Why does a dentist have to drill a tooth cavity?

To fix a cavity, the decay must be removed from the tooth. The traditional procedure involves using a dental drill to break through hard tooth structure and to remove the softer decay.

Some dentists now use dental lasers rather than or in addition to the traditional dental drills. Some dental lasers produce their own anesthetic-like effects, meaning that you may not need to be numbed for the procedure.
A tooth "cavity" is termed when you have decay on your tooth. The dentist has to "drill" it to remove the decay area. After that, tooth can be directly filled with amalgam "silver" material or resin "white" composite. In cases where the cavity occupies majority of the tooth, repair can be made indirectly with a porcelain inlay, onlay, or crown. 
When someone gets "plaque" on their teeth, they are getting a "film of bacteria." These bacteria stick to teeth. The bacteria then create acid as a by-product of their metabolism. That acid can eat through the tooth's enamel causing a cavity -- a hole through the enamel.

A tooth has three parts: The outer, very hard enamel, the inner, much softer dentin (think of dentin like ivory) and the core pulp, which consists of nerves and blood vessels.

Once acid breaks through the enamel, bacteria can invade the dentin and will eat through it fairly quickly. If the bacteria make it to the pulp, it causes an infection. You will need a root canal.

When dentists drill into teeth, they are shaping an opening. This is to ensure all of the rot in the dentin is removed entirely. Another reason for drilling is to create a shape for the filling to bond and fill the cavity completely. Fillings can be either a silver/mercury amalgam or they can be plastic.
Tooth decay is actually a disease. The preparation ("drilling") of the tooth is necessary to clean out the tooth structure that has been destroyed by that disease process, and properly shape the remaining tooth for a filling. The filling rebuilds the portion of the tooth that was broken down by the decay. 

For many years, developments have evolved with lasers for preparing "tooth cavities", as well as a process called air abrasion--essentially, like a tiny sand blaster for use on teeth. These both have their uses, but have not yet reached the point where they can replace the dental hand piece ("drill").
Because some of the infection of the cavity is hidden below the hard-surface enamel. The dentist has to remove some of the enamel just to get access to the decay to remove the infection. There are nerves in the tooth under the enamel surface; that's why your tooth has to be made numb to be comfortable.
Lucia Yau, DDS
When a tooth has cavity, bacteria is inside the tooth and breaking down tooth structure. The "drill" is used to remove the broken-down part of the tooth and clean away the bacteria leaving healthy tooth structure. 

Teeth are the hardest substance in the human body. When a tooth gets a cavity it means that part of the tooth is soft where it was once hard. This soft part of the tooth needs to be removed otherwise, with time, it can get bigger and spread to the rest of the tooth and need a more invasive procedure. Usually a drill is used to remove this soft part of the tooth until only hard tooth structure is left. The dentist will prepare the tooth according to the type of filling that will be used to fill the space of where the cavity use to be.   

Picture of Teeth Anatomy
A tooth cavity contains the bacteria which caused the decay. This needs to be removed so it will not spread. Your dentist will use a dental hand piece to remove this decay. Once it is removed, the tooth can be filled with a filling.
The dentist has to drill a tooth cavity because the cavity (the hole in the tooth) is not just an empty space -- it is actually filled with decayed tooth material. In order to correctly prepare the tooth for a filling, the dentist must remove this decayed material. Once the dentist uses the drill to clean up the cavity, he or she can fill in the clean hole with either an amalgam filling (containing mercury, copper, silver, and other metals) or a composite resin filling.

Well, we have to drill because a cavity is a hole in your tooth, and there's nasty stuff (decay) in that hole that needs to be removed. However, the "cavity hole" is really not one we can work with - it's often too small for us to get in there and do what we have to do. So we need to further drill the hole to make ourselves a workspace big enough to clean out the decay. Then we use a filling on the (larger) hole we made.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.