Dear tlcha
When a tooth has a “cavity” it really means there is bacteria inside the tooth. Once the bacteria get past the enamel part of the tooth the body has no way of naturally getting the bacteria out. The bacteria will keep eating until they eventually reach the nerve of your tooth. Dentists combat this infectious spread by using a small drill to remove the infection and then fill the hole with either composite or amalgam. Any time a Dentist works on a tooth there are risks associated with treatment. The risk is usually low around 1% that treatment can result in a “complication.”
To fully answer your question I will explain a little bit of the anatomy of a tooth. A tooth is composed of three parts, the enamel (hardest substance in the body), the dentin (similar to bone) and the pulp (nerve of the tooth). The pulp is the part that supplies blood and nutrients to the cells inside our teeth. In a normal tooth this nerve helps us detect hot and cold, as well as provide sensory information so that we don't bite down on something hard such as a rock and break our teeth. The nerve is a relatively large space inside the tooth, but the only way blood and nutrients are allowed into a tooth is through a tiny hole at the very end of the tooth called the "apical foramen." In a normal tooth there is a tiny artery and a tiny vein that travel through this tiny hole and supply everything the tooth needs for survival.
If you hurt yourself (such as a cut) your body will release signals to other cells calling for help. These cells will rush to the site and cause inflammation and swelling. This inflammation is a good thing because it normally helps your body repair itself faster. The problem is it has an opposite effect in a tooth.
When a tooth gets hurt it sends out the same call for help. The problem is that tiny hole we talked about earlier called the “apical foramen.” As the pulp of the tooth becomes inflamed it builds up pressure inside the tooth. This pressure cuts off the blood supply at the tiny hole strangling the tooth. Once the tooth loses blood supply it’s only a matter of time before it dies.
What happened in your situation?
The Dentist removed your amalgam filling and replaced it with a composite filling. During this process your pulp became inflamed which cut the blood supply to the tooth. Right now your tooth is in the process of dying and there is nothing that can be done to stop it. You could put amalgam back in, but it would more than likely accelerate this process. Once the tooth dies it will stop hurting, but at any time it could abscess and cause potentially life threatening issues. My recommendation would be to get the root canal and avoid further pain.
Sorry for the bad news…
Dr. Steve