Root canal isthmus(RCI) is defined as a narrow, ribbon-shaped communication between two root canals that contains pulp or tissue derived from pulp. Any root that contains two or more root canals has the potential to contain an isthmus. The incidence of RCI from different tooth positions varies, with the highest RCI incidences usually found in the mesial root of the mandibular first molar and the mesiobuccal root of the maxillary first molar. The presence of RCI increases the difficulty of root canal therapy and introduces uncertainty regarding the prognosis for dental treatment. It is recommended to use CBCT and dental microscopy to identify teeth with suspected RCI in clinical practice. At the same time, for treatment of teeth with RCI, appropriate instruments should be selected, and enhanced root canal irrigation assisted by ultrasound should be considered to improve the success rate of root canal treatment and endodontic root-end surgery. The current technology still has some limitations regarding the cleaning and filling of RCI and additional research and development. Improvement of the corresponding technology and equipment is a current research hotspot and a future research direction.

The anatomical morphology of root canal systems in maxillary second molars (MSMs) is complex and shows diverse variations, which leads to challenging root canal treatments that could end in failure. Therefore, understanding the diverse nature of root canal anatomy is an important prerequisite for the successful treatment of MSMs. The fused root is among the most prevalent root variants of MSMs, exhibiting a distribution rate ranging from 23.9% to 42.25% within the Chinese population. Fused roots are often accompanied by complex root canal fusion, as the incidence of root canal fusion in type Ⅵ fused roots of MSMs is 90.5% within the Chinese population. There are several treatment strategies for the root canal variations in MSMs, including the location of the root canal orifice and clearing and filling of the isthmus within the fused root canal. Cone-beam computed tomography, dental operating microscopy, and digital dynamic and static navigation technologies can be employed to accurately identify the root canal variations associated with fused roots. In the treatment of fused root canals, for the cleaning and filling of the isthmus in fused canals, enhanced chemical preparation, timely application of laser-assisted irrigation, photodynamic therapy, and microapical surgery significantly improve the success rate of endodontic procedures. The treatment strategies and prevalence of root canal variation of MSMs from a native Chinese population were summarized in this paper to provide guidance and reference for the successful treatment of MSMs.