Dental crown. Coloured scanning electron microgr- aph (SEM) of the join between a tooth (green) and a dental crown (light blue). The tooth and ceramic crown are coated in debris (orange) which became attached to the tooth when it was extracted for this SEM. 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. Here, the crack between the tooth and crown allows bacteria to enter and cause secondary tooth decay. Magnification: x50 at 6x7cm size.