Dental school requires four years, and specialists like orthodontists and endodontists generally need another two to four years.

http://commons.wikimedia.org/wiki/File:Dentist%27s_sign,_Hillsborough_-_geograph.org.uk_-_1591235.jpg The scientists supplied 8 specialist dentists and endodontists, who served as experimental subjects, with a large pool of extracted teeth.