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Except with dentures, missing teeth can be replaced with dental implants. People made and used dental implants since some 4000 years ago, using bamboo pegs and seashells instead of missing teeth.
Dental floss has a history that his maybe longer than of the toothbrush but we know for sure that it is in use since the 19th century. It is in the wider use since the invention of synthetic materials like nylon.
Since ancient times we had problems with crooked teeth. They interfered with proper eating and were not too aesthetically pleasing. Because of that, we tried to invent mechanical aids that helped us to fix them. Read about history of orthodontics.
Since the ancient times, we try to keep our mouth and teeth clean. Until we found out how bacteria work, we used some pretty unsanitary methods for that but in time we learned and today our hygiene standards are pretty high. Read more about history of oral hygiene.