Tooth pain or sensitivity caused by a cavity should stop bothering you once that cavity is filled. The decayed portion of the tooth has been removed, and the resulting void has been filled with a tooth-coloured composite resin, which blends seamlessly into the surrounding tooth. It looks as though the cavity was never there. So why does your tooth still feel uncomfortably sensitive?
Composite Dental Resin and Shrinkage
Tooth-coloured composite dental resin is applied to your cavity in a soft, flexible form. It dries quickly, and this process can be accelerated with a special curing light. Composite resin can be affected by something called polymerisation shrinkage—in which the polymers contained within the resin shrink during the drying phase. This basically means that your filling might not be as large as it needs to be.
An Inadequate Seal for Your Cavity
Although you don't want a filling to be too large for the cavity, it must create a seal on the tooth's outer surface, with the seal slightly overlapping the sides of the cavity. If your filling experienced polymerisation shrinkage, its seal might not be intact. The gap will be tiny, and you won't be able to see it yourself. But this gap can still allow bacteria and other irritants to enter the tooth. This means that the tooth's nerve is irritated, which is causing your ongoing sensitivity.
Deepening Cavities and Root Canals
Your filling will need to be corrected. If bacteria continue to enter your tooth, its nerve may become infected. This will require root canal treatment, which involves the removal of the infected nerve. Bacteria can also cause the redevelopment of the original cavity, hidden beneath the filling. Even if you don't need a root canal, re-treatment of your cavity can be needed. Obviously, it's best to avoid this problem by having your filling checked.
Sealing Your Existing Cavity
Your dentist won't need to start over with your filling, and can just apply some extra composite resin to its surface so that it's properly sealed, before polishing its outer layer again—which helps it to blend in with the surrounding tooth surface, making it look like a natural part of the tooth's dental enamel. Your cavity is now completely sealed, and your tooth has been restored.
If your tooth continues to be sore or sensitive after your dentist has filled your cavity, you'll need to report the problem—otherwise, the problem will become a lot worse.
Speak to your dentist to learn more.