If there is black color in the sulcus of the big teeth, it is very likely that there is caries, but not as long as there is black color, we have to carry out filling treatment, but must form cavity. Most of people have some sulcus caries on the surface of teeth, but there is no cavity formation, you can consider to observe, as long as you brush your teeth well, usually the sulcus caries will become static caries and no longer develop in more serious direction. If the sulcus caries is getting bigger and bigger, and there is already hot and cold sensitivity or painful feeling when eating sour or sweet food, then it is necessary to carry out filling treatment as soon as possible, and the situation will be improved after filling. Because the sulcus is the location where food residue is easily accumulated, so now children can have sulcus closure in advance around 6 weeks old to prevent the occurrence of dental caries.