Why a crown?

First and foremost for strength. We age and the repairs often carried out on back teeth, for reasons of childhood decay, begin to break down. Teeth need to be strong enough to cope with forces of chewing and grinding. The best repair is often a crown.  It can be that strong.

Crowns can be made of metal for ultimate strength.  They can be made of metal with porcelain and are mostly appropriate for back teeth where strength and appearance are important. 

Pure porcelain teeth are mostly used in the front part of the mouth, where aesthetics are paramount and strength isn't necessarily an issue.