牙根外吸收的發生不論是在有牙髓活性的牙齒或無牙髓活性的牙齒都是常見到的。本篇是報告一位25歲男性患者，因右下第三大臼齒為水平阻生齒想要拔除，於牙科門診做口腔X光攝影檢查時發現其右下第二大臼齒之遠心側牙根靠近第三大臼齒處有明顯牙根外吸收之情形，放在拔除右下第三大臼齒，且等傷口復原之後，進行右下第二大臼齒之根管治療。經由髓腔開擴、清創、氫氧化鈣糊劑量放，控制牙根外吸收的進行，之後再以馬來膠及MTA（三氧礦化物）封填根管及修補牙根吸收處，咬合面再以複合樹脂填補完成治療。經由長期的追蹤，到目前為止右下第二大臼齒仍良好保存著。相似的病例也發生在左下第二大臼齒且採用相同的治療方式。

External root resorption was usual on vital and nonvital teeth. The cause and mechanism was descried in many studies. This case is a 25-year-old male. He visited our department for dental extraction of the lower right third molar. After the radiographic examination, it was found that there was obvious external resorption of distal root of lower right second molar close to the crown of lower right third molar. Extraction of the lower right third molar was suggested and performed at the next appointment. Endodontic treatment was then performed on the lower right second molar. The canal enlargement, debridement, calcium hydroxide paste dressing was done and external root resorption was under controlled. Then, the root canal was filled by Gutta-Percha and repaired the resorption area by MTA (Mineral Trioxide Aggregate). Occlusal surface of this tooth was restored with composite resin. After long term follow-up, the lower right second molar is still preserved well up to now. The similar case occurred in the lower le ft second molar by the same treatment method.