When a tooth is heavily decayed it is more likely to fracture and lose considerable tooth substance. The greater risk is that a fracture line may go below the margin of the gum which makes restoration more difficult or near impossible. A dental crown is an artificial tooth or cap, which covers a tooth weakened by decay.
Crowns may be made from several different materials, gold and porcelain being the most common types.
A bridge allows the dentist to replace lost teeth without the use of a denture or dental implant. Basically a false tooth is held in place by being attached to a tooth next door.