Q. Hi, Dr. Glaister: Dentist said I had a vertical crack in my lower left molar (tooth number 18) in the back of the tooth. It doesn’t hurt although sometimes I feel sensitivity when I chew. He said I should have a crown. I heard the cheapest crown is a resin composite. What is the life expectancy of that kind of crown and can it be done quickly in one visit? Is it different than a metal or porcelain crown inthat they don’t have to grind your teeth?