| Root canal preparation, or canal instrumentation, is one important step in endodontic therapy, or root canal therapy, which is designed to remove intracanal microorganism and all of the infected pulp by mechanical removal and chemical cleaning, and to prepare the canal into the cone shape with the bigger size of the apical part and smaller size on the bottom, so as to facilitate the filing. Complete procedure of canal instrumentation includes shaping and cleaning. And the ideal canal instrumentation requires not only the continuous cone shape, but also the maintenance of the original form and direction to prevent root canal transportation.The preparation for curved root canals has always been really difficult in endodontic therapy, because of the risks of potential loss of working length, the exposed apical part, ledging, zipping and transportation. With good flexibility, the nickel-titanium devices can theoretically maintain the original shape of the canals quite well, and have presented very unique advantages in the instrumentation for curved root canals. However, there haven’t been standardized specifications on the mechanical nickel-titanium endodontic files. Due to the unique designs of the components, motions, cross sections, processing techniques, core mass, tapers and cutting edge degrees of the commonly used systems in clinical practices, there are quite a few differences of their shaping and cleaning functions and safety among them.Besides, canal instrumentation can damage the structure of the dentin, create fissures on the surface of the tooth root, and lead to vertical root fracture (VRF). The design of the nickel-titanium files including the core diameter, the cross sections, the cutting edge degrees, the groove depth and the tapers can all influence the performance of the endodontic files, hence impact the occurrence of the dentin fissures. The small dentin fissures can develop into root fractures with the straines caused by repeated occlusions, and cause the failure of the endodontic therapy.In recent a couple of years, a new type of nickel-titanium endodontic file called Twisted File Adaptive (TFA, made by SybronEndo, a US company) has been marketed. With the special motor of Elements Adaptive Motion, TFA system can generate a special motion called self-adaptive adjustment, with the reciprocating angles changing all the time:unidirectional rotation without external forces (600°clockwise and O°counterclockwise), and reciprocating motion with external forces(370°clockwise and 0～50°counterclockwise). The self-adapting endodontic file system of Twisted File, is an application of a patented intelligent algorithm, which makes it possible to adjust the torsion force driven by the motor of Twisted File Adaptive to the best degree with the intelligent algorithm according to the resistance during the canal instrumentation. And this means preparation by Twisted File Adaptive can be performed according to the intracanal resistance:it can rotate or step back when it is needed to rotate or step back.In 2011, the single-file nickel-titanium reciprocating endodontic system in the in-and-out retrieval motion with specific motors:Reciproc (made by VDW, a German company) and WaveOne (made by Dentsply, a Swiss company) were marketed and promoted in clinical practices. The reciprocating NiTi files are the endodontic files in the clockwise and anticlockwise motions alternately in the canal instrumentation, which is totally different from the motion system of mechanic NiTi file systems. The idea of the reciprocating motion comes from the "balanced force" of manual instrumentation. In the canal instrumentation with this type of endodontic file, during the counterclockwise rotation, the cutting edge of the endodontic file can produce a cutting force on the dentin wall with a direction deep into the inferior canal root; while the clockwise motion can relieve the twisting stress and the screwing force to the inferior canal root. The section of this endodontic file is symmetric and can produce cutting force both clockwise and counterclockwise, with the angle of counterclockwise bigger than that of clockwise motion. With this design, the endodontic file now can effectively go deeper into the apical part of the canal root. This type of endodontic file is more applicable for the preparation for the curved root canals, with the purpose of reducing intracanal pressure strain and stretching strain, so as to decrease the cyclic fatigue of the NiTi file.ProTaper Next is a new type of NiTi rotating system for canal instrumentation as a revised version of ProTaper Universal made by Dentsply. Compared with ProTaper Universal, ProTaper Next has three major advantages as follows:firstly, the eccentric rectangular cross-section of ProTaper Next provides enhanced strength; secondly, compared to the traditional NiTi material, the M-Wire technology of ProTaper Next can improve the flexibility and the resistance to cyclic fatigue. M-Wire is produced by the thermomechanical treatment of NiTi alloy, which has better flexibility, strength and resistance to abrasion with the unique martensite nanocrystals; and thirdly, ProTaper Next has a unique asymmetrical rotary motion which can enhance the shaping efficiency to get more cross-sectional space with smaller file, with fewer files to complete the shaping.In recent years, the studies on the NiTi systems for canal instrumentation have been mainly focused on the comparisons on the shaping and cleaning, the related tooth root fissures and the fatigue tests between the traditional rotating motion and the reciprocating motion of fixed tapers. In this study, the canal transportation of simulated resin curved root canals in the canal instrumentation was compared by micro-CT. And the strain tests were applied to compare the strengths, the distributions with time of the strain caused by self-adapting endodontic file system of Twisted File Adaptive, fixed reciprocating system of WaveOne and continuous rotating system of ProTaper Next on the simulated curved root canals. With the combination of the in vitro experiment of teeth, this study also investigated the influences of different motions on the formation of the tooth root fissures after the preparation, so as to provide experimental evidences for the clinical applications of the new NiTi systems.This dissertation mainly included the three chapters as follows:Chapter 1:comparisons on the canal transportation of simulated resin curved root canals in the canal instrumentation by Twisted File Adaptive, WaveOne and ProTaper NextMaterial and methods:10 simulated resin curved root canals with the curvature degree of 28° were prepared respectively by Twisted File Adaptive, WaveOne and ProTaper Next. Before and after the canal instrumentation, the resin was placed in the embedding model made by silicone rubber, so as to ensure the consistency of the sample locations of the twice Micro-CT scanning before and after the canal instrumentation. After the scanning, the data with the file format of DICOM were imported into the software Mimics 10.0 for 3D reconstruction, and the 3D images before and after the canal instrumentation were overlapped with the software Solidworks for location matching, then the root transportation values were calculated and analyzed.The results were:Continuous taper was obtained after preparation in all of the three groups. In all the detecting point 1mm,2mm,3mm......12mm, there were statistically significant differences among three groups(P<0.05).The average root transportation of the Twisted File Adaptive grout in the apical part (1-4 mm from the apical foramen) was 0.100±0.009 mm, and the transportation values of the WaveOne group and the ProTaper Next group were respectively 0.259±0.022 mm and 0.150±0.034 mm; for the middle part of the canal (5-8 mm from the apical foramen), the average transportations caused by the three devices were respectively 0.675±0.021 mm,0.149±0.027 mm and 0.563±0.009 mm; and for the upper part of the canal (9-12 mm from the apical foramen), the average transportations were respectively 0.121±0.0336 mm,0.133±0.024 mm and 0.081 ±0.012 mm. For the apical part, the average transportation caused by Twisted File Adaptive was lowest, and the differences of the transportations caused by the three devices were statistically significant (P<0.05); for the middle part of the canal, the average transportation caused by WaveOne was lowest (P<0.05), while the differences between Twisted File Adaptive and ProTaper Next were not statistically significant (P>0.05); and for the upper part of the canal, the average transportation caused by ProTaper Next was lowest (P<0.05), while the differences between Twisted File Adaptive and WaveOne were not statistically significant (P>0.05).Chapter 2:biomechanical analyses of simulated resin curved root canals in the canal instrumentation by Twisted File Adaptive, WaveOne and ProTaper NextMaterial and methods:ten simulated resin curved root canals with the curvature degree of 28° were prepared respectively by Twisted File Adaptive, WaveOne and ProTaper Next. Before the instrumentation, the strain gauge foils were pasted on all the surfaces of the samples, with the sites:the starting point of the curvature on the inner wall (site 1), the most curved point of the curvature on the inner wall of the canal (site 2), and the point with 1 mm above the apical foramen of the root canal (site 3). The strain gauges were then connected with Electrical Resistance Strain Gauge, so as to monitor and to record the real-time strain changes on each site during the canal instrumentation with the software, and then the curves of the changes of micro-strain values (μstrain) with time were conducted. On this curve, the maximum micro-strain value (μstrain) of the three sites of Twisted File Adaptive, WaveOne and ProTaper Next could be read directly.The results indicated that three types of mechanic NiTi devices of Twisted File Adaptive, WaveOne and ProTaper Next all caused strains of different strengths to the canal walls during canal instrumentation. On site 1, i.e. the starting point of the curvature on the inner wall, the average values of the maximum micro-strain value produced by Twisted File Adaptive, WaveOne and ProTaper Next were respectively 101.92±17.17,170.81±16.44 and 352.29±43.24, and the differences of the average values of the maximum micro-strain value produced by the three devices were statistically significant (P<0.05). At site 2, that is, the most curved point of the curvature on the inner wall of the canal, the average values of the maximum micro-strain value produced by Twisted File Adaptive, WaveOne and ProTaper Next were respectively 140.68±9.20,293.22±32.06 and 393.95±37.51, and the differences of the average values of the maximum micro-strain value produced by the three devices were statistically significant (P<0.05). And for site 3, the point with 1 mm above the apical foramen of the root canal, the average values of the maximum micro-strain value produced by Twisted File Adaptive, WaveOne and ProTaper Next were respectively 231.74±44.16,477.42±105.04 and 396.73±49.95.The Twisted File Adaptive system produced the lowest average value of the maximum micro-strain value (P<0.05), while the WaveOne single file and the ProTaper Next device both caused higher results than the Twisted File Adaptive system, but the difference between the latter two groups were not of statistical significance (P>0.05).Chapter 3:comparisons on the dentin fissures of mandibular premolars in vitro canal instrumentation by Twisted File Adaptive, WaveOne and ProTaper NextMaterial and methods:thirty human mandibular premolars with the curvature degree less than 10° were prepared respectively by Twisted File Adaptive, WaveOne and ProTaper Next. The silicone rubber impression material was used to wrap the surface of the tooth roots so as to simulate the periodontal membrane. The epoxy resin was employed for the embedding of the tooth roots after canal instrumentation. The tooth roots were transected at 3,6 and 9mm from the apical part on the slow precision sawing machine with cooling water flow. Then the cross-sections of the tooth roots were photographed and evaluated with 12×stereomicroscope. If with any flaw of the dentin on any cross-section, this tooth should be determined as a fractured tooth.The result showed:the control group had no fissures. At the 3 mm level, the occurrence rates of dentin fissure in the groups of Twisted File Adaptive and ProTaper Next were respectively 10.0% and 13.3%, the difference between the control group and the two latter groups was not statistically significant. And there was also no statistically significant difference between Twisted File Adaptive and ProTaper Next groups (P>0.05). While the occurrence rate of dentin fissure in the group of WaveOne was highest than that of the control group, Twisted File Adaptive and ProTaper Next group (P<0.05). At the 6 mm level, the occurrence rates of dentin fissure in the groups of Twisted File Adaptive, WaveOne and ProTaper Next were respectively 23.3%,33.3% and 40.0%, there was no statistically significant difference among the experimental groups (P>0.05), and the difference between the control group and the experimental groups was statistically significant (P<0.05). At the 9 mm level, the occurrence rates of dentin fissure in the groups of Twisted File Adaptive, WaveOne and ProTaper Next were respectively 30.0%,36.7% and 33.3%, there was no statistically significant difference among the experimental groups (P>0.05), and the difference between the control group and the experimental groups was statistically significant (P<0.05). The total incidences of dentin fissure of the experimental groups were respectively 21.1%,35.6% and 28.9%, and the difference between the control group and the experimental groups was statistically significant (P <0.05). Meanwhile, the total incidence of Twisted File Adaptive was lower than WaveOne group (P<0.05), and has no statistically significant difference with ProTaper Next group (P>0.05). And there no statistically significant difference between WaveOne and ProTaper Next group (P>0.05).In summary, for the shaping functions, the apical transportation caused by Twisted File Adaptive was lowest. It was concluded that the shaping ability of new nickel titanium instrument WaveOne and ProTaper Next is good with little transportation in the middle and coronal part of the canal when preparing curved canal. For the biomechanical mechanism, the new self-adapting NiTi Twisted File Adaptive caused the lowest strain to the whole root canal in the canal instrumentation for simulated resin curved root canals, the WaveOne single file caused less force to the middle and upper part of the canal than that of the ProTaper Next device, while the strains caused by the WaveOne single file and the ProTaper Next device to the apical part were not significantly different. For the occurrence rate of dentin fissure, it could be concluded that the process of canal instrumentation can damage the dentin and cause the dentin fissures of the tooth root. In the apical part, the occurrence rates of dentin fissure in the groups of Twisted File Adaptive and ProTaper Next were lower than WaveOne group. And compared with WaveOne, the new NiTi Twisted File Adaptive can cause less total occurrence rate of dentin fissure after canal instrumentation, and can lower the risk of the tooth root fracture after the endodontic therapy. |
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