When a tooth loses a large filling, the best way of restoring a large filling is to place a permanent crown over the tooth.  Because the large cavity has already destroyed so much of the tooth, a filling alone will not be enough, generally, to give the tooth protection and strength. 

A tooth that loses a filling is in bad shape from the moment the filling comes out.  Bacteria and plaque will attack the old cavity and start working their way down through the dentin and spread to the root tips and root chamber.  When this happens, an abscess is inevitable, and a root canal is more than likely, if not mandatory. 

Restoring a large cavity is top priority, therefore, in order to maintain the cosmetic value of the tooth and the health of the entire mouth. 

When it comes to restoring large cavities, a replacement filling is not enough.   This is not to say there is no need for some sort of filling material to be used to a certain extent.  Cosmetic dental bonding resin can be very helpful in the form of a core buildupbut the patient needs more than this is the tooth itself was once attacked by a very large cavity.

If dentists try to replace a filling with another filling alone, what ends up happening is that more tooth matter is destroyed in the process of removing the decay and reshaping the tooth.  This weakens the tooth even more than it was already weakened by the cavity. 

Dentists, therefore, strengthen and protect a tooth damaged by a large cavity by restoring both its original size and shape, along with its ability to bite and chew food, with a porcelain crown.

The crown will work as a shield by covering the tooth and keeping food debris away from its surface.  Bacteria will not be allowed to grow on the surface of the tooth.  A crowns hard surface can chew food like a normal tooth.  It can also be brushed and flossed like any other tooth.   

A patient with a crown must pay attention, though, to that hairline area between the base of the crown and the gum line.  This very small space is an entry point to bacteria.  If they get under the crown, they can attack the tooth again and start the process of decay all over again.  This is why this particular area has to be brushed and floss with extra care.    

Dentists do more than simply visually examine a tooth to determine if they need to restore a large cavity with a permanent crown.  They look at how large the cavity is, how much of the tooth is still left, and where the tooth is located in the persons mouth.  Both traditional hand tools and sophisticated new dental technologies are used to diagnose the viability of a tooth. 

The patient is then informed of options, and if a permanent crown is the agreed upon course of action, molds are taken in the office and a dental lab custom makes a crown as a protective fitting for the tooth.