Abstract
The purpose of this study was to evaluate the incidence and microscopic anatomy of the isthmus to provide more precise anatomical information about the mesiobuccal (MB) roots of the maxillary first molars and the mesial (M) roots of the mandibular first molars. Twenty-eight maxillary and 31 mandibular first molars were embedded, sectioned, stained, and observed at 30× magnification to evaluate the incidence and microscopic anatomy of the isthmus. The incidence of an isthmus 3 mm from the apex was 89.3% and 100% in the MB roots of the maxillary first molars and in the M roots of the mandibular first molars, respectively. The mean dentin thickness between the isthmus and the distal root surface was <1 mm at a distance of 3 mm from the apex in both types of roots. In this study, whenever two main canals were located in the MB roots of the maxillary first molars and in the M roots of the mandibular first molars, the likelihood of the presence of an isthmus increased. Therefore, clinicians should be aware of the thinnest dimensions in the distal surface of the MB roots of the maxillary first molars and the M roots of the mandibular first molars during nonsurgical and surgical root canal treatment.
Highlights
One of the main anatomical complexities of the molars is the presence of an isthmus, which is defined as a narrow, ribbon-shaped communication between two root canals that contains pulp tissue[5]
In a recent retrospective clinical study, we reported that the success rate of endodontic microsurgery for isthmus-absent teeth was higher than that for isthmus-present teeth in molars[10]
Additional reductions in root dentin are required for isthmus-present teeth during surgical root-end preparation; further root weakening is unavoidable in isthmus-present teeth compared with isthmus-absent teeth
Summary
One of the main anatomical complexities of the molars is the presence of an isthmus, which is defined as a narrow, ribbon-shaped communication between two root canals that contains pulp tissue[5]. Sathorn et al.[11] reported that fracture susceptibility increased as the thickness of the remaining root dentin after canal preparation decreased. Finite element analyses and histologic studies have reported that a root with an isthmus is more likely to be susceptible to fracture than a root without an isthmus[11,12,13]. The purpose of this study was to evaluate the incidence of an isthmus and to assess the microscopic anatomy―minimum dentin thickness, in particular―of root canal systems to provide more precise anatomical information about MB roots of the maxillary first molars and M roots of the mandibular first molars
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