Sometimes your tooth can be sensitive after a filling, even if it was not painful before it was filled. This can be upsetting to the patient and to the dentist. Pain can be a complicated topic. I have seen huge cavities, where the nerve is still alive, but they do not hurt. Sometimes even the smallest areas of decay are sensitive. Cavities, however, need to be filled when decay has penetrated the enamel, whether they hurt or not.
Enamel is usually 1 to 1.5mm thick. Under the enamel is the dentin. Dentin is hard and similar to bone, and it is porous. In each pore, there is a very small amount of live tissue which brings pain sensations down to the nerve. If the dentist is removing decay laden with bacteria, the dentin is already infected with bacteria. The tissue in these pores is damaged, both by the bacteria and the drilling. The deeper the decay and the more drilling that is needed, the more damage there is to the dentin. This damage to the dentin can cause pain and inflammation in the nerve, called a pulpitis.
“Desensitizers” and healing coatings are applied under the filling to kill bacteria and to help calm the inflamed nerve. These applications usually work pretty well. However, like anything in health care, you do not know for sure what the outcome will be. Sometimes the new filling will be sensitive for a while, and then the nerve recovers. Sometimes bacteria have traveled far enough down the dentin pores to infect the nerve. This becomes an abscess, and it can occur even months later. The deeper the decay, the more likely the nerve will become abscessed, even after the decay has been removed and the filling placed. Obviously, the sooner decay and bacteria are removed, the better!