| Root canal preparation is the key step in root canal treatment. The purpose of root canal preparation is to remove the infected pulp tissue, microorganisms and their metabolites mechanically and chemically in order to form a root canal cavity beneficial for obturation while maintaining the root canal anatomy. The outcome of root canal treatment is closely related with the root canal anatomy. Oval-shaped canal is defined as having a maximum diameter of up to 2 times greater than the minimum diameter. Previous study has reported a high prevalence of oval root canal in human teeth. The prevalence of oval-shaped root canal is 42-92%. However, cross sections of most root canal treatment instruments are round, which is not consistent with the cross sectional form of oval-shaped root canals. As a result, it is hard to clean and shape the oval-shaped root canals completely since the instruments do not contact the entire canal wall. After preparation, uninstrumented recesses may be left in many oval-shaped root canals, harboring pulp tissue and bacteria, thus jeopardizing the outcome of the root canal treatment. Preparation width should be increased when using conventional instruments in order to clean and shape the oval-shaped root canals completely. However, that may lead to ledge formation, zipping and perforation. Resistance to root fracture may also decrease. After preparation of oval-shaped canals, more dentin was removed mesiodistally, leaving weaker mesiodistal wall than buccolingual wall. To clean the recess completely without removing too much dentin mesiodistally represent a critical challenge.The self-adjusting file was introduced by Metzger et al. in 2010. It is a hollow and flexible root canal preparation instrument in the form of a compressible, thin-walled, pointed cylinder composed of 120μm thick nickel titanium lattice either 1.5 or 2.0mm in diameter. The 1.5mm file may easily be compressed to the extent of being inserted into any canal previously prepared or negotiated with a#20 K-file. The 2.0mm file will easily compress into a canal that was prepared with a#30 K-file. When the file is inserted into the root canal, the instrument adapts three dimensionally to the root canal and attempt to regain its original dimensions, thus applying a constant delicate pressure on the canal wall. When inserted into a root canal, it adapts itself to the canal’s shape, both longitudinally and along the cross section. It will attain a round cross section in a round canal whereas it will attain an oval cross section in an oval canal, providing a three-dimensional adaptation. The self-adjusting file is operated with vibrating handpieces with 3,000 to 5,000 vibrations per minute and amplitude of 0.4mm. The surface of the lattice threads is lightly abrasive, which allows it to remove dentin with a back and forth grinding motion. Another characteristic lies in the hollow design which allows for continuous irrigation throughout the procedure of mechanical preparation. It is reported that the self-adjusting file removed the smear layer more clearly. Since irrigates could flow freely through the hollow-designed shaft, amount of irrigate passing the apical foramen during operation is reduced, which could reduce incidence of post operative pain. The self-adjusting file is inserted carefully into the root canal carefully with in and out manual motion until it reaches the working length using two cycles of 2 minutes each for a total of 4 minutes per canal. Such procedure will remove a uniform dentin layer 60-75μm thick from the canal circumference. When comparing the shaping ability, the self-adjusting file could adapt itself to the root canal. It maintains the original shape of root canal with less straightening and perforation. When comparing the cleaning ability, since irrigation is performed during root canal preparation using self-adjusting file, debris and smear layer is easier to be removed.ProTaper Next is a brand new set of rotary instruments based on the convergence of a variable taper design on a given file ProTaper Universal from the company of Dentsply Maillefer. Compared with the ProTaper Universal instruments, there are three advancements in the ProTaper Next. The first one is the cross sectional design. Cross sectional shape of ProTaper Universal is convex triangle while the ProTaper Next is characterized by an innovative off centered rectangular cross section with greater strength. The second one is the way of motion. ProTaper Universal is working in a symmetrical rotation way while ProTaper Next works in a snake like swaggering movement, which makes it possible to completely prepare root canals using fewer instruments than the number required by the ProTaper Universal. Moreover, an offset design maximizes the auguring of debris out of the canal, compared with a file with a centered mass and axis of rotation. The third one lies in the instrument material. Files of ProTaper Next are made of special nickel titanium called M-Wire that is created by a thermal treatment process. The benefits of M-Wire nickel titanium are increased flexibility of the instruments and improved resistance to cyclic fatigue. Previous study on ProTaper Next focused on the shaping ability, cleaning ability of preparing curved canal, microcracks formation after root canal preparation and cyclic fatigue of instruments.Up to now, there are neither study on the ProTaper Next in preparing of oval-shaped root canals, nor study about comparison of preparation effect with self-adjusting file and ProTaper Next. As a result, this study aims to compare the shaping ability, cleaning ability and microcracks formation of self-adjusting files, ProTaper Next and ProTaper Universal when preparing oval-shaped root canals of mandibular anterior teeth, providing the theoretical basis for clinical use.The present paper mainly consists of the following four chapters:Chapter 1:shaping ability of self-adjusting file, ProTaper Next and ProTaper Universal in preparation of resin canals30 resin canals with curvature of 35 degree were prepared with self-adjusting files, ProTaper Next and ProTaper Universal. Black ink and red ink was injected into the canal and pictures were taken using a fixing set made of silicone rubber respectively before and after preparation. Software of Adobe Photoshop v7.0 was applied to overlay the figure before and after preparation. Configuration was performed using AutoCAD. Results show that amount of resin removal increased while the distance from the apex increased. Continuous taper was obtained after preparation in all of the three groups. In the detecting point 0mm and 1mm, self-adjusting files removed more resin than ProTaper Universal and ProTaper Next (P<0.05). In the detecting point 3mm,4mm,5mm,6mm,7mm,8mm,9mm and 10mm, self-adjusting files removed less resin than ProTaper Universal and ProTaper Next(P<0.05). In the detecting point 2mm,11mm,12mm and 13mm, there were not statistically significant differences among three groups (P>0.05). In the Omm and 1mm, ProTaper Next transported less than self-adjusting files and ProTaper Universal (P<0.05). In the 3mm, ProTaper Next transported less than ProTaper Universal (P<0.05), but not different from self-adjusting files(P>0.05). In the detecting point 5mm,6mm,7mm,8mm,9mm,10mm and 11mm, ProTaper Next and self-adjusting files transported less than ProTaper Universal (P<0.05). There were no statistically significant differences between ProTaper Next and self-adjusting files. When comparing the preparation time, the differences were statistically significant. Time for self-adjusting files was longest (300.3±5.5s). Time for ProTaper Universal was 211.7±10.5s. ProTaper Next cost least time(178.5±5.1s). It could be concluded that ProTaper Next and ProTaper Universal removed considerable amount of resin but ProTaper Next transported less and cost less time than ProTaper Universal. Self-adjusting files removed more resin in the apical but less in the middle than ProTaper Next and ProTaper Universal. In the coronal part, there were no differences among three groups. Self-adjusting files transported more than ProTaper Next in the apical part, which was the same as ProTaper Universal. However, it transported the same as ProTaper Next in the middle and coronal, which was less than ProTaper Universal. It was concluded that the shaping ability of new nickel titanium instrument Self-adjusting file and ProTaper Next is good with little transportation when preparing curved canal. Time in preparing canal by ProTaper Next is less.Chapter 2:in vitro shaping ability of self-adjusting file, ProTaper Next and ProTaper Universal in preparation of oval-shaped root canals:a micro-ct study45 extracted mandibular anterior teeth with oval canals were prepared by self-adjusting files, ProTaper Next and ProTaper Universal, respectively. The teeth were scanned by micro-CT fixed by a set made of silicone rubber before and after preparation. Files of DICOM format were obtained and input into Mimics11.0 and Mevislab 2.4 to overlay canals and reconstructed root canals three dimensionally in order to compare area of canal before and after preparation, area of instrumentation, volume of canal before and after preparation, volume of resin removal and unprepared surface ratio. It showed that both volume and area increased after preparation in all of the three groups. Neither preparation complication nor instrument separation occurred. After preparation, area of root canal increased to 5.13±2.84mm2 by removing 1.73±0.83mm3 root dentin in self-adjusting files. In ProTaper Next, area of root canal increased to 5.86±2.14mm2 by removing 1.95±0.58mm3 root dentin. In ProTaper Universal, area of root canal increased to 6.79±2.12mm2 by removing 1.77±0.57mm3 root dentin. There were no statistically significant differences among three groups about the area and volume of root canal before preparation, amount of area increased and amount of dentin removed (P>0.05). The unprepared surface ratio was 24.44±3.56%、41.82±4.92%、39.38±3.80% for the self-adjusting files, ProTaper Next and ProTaper Universal respectively. Unprepared surface ratio was less in self-adjusting files (P<0.05). There were no statistically significant differences between ProTaper Next and ProTaper Universal (P>0.05). There were no statistically significant differences among three groups about the area and volume before preparation, volume increasing and amount of resin removal (P>0.05). Time for self-adjusting files was longest (285.4±11.7s). Time for ProTaper Universal was 201.0±9.4s. ProTaper Next cost least time (176.7±8.6s). It could be concluded that self-adjusting file, ProTaper Next and ProTaper Universal could not completely shape oval-shaped root canals. Self-adjusting files removed dentin more homogeneously, with a little uninstrumented root dentin. The shaping ability between ProTaper Next and ProTaper Universal was the same. It was concluded that Self-adjusting File removed root dentin more homogeneously with a little unpreparaed uninstrumented surface when preparing oval-shaped root canal. Shaping ability of ProTaper Next and ProTaper Universal was not of difference when preparing oval-shaped root canal. Time in preparing oval-shaped root canal with ProTaper Next is the least.Chapter 3:in vitro cleaning ability of self-adjusting file, ProTaper Next and ProTaper Universal in preparation of oval-shaped root canals:a scanning electronic microscopic study30 extracted mandibular anterior teeth with oval canals were prepared by self-adjusting files, ProTaper Next and ProTaper Universal. A longitudinal groove was cut along the root. The root was split longitudinally. Scanning electronic microscopy was used to observe the debris and smear layer in the root canal surface in the coronal 1/3, middle 1/3 and apical 1/3. Results showed that there were no statistically significant differences in debris and smear layer in the coronal third among three groups (P>0.05). In the middle1/3, debris was less in self-adjusting files than ProTaper Next (P<0.05), less in ProTaper Next than ProTaper Universal (P<0.05). Smear layer was less in self-adjusting files than in ProTaper Next and ProTaper Universal (P<0.05). There were no statistically significant differences between ProTaper Next and ProTaper Universal (P>0.05). In the apical 1/3, debris and smear layer scored less in self-adjusting files than ProTaper Next and ProTaper Universal (P<0.05). There were no statistically significant differences between ProTaper Next and ProTaper Universal (P>0.05). It was concluded that the cleaning ability of Self-adjusting file, ProTaper Next and ProTaper Universal was in coronal third when preparing oval-shaped root canal. In the middle third, debris was less in self-adjusting files than ProTaper Next, less in ProTaper Next than ProTaper Universal. Smear layer was less in Self-adjusting files than in ProTaper Next and ProTaper Universal with no difference between ProTaper Next and ProTaper Universal. In the apical 1/3, Self-adjusting files cleaned the oval-shaped canal with less debris and smear layer than ProTaper Next and ProTaper Universal.Chapter 4:microcracks formation after preparation of oval-shaped root canals in vitro by self-adjusting file, ProTaper Next and ProTaper Universal90 extracted mandibular anterior teeth with oval-shaped root canals were prepared by self-adjusting files, ProTaper Next and ProTaper Universal. Roots were embedded by epoxy resin and cut horizontally at 3,6 and 9mm from apex. Cross sections were obtained and observed under stereomicroscope with 12times magnification to see whether there were microcracks. Evaluation criteria were as follows:"no defect" was defined as root dentin without cracks or craze lines either at the internal surface of the root canal wall or the external surface of the root. Defects referred to all lines observed on the slice that either extended from the root canal lumen to the dentin or from the outer root surface into the dentin as well as other lines observed that did not seem to extend from the root canal to the outer root surface. Teeth were judged as cracked teeth as long as one section had dentinal defect. Results show that incidence rate was higher in ProTaper Universal than ProTaper Next and self-adjusting files (P<0.05). There were no statistically significant differences between ProTaper Next and self-adjusting files (P>0.05). It could be concluded that root canal preparation could destroy dentin leading to dentinal microcracks. Compared with ProTaper Universal, ProTaper Next and self-adjusting files produced less dentinal microcracks, which could decrease risk of root fracture after root canal treatment. It was concluded that microcrack formation after root canal preparation was less in the new nickel titanium instrument Self-adjusting file and ProTaper Next, which could decrease risk of root fracture after root canal treatment.In conclusion, the new nickel-titanium instrument self-adjusting file and ProTaper Next shaped the resin canal well. ProTaper Next removed similar amount of resin to the ProTaper Universal with less transportation and less time. Self-adjusting files removed more resin and transported more than ProTaper Universal in the apical while they removed less resin and transported less in the middle and coronal. When preparing oval-shaped root canals in extracted teeth, self-adjusting files removed root dentin more homogenously with a little root dentin untouched. The shaping ability between ProTaper Next and ProTaper Universal is similar. When comparing cleaning ability, self-adjusting files were better than ProTaper Next and ProTaper Universal in debris and smear layer cleaning in middle and apical part. ProTaper Next removed debris more clearly than ProTaper Universal in the middle. Microcracks formation was less in self-adjusting files and ProTaper Next than ProTaper Universal, which could decrease risk of root fracture after root canal treatment. |
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