Reasons why a single tooth might become discolored.
Root canal treatment
There are times when a single tooth has experienced an event or undergone changes and as a result has become individually darker than its neighboring teeth. As an example, it is commonplace that a tooth that has had root canal treatment will darken over time.
Tooth trauma
Another example is a tooth that has been traumatized, such as having been bumped in an accident. In this case the darkening of the tooth can be an indication that there is a problem with the health of the nerve inside the tooth. And for this reason, any individually darkened tooth should always be evaluated by a dentist.
Some tooth discolorations are associated with existing dental work.
Sometimes it is a person's existing dental work that is the source of the tooth staining, or relative tooth discoloration, that they have noticed.
Existing white dental fillings may stain.
It is the nature of white dental fillings that they will discolor as they age. While teeth whitening treatments can be used to lighten the color of tooth enamel, they
cannot be expected to produce a color change in dental work (with a few exceptions). In the case of white fillings that have stained, the solution your dentist will have to offer is to replace them.
A person's natural teeth may discolor over time and therefore no longer match previously placed dental work.
There can be cases where the color of a person's existing dental restorations has remained stable over time but the person's untreated teeth have become stained or discolored. The net result is a cosmetic mismatch.
This type of situation is especially commonplace in situations where dental crowns are involved. The glass-like nature of porcelain will not pick up stains over time like tooth enamel can. This means that a dental crown that matched its neighboring teeth perfectly when it was first place will, as the person's untreated teeth undergo the natural and gradual staining process, appear too light.
Tooth enamel color changes caused by poor oral hygiene.
White spot lesions.
Some people will notice, in comparison to that which was present in previous months or years, that certain locations on the surface of their teeth have developed an opaque whitish hue. Often this change has occurred on that portion of the tooth that lies nearest the gum line. This type of tooth discoloration is often a "white spot lesion."
White spot lesions are the earliest visually detectable stage of cavity formation. They form in those regions where dental plaque has accumulated on a tooth's surface with regularity. White spot lesions are literally those areas on the tooth's enamel surface where the acidic waste products excreted by the bacteria living in the dental plaque have dissolved away a portion of the enamel's mineral content. This loss in mineral content causes the visible change in the enamel's appearance. It causes it to appear chalky white.
White spot lesions associated with orthodontic treatment (braces).
This is a serious type of tooth staining in the sense that cavity formation has begun. Possibly the advancement of the decay process can be kept in check, even indefinitely, with proper oral home care, the appropriate use of fluoride, and regular evaluation by your dentist. But due diligence should be paid to the formation of any white spot lesion.
There can be an especially disappointing phenomenon associated with the formation of white spot lesions during orthodontic treatment. When a person's braces are placed brackets are attached to their teeth. These brackets make it more difficult for the orthodontic patient to adequately clean their teeth. If they do not practice diligent oral home care and do allow plaque to accumulate and persist on the tooth enamel surfaces surrounding these brackets, white spot lesions can form. If this does occur, once the braces have been removed the patient will find that their teeth appear as if they have small white "bull's eyes" on them due to the appearance of those regions surrounding the brackets where the white spot lesions have formed.
Tray-based tooth bleaching systems.
