A dental sealant is a thin plastic coating (clear or white) that's bonded into the pits and grooves of a tooth. (Dentists formally refer to them as "pit and fissure" sealants.)
They're most frequently placed on the chewing surface of teeth (where most of a tooth's grooves lie).
But they can also extend onto their cheek or tongue sides too (either as a continuous or separate piece) depending on what additional pits and fissures are found there.
Sealants help to protect teeth from the formation of tooth decay. Sealants are considered a preventive measure, not a corrective one (like placing a filling).
The most predictable sealants are those that are placed before any sign of a cavity has had a chance to form.
For this reason, once a tooth has been identified as a candidate, it's a good idea to have it sealed as soon as is reasonably convenient.
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Even if vague signs of cavity formation have started to appear ("incipient" decay), a dentist may determine that it's still acceptable that a sealant can be placed.
This simply needs to be determined on a case-by-case basis.
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