We often receive many referrals from sister hospitals. Among these referrals, a large percentage of patients cite “root canal failure” as the reason. Patients often say to us: “My root did not pass in XX hospital, and I heard that you have the equipment here, so I transferred it to you to have it passed”. We can fully understand the patient’s thoughts, because for the patient, who has to go around for many times, spending a lot of time and money, naturally has a great hope. However, it is important to clarify that not all root canals are always clear, and not all root canals that are clear will be problem-free. Below I will give a brief introduction to several common causes of root canal failure, so that you can have a better understanding of root canal treatment. The first cause of root canal failure is the natural physiological changes of teeth: as we grow older, there will be some calcified material deposited in the root canal system, which is an ageing change of teeth and is a normal physiological phenomenon. We call the root canal inaccessibility caused by the deposition of this calcified material root canal calcification, which is more common in the teeth of middle-aged and elderly people. Whether or not this type of root canal dilation is necessary depends on the actual condition of the tooth. If the calcification is complete and there is no inflammation, there is no need to forcibly enlarge the canal if there is no problem after a long period of observation. The second cause of root canal failure comes from the effects of other treatments: in the days when root canal treatment was not popular, some other methods of treating pulpitis and periapical inflammation were used, such as dry pulp surgery, plastination, etc. For root canals that have been treated with these methods (especially plastination), dilation is difficult and sometimes special equipment (such as ultrasound, microscope, etc.) must be used to achieve the treatment goal. Sometimes special equipment (e.g. ultrasound, microscope, etc.) must be used to achieve the purpose of treatment. The third type of root canal failure is due to some human factors: because the root canal of some teeth has a special morphology (too long or excessively curved, etc.), it is difficult for the doctor to find the correct root canal access, forming a step or root canal offset, and sometimes even some treatment instruments are broken in the root canal, so naturally it will not be able to be expanded. In this case, the treatment is very difficult, and even with the use of professional auxiliary equipment, sometimes it is not possible to achieve the purpose of treatment, which eventually leads to treatment failure or tooth extraction. Several of these causes can occur individually, but more often than not, several conditions exist in combination, causing greater treatment difficulties, and there are certain risks associated with ultrasound and other treatment equipment, so in some cases, treatment can only be an attempted treatment. Very often, the doctor spends a lot of energy and the patient pays a lot of time and money, but still cannot achieve the purpose of treatment, which is a result that neither the doctor nor the patient wants to see. Therefore, in the face of such a complex root canal treatment, it is necessary for the patient and the doctor to fully communicate in order to get a treatment plan that is most suitable. Finally, it is important to emphasize that root canal access is only one of the important factors affecting the outcome, but not the decisive one. Root canal treatment is a very standardized and strict procedure, and it is only by ensuring that the preparation, disinfection and filling are in place that root canal treatment can be as effective as it should be. In many cases, the failure of the entire treatment can be caused by the omission of one small step. As Mozi said, “People get their disease from many sources, some from cold and heat, some from labor and suffering. If a hundred doors are closed and one door is closed, why would a thief be unable to enter?”