When a cavity is too large to hold a filling securely, then a crown may be placed. A crown is technically called a “full jacket crown”, and sits like an external support around the inner tooth and core – just like a “bucket on a sandcastle”.
The result is often excellent and very strong, but the tooth needs to be drilled smaller in size on every surface to make room for the crown – so this is not usually done on healthy teeth with small fillings. A laboratory needs to make the crown so a temporary crown is in place while they construct the new crown.
Most crowns are made with porcelain bonded over an inert metal coping, which gives both good strength and good appearance.
Other crowns can be “all porcelain” which gives optimised cosmetic appearance, though not so strong; or ‘all gold’ crowns where the whole crown is made of a gold alloy – giving optimised strength but not tooth coloured.