Endodontic treatments of teeth have been suggested because cavity access and root canal enlargement risk root canal fracture, and in particular, the relationship between the distribution of occlusal stress and root canal fracture has been reported (28). To reduce the risk of root canal fracture, MI Endo is recommended for MI access of a cavity and root canal preparation using Ni–Ti files. It is essential to use a K file when preparing a root canal. Anatomical root canal information, such as curvature, calcification, apical foramen diameter, and root canal length, can be obtained at this step. Furthermore, in curved root canals, removal of the orifice dentin ridge is the most important step in root canal preparation, and this operation facilitates file insertion to the apical foramen. Specifically, removal of dentin ridges by straight-line access and use of glide paths have been regarded as essential techniques for the safe use of Ni–Ti files.
Straight-line access, which is a basic concept of root canal preparation, reduces fracture resistance and causes root fracture because of excessive cutting of orifice dentin. Endodontically treated teeth have high stress concentration in the cervical region (29, 30), and excessive cutting of the orifice dentin is considered to be a factor that increases fracture susceptibility (31). These study reports have shown that minimizing flare formation by straight-line access may reduce the risk of cervical fracture. In addition, the root canal without straight-line access causes stress dispersion in the orifice dentin when normal stress is applied to the occlusal surface, whereas the root canal with straight-line access concentrates stress and has weak fracture resistance (19). The results in this study showed that teeth treated with MI Endo were less susceptible to cervical fracture. Previous studies have shown the preservation of orifice dentin correlates with long-term occlusal function maintenance in endodontically treated teeth (14, 15).
These research reports have discussed the need for straight-line access in root canal preparation. For straight-line access in unformed root canals, it is assumed that it will be difficult to limit the instrument operation and have a direct view of the site because of the reduced surgical field at the start of root canal preparation. As a result, inadequate morphological tracking is predicted. This study analyzes the relationship between the final canal morphology by Ni–Ti preparation and straight-line access, and enables accurate root canal preparation even in MI Endo, which reduces the risk of root canal fracture.
The TN used in this study was developed with the concept of MI Endo, which maintains a high occlusal strength of endodontically treated teeth by forming a root canal with MI shaping. TN also enables proper root canal preparation without straight-line access. The increase in root canal width and the median displacement were measured in TN canal preparation. Even when comparing the no straight-line access (Group A) and the straight-line access (Group B), no significant difference was observed between the increase in root canal width and median displacement when using TN. As a result, straight-line access was not required for accurate canal preparation.
Interesting results were obtained with the PTN no straight-line access (Group C) and straight-line access (Group D). The increase in root canal width and the amount of median displacement in the PTN without straight-line access (Group C) showed the same amount of displacement as in Groups A and B with TN at points 1–3 mm from the apex. Furthermore, the displacement by TN in Groups A and B increased only on the inner curve at 5 mm and the outer curve at 8 mm; however, no significant difference was observed, and straight-line access was not formed even in the root canal formation with PTN. As a result, proper root canal preparation is possible. Because the PTN X1 and X2 files have a large taper, flare of the upper 1/3 of the root canal occurs even in the no straight-line access (Group C) similar to the straight-line access formation at the apex. The displacement of the inner curve at 3, 5, and 8 mm was significantly increased, showing a tendency of straightening on the inner curve of the root canal in the PTN straight-line access (Group D). The root canal preparation by PTN with straight-line access, which is considered to be essential for maintaining the anatomical root canal morphology, straightened the inner curve of the root canal.
The PTN has two files, X1 and X2, that can be used to obtain the final root canal morphology for sufficient root canal cleaning and root canal filling. At the same time, the elasticity of the file itself is improved. The file operability is easy and it is expected to reduce clinical accidents. The file shape of PTN is characterized by a rectangular cross section, which allows for cutting at only two points in contact with the root canal wall to reduce transportation. Therefore, PTN is the most frequently used file system in Europe and the United States because there are few accidents during root canal preparation and accurate root canal preparation is possible even in curved root canals. The TN file system was constructed with a system similar to PTN, and the system was designed to have a final root canal morphology that can be filled with root canals using two files, Small and Prime. However, because there is no straight-line access, there is concern that tracking of the apical curved root canal may be inadequate because of restricted instrument operation in the reduced operation field. The results of this study proved that there is no need for straight-line access, which had previously been considered an essential procedure for accurate root canal preparation. Insufficient flare preparation in the upper part of the root canal when straight-line access is not formed suggests the difficulty of root canal cleaning and the effect of insufficient pressurization of root canal obturation. However, for root canal preparation by MI shaping type TN, a dedicated needle with an improved tip is recommended for root canal cleaning and pressure root canal obturation or single point root canal obturation with a dedicated gutter core by root canal obturation. In recent years, a bioceramic sealer with excellent biological properties, such as hard tissue forming ability, biocompatibility, and non-cytotoxicity, has been developed for root canal obturation (32). Such bioceramic sealers are expected to have a tighter sealing chain than other sealers by forming hard tissue at the dentin interface and apical foramen. The root canal obturation method currently uses multiple chemically and physically stable gutta-percha cones to tightly seal the root canal by lateral and vertical obturation. An obturation method has been established and promising clinical results have been reported (33). However, the single-point root canal filling method using a bioceramic sealer capable of root canal preparation and hard tissue formation (34), which reduces the risk of root canal fracture caused by the minimally invasive cutting type TN, will follow the flow of endodontic treatment in the future. The possibility of accelerating MI Endo was shown in this study.