Every person has certain anatomic features. Some children have permanent molars with very thin and deep grooves that, even with the most thorough and exceptional at home oral hygiene, will still develop cavities. Why does that happen? Simply because the grooves are too thin and too deep for the bristle of the toothbrush to physically reach inside of them.
A common treatment recommended in these cases are dental sealants.
The procedure is simple, the dentist or hygienist will clean the tooth of any debris and impurities, and a thin liquid type resin is placed inside of the groove (similar to a dental filling but without drilling). Once placed, the grooves are now sealed – the bacteria cannot penetrate them anymore and the bristles of the toothbrush can now clean every area.
As these teeth have a much higher risk of developing a cavity, sealants must be place as early as the teeth erupt. If a cavity has developed already, the tooth cannot be sealed anymore as the cavity must be cleaned out and a regular filling must be placed.