Fillings are fabricated in the office by the dentist to replace tooth structure missing due to breakage of the tooth or removed from the tooth when decay is excavated. It is important to treat lost tooth structure early because chemicals in the mouth are able to permeate into the tooth structure and can initiate death of the tooth's nerve. Untreated these conditions will lead to either the need for root canal or an extraction.
Today we use either composite (plastic and glass) fillings completed in the dental office or castings of porcelain or gold that are fabricated by the dental technician. We do not use silver amalgam fillings which do contain mercury. The fillings are all bonded into the tooth which helps replace the strength of the tooth and decreases the chance of new decay in the future.
If the filling will be very large and compromise the tooth's integrity the dentist will recommend a crown.