There are nine dental specialties recognized by the American Dental Association and require 2-6 years of further formal university training after dental school. The specialties are orthodontics (straightening of teeth), oral and maxillofacial surgery (extractions and facial surgery), pedodontics (treatment for children), periodontics (treatment of gum disease), prosthodontics (replacement of missing facial anatomy by prostheses such as dentures, bridges and dental implants), endodontics (root canal therapy), dental public health (study of dental epidemiology and social health policies), oral and maxillofacial radiology and pathology (study of oral and dentally related diseases). Specialists in these fields are designated registrable (U.S. "Board Eligible") and warrant exclusive titles such as orthodontist, oral surgeon, pedodontist, periodontist, or prosthodontist upon satisfying certain local (U.S. "Board Certified") registry requirements.