Endodontics is moving forward to minimally invasive endodontics, including contracted endodontic cavities and periapical microsurgery. Technological advances such as CBCT imaging, operating microscopes, nickel-titanium instruments, and Single-Cone Technique provided a great deal of convenience. The contracted endodontic cavity may lead to difficulty in operative procedure due to smaller operating space, and have the potential to cause apical transportation, ledge or shelf and instrument fracture in consideration of the curvature inside root canals. This study suggested that apical transportation after instrumentation for CECs was significantly bigger (P < 0.05) than TECs in the premolars with two dental roots, and no instrument fracture was experienced. This means although the apical transportation of CECs was bigger in premolars with two dental roots, with the appropriate straight path to a root canal, there is a small probability that instrument fracture occurred in premolars. There was no significant difference (P > 0.05) between the TEC and CEC groups in the increased sectional area and the deviation of the central point in single-rooted premolars. Which means in single-rooted teeth, it is safe and effective.
It is reported that guided endodontics printed templates have been used to locate all root canals in the apical third of teeth with pulp canal calcification and apical pathology with the aid of 3D printing technology and digital dentistry [15,24]. In this research, we used guided endodontics printed templates for minimal cavity access, which acquired the least tooth structure removal and projected the access trajectory to each canal orifice. Contracted endodontic cavity with the aid of 3D-printed template seems to be a safe, clinically feasible method for locating root canals.
A previous study reported that CEC seems to exhibit better preservation of the original canal anatomy, particularly at the crown level, including incisors, premolars, and molars with TECs . The conservative endodontic cavity, which could keep back the truss of dentin between the cavities, could save more dental tissue in premolars with two dental roots. Although more tooth tissue was retained, there was no obvious increase in the fracture resistance. It is probably because the volume of premolars was small, although CECs could conserve more corona dentin, and dental cervix dentin and the load at fracture for premolars with CEC was bigger than TEC. It couldn’t lead obvious statistical differences, and more data need to be studied.
The complete cleanness of the root canal was still the primary objective for nonsurgical root canal therapy. On the basis of the experimental data, the instrumentation efficacy was more effective for TECs than for CECs in single-rooted premolars. Because contracted endodontic cavities are smaller than traditional endodontic cavities, the instrument could not shape the entire wall of the canal into a straight line. With the aid of the 3D printed template and the straight line of contracted endodontic cavities, the instrumentation efficacy of TECs and CECs showed no significant difference in premolars with two canal roots (P > 0.05). According to the report , the instrumentation efficacy did not differ significantly (P >.15) between CECs and TECs in any of the roots or canal levels in maxillary molars. Another research found there was no significant difference in remaining pulp tissue between the TEC and CEC groups within the MB or ML root canal at any of 1/3 of the root .
In the premolars with two dental roots, the increased sectional area was significantly bigger (P < 0.05) in the CEC group than in the TEC group. It might be due to the design of canal orifice in the premolars with two dental roots. The drill was virtually superimposed on the root canal with inclination angle in the corona. When we design a straight-line pathway into canals, the orifice would be in the buccal cusp. Therefore, we shifted a little toward the middle to avoid the destruction of marginal ridges. Because of the inclination angle in coronal, the deviation of the central point after instrumentation for CECs was more significant (P < 0.05) than that for TECs. Mario A et al. also reported that TECs show more preservation of the original root canal anatomy with less apical transportation than CECs .
Micro-CT allowed the 3D anatomy assessment of root canal fillings and voids, and the results obtained in our study did not show obvious differences in the percentage of root canal filling in the CEC and TEC groups. Although the entrance of the pulp chamber is smaller in CECs, with the straight-line pathways into canals, the root canal filling can be completed for both CECs and TECs equally.
Numerous studies provided CEC preparation did not increase the fracture strength of teeth compared with TEC preparation [25–27]. This result corroborate with those above research. The researchers have found the endodontic procedures do not weaken teeth with intact marginal ridges ,the CEC and TEC groups were both prepared with intact marginal ridges, and there were no significant differences (P > 0.05) on biomechanical responses between the premolars in the CEC and TEC groups. At the same time, the result was opposite to Krishan’s research , which reported CEC increased fracture resistance in premolars and mandibular molars. The following 4 explanations clarify this contradiction: (1) the simulated clinical treatment procedure to restore the access cavities with resin before fracture resistance test; (2) the angle of the tooth loaded at the central fossa from the tooth long axis and the spherical crosshead was different in this study; (3) single-rooted premolars and premolars with two dental roots were differentiated; (4) each sample has been tested by fatigue cycle test. All of these factors have a potential effect on the final results. In addition, the load type was comparable to that experienced in the mouth, and human teeth were subjected to forces in different directions at the same time, the tested teeth have irregular shapes, and the experimental data acquired were just in one direction, and the results were for reference. Besides, when we talk about CECs, the extended preparation time and materials should be considered.