Dental crown fitting. View from inside the mouth of a dentist fitting a dental crown. The dentist is using a glue which sets under ultraviolet light to attach the crown. The blue light enables the dentist to aim the ultraviolet light correctly. If the top of a tooth is badly cracked, decayed or broken, it must be replaced by an artificial crown. The remaining undamaged part of the tooth is cut down to a peg to receive the hollow crown. This crown is then shaped to form a replica of the original tooth and cemented in place. This photograph was taken inside a special dental simulator.