Discolouration of a tooth can be Extrinsic or Intrinsic in origin. Extrinsic discolouration results from the topical effect of certain foods, drinks, tobacco as well as neglected oral hygiene or even some dental hygiene products (e.g. excessive use of Chlorhexidine mouthwashes).
On the other hand, Intrinsic discolouration is one whose origin is from the dental pulp or dentine of the tooth. This includes internal bleeding or nerve death (necrosis) due to trauma, internal blockage of the dentine (sclerosis) resulting in a darker, more opaque tooth colour; and also, discolouration due to dental materials which affect the overlying tooth shade. It is not uncommon to see a tooth darken significantly after a traumatic injury and this process can be gradual so that treatment can often be years later after the injury.
Internal Bleaching is used in the case of internal discolouration and traditional Tooth Whitening (Bleaching) is reserved for more extrinsic causes.
It usually involves exposure of the deeper dental tissues and pulp so in almost all cases the tooth has been devitalised or Root- Treated beforehand. The Root Canal is sealed carefully to avoid irritation of any deeper structures and usually a peroxide-based agent is placed into the tooth where it can remove organic stains etc from the deeper dentine structure and this is then sealed in temporarily. Usually a course of treatments is needed to achieve the correct colour and when finished the access hole is filled in with composite resin material.
It can be a very effective means of achieving harmony in the smile after an individual tooth has darkened, leaving a completely natural result with no artificial materials exposed during smiling (unlike crowns/veneers). The access hole is always at the back of the tooth! The costs are less than hiding the discolouration with porcelain veneers/crowns and it avoids excessive weakening of the tooth structure.