Sometimes crowns and fixed bridges are used to restore badly worn teeth. If the natural teeth were worn from clenching and grinding the teeth (bruxism), the new crowns and fixed bridges may be subjected to the same wear. In general, dental porcelain and metal alloys wear at a slower rate than tooth enamel. However, excessive wear of the crowns or fixed bridges may necessitate an acrylic resin mouth guard (also called a protective occlusal splint or night guard.)