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A radiolucency present at the root tip of a tooth that had an endodontic treatment 5 years ago indicates that that particular root was not sealed completely. Ideally, the material used to fill the canal should extend all the way to the tip, and include any accessory channels and small canals present, thereby eliminating any possibility for bacteria to remain inside the tooth. The radiolucency indicates that somewhere at that root tip remains an area inside the tooth canal that still harbors bacteria and your body has established a small chronic low grade response to that bacteria in an attempt to eliminate the problem. The radiolucency is an area where there is an absence of bone. This small space contains white blood cells and bacteria. In order to get rid of the radiolucency, that particular root needs to be re entered and resealed. I hope this answers your question.
If you have any concerns regarding this issue that I have not addressed, please let me know and I will get back to you with more information.
Sincerely, XXXXX XXXXX DDS