the clear 4 to 8 shade improvement. Brownish discolouration responds more slowly and needs more cycles.

Greyish discolouration, particularly from childhood tetracycline exposure, responds worst. The grey is banded through the dentine and may lighten somewhat, but the bands remain. For this group the honest advice is to consider veneers or crowns, not to sell more whitening courses.

A single tooth noticeably darker than the rest is usually a tooth whose nerve has died. External whitening does almost nothing in that case; the correct approach is internal bleaching — placing the whitening agent inside the pulp chamber after the root canal treatment has been completed properly.