14 DENTAL TRIBUNE Pakistan Edition January 2018 Study shows positive ... Continued from page 4 lower risk after receiving CAMBRA preventive therapies. Dental caries was low in both groups. A m o n g 2 4 2 p a t i e n t s ( 1 3 7 intervention, 105 control) initially identified as high risk for caries, only a quarter of the patients remained at high risk in the CAMBRA group at 24 months, and just over half (54 percent) in the control group. Of the 192 low-risk participants (93 intervention, 99 control), most participants remained low risk. The researchers believe this points to the assessment correctly identifying those at risk for caries. Study shows positive ... Continued from page 6 Acknowledgements Andy Morton and Ian Murch, the fantastic laboratory technicians at Borough Crown and Bridge that I work closely with. -DT UK Dentinal defects after root ... Continued from page 8 taken from 240 tooth slices were examined. The distribution of dentinal defects caused by the tested NiTi file systems between apical, medial, and coronal regions is presented in Table 1. In the present study, no statistically significant difference was found among the NiTi files tested and between them and control group in terms of the total number of dentinal defects (P > 0.05). Discussion In this study, the dentinal defects created by HEDM, WOG and PTG NiTi file systems on mandibular molar teeths mesial canals were evaluated. The Number and Percentage of Slices with Defects at Each Level (n = 20) Group Control 3mm n(%) 6mm n(%) 9mm n(%) Total of specimens of presenting defects n (%) 4 (20 %) 8 (40 %) 6 (30 %) 11 (55 %) Pro Taper GOLD 8 (40 %) 10 (50 %) 8 (40 %) 14 (70 %) WaveONE GOLD 6 (30 %) 9 (45 %) 8 (40 %) 12 (60 %) HyFlex EDM 5 (25 %) 10 (50 %) 7 (35 %) 11 (55 %) According to the results of the present study, it was determined that all of the tested NiTi files created dentinal defects but no statistically significant difference was found when compared to thecontrol group. For this reason, the null hypothesis of the present study was accepted. In many in vitro studies, the dentinal defects created by the NiTi file systems during root canal preparation were investigated using single- and straightrooted teeth (Karata et al. 2015b, Kfir et al. 2017). However, the increasing root canal curvature would increase the stress on the files, which were used in preparation, and consequently on the dentine. An increase in the stress on dentine would cause increasing irregularities (transportation, straightening, etc.) within the canal and lead to thinner dentinal structure in certain regions. Thinner dentine would weaken the root structure and prepare the ground for vertical root fracture formation (Kim et al. 2013). In previous studies, it has been reported that the highest level of stress occurred on the curved root canals during the root canal preparation by NiTi rotary files (Kim et al. 2013, Medha et al. 2014). For this reason, the mesial canals of mandibular molar teeth were used in present study. NiTi file manufacturers generally recommend using the files on single. Based on four-canal maxillary first molar tooth in present study, the files were discarded after use in four canals (two specimens) in order to prevent the deformation from influencing the results (Hin et al. 2013). It has been reported that use of larger files in shaping the root canals increase the incidence of dentinal defect formation (Capar et al. 2015). For this reason, in the present study, the apical diameter of files was determined to be 0.25 mm, and no larger file was used. Moreover, in order to protect the dentine microstructure, 1 % NaOCl was used as an irrigation solution. Thus, it was ensured that most of the dentinal defects to be related with the mechanic preparation. It has been reported that the forces applied while extracting the teeth, and the stress during storing the teeth and obtaining the slices might cause dentinal defects (De-Deus et al. 2014). This may explain the formation of dentinal defects in the negative control group, where no intervention was made. Studies using conventional methods of sectioning have failed in determining these defects in negative control groups (Capar et al. 2014, Karata et al. 2015b, Li et al. 2015). When illumination was applied on the obtained dentine slices, the light moved along the dentine, but stopped at the point of any crack on dentine and thus the presence of crack and/or fracture could be determined ( A m e r i c a n A s s o c i a t i o n o f Endodontists, 2008). In Coelho and colleagues study (2016a, 2016b), dentinal defects could be determined in many specimens in negative control groups by employing light-emitting diode (LED) trans-illumination. Moreover, Arslan et al. (2014) have also used methylene blue in order to determine dentinal defects, and they reported non-significant differences between the experimental group and negative control group in terms of dentinal defects. Arias et al. (2014) reported in their study that masking is important for eliminating observer bias because of observing which of the specimens had been shaped or not. For this reason, the canal-masking method was used in the present study in order to eliminate any bias. The movement kinematic of NiTi files may affect the amount of dentinal defects during canal root shaping by files. Under favour of the reciprocation movement in clockwise and counterclockwise directions, the file is protected from being stuck within the canal while shaping the root canals (Yared 2008). Some of the studies have reported WaveOne files to cause less dentinal defects than ProTaper Universal files (Kansal et al. 2014, Li et al. 2015), while some other studies have reported that reciprocation systems create more dentinal defects (Bürklein et al. 2013). Besides that, in some studies, no statistically significant difference between the reciprocation systems and rotary systems has been reported (Arias et al. 2014, Karata et al. 2015a, Coelho et al. 2016b). Li et al. (2015) have examined the dentinal defect formation incidences of ProTaper Universal, ProTaper Next and WaveOne files in curved root canals of molar teeth. The researchers have reported that ProTaper Next file system created less dentinal defects than other files. El Nasr and El Kader (2014) have reported ProTaper Universal F2 files operating based on the same movement kinematic with WaveOne system to cause less dentinal defects. Similar to the results of other studies (Capar et al. 2014, Li et al. 2015), the researchers attributed these results to the heat treatment, to which WaveOne files are exposed. Karata et al. (2015a) have examined the dentinal cracks created by ProTaper Universal, ProFile Vortex (Dentsply Maillefer), ProTaper GOLD, Reciproc (VDW, Munich, Germany), and F360 (Komet Brasseler, Lemgo, Germany) files systems in mandibular incisor teeth, and reported that there was no statistically significant difference between ProTaper Universal, ProFile Vortex, ProTaper GOLD and Reciproc groups in terms of dentinal defect formation. It is believed that the reason for the difference in the present study originates from the differences in methodologies used. Similar to the present study, Coelho et al. (2016b) have used LED in investigating the dentinal defects created by ProFile (Dentsply Maillefer), TRUShape (Dentsply Maillefer) and WaveOne GOLD systems on mandibular molar teeths mesial canals, and reported statistically non-significant differences between the negative control group and experimental groups in terms of dentinal defects. Capar et al. (2014) have examined the dentinal defects created by HyFlex CM, ProTaper Universal and ProTaper Next NiTi file systems on mandibular premolar teeth during preparation procedure. The researchers have reported ProTaper Next and HyFlex CM files to cause less dentinal defects than the ProTaper Universal files. Ashraf et al. (2016) have examined the dentinal defects created by ProTaper Universal, ProTaper Next and HyFlex CM NiTi file systems in mandibular premolar teeth by using sectioning method. Researchers have reported that HyFlex CM files caused less cracks than ProTaper Universal and ProTaper Next files did. In our literature review, it was determined that HEDM files dentinal defect incidence had not been studied before. For this reason, it is not possible to directly compare the results of the present study to those of others. In a finite elements analysis, it has been shown that increasing the taper of files also increased the stress on root canals during shaping procedure (Kim et al. 2010). Bier et al. (2009) reported that the taper of files might influence dentinal defects on roots during the shaping procedure. Yolda et al. (2012) have alleged that the tip design, cross- section, constant or variable taper, and groove and pitch structure of NiTi files might be related with the formation of dentinal defects. However, it is not exactly known how the taper of files affected the results of present study, because the taper of files used were not same and the taper of HEDM file was not known. The similar results obtained are thought to originate from the fact that the files were made of alloys having no shape memory (Gold and CM). Versluis et al. (2006) have reported that the level of stress on coronal and medial third during root canal shaping was three times more than that on the apical third. Despite that, Kim et al. (2010) have reported the stress on the apical third during shaping of the curved root canals to be more than that on the middle and coronal third. According to the results of the present study, there was statistically non- significant differences between the dentinal defects created by NiTi file systems on the apical, middle, and coronal third, and this finding is believed to originate from the fact that the files were made of alloys having no shape memory (Gold and CM). Even though it was important to simulate the clinic conditions in a laboratory environment in the present study, especially in the studies on examining the mechanical properties of teeth, many external factors such as storing the teeth after extraction and until the sectioning procedure affected the results of study (Bürklein et al. 2013). For this reason, as stated by Coelho et al. (2016b) in their study, the use of teeth extracted using periodontal reasons, which require very low level of force during extraction, and the careful storage of these teeth until the sectioning procedures would allow for more successful outcomes. Another limitation of present study is the difficulty of standardisation of apical pressure applied by the operator during root canal shaping procedure and that this may influence the results. Conclusion Within the limitations of the present study, no statistically significant difference was found among the HEDM, WOG, PTG and the control group in terms of the total number of dentinal defects.