Our tooth has three layers outer layer is Enamel, the middle layer is Dentin, and the innermost layer is pulp.

Whenever there is any cavity, decay involving outer two layers then filling is possible,

but in case cavity/decay/irreversible damage involving pulp chamber, then it can be treated by Root Canal treatment.

Root Canal treatment also called RCT. It is an art of dentistry to save a tooth from removal when the infection has reached its nerve portion (Pulp Chamber).
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