## 4 Discussion
Similarly, [PERSON] et al., while assessing the possibility of molar teeth canals shaping with the usage of ProTaper and Hero Shaper, maintained the patency of all prepared canals regardless of the system used [3]. [PERSON] et al. has also conducted the research on canal shaping with ProTaper and Hero 642 with the usage of training blocks. In that research, they also have not observed the obliteration of the canals [18]. [PERSON] et al. comparing the efficiency of molar teeth's canals preparation with the usage of 6 rotary systems and hand instruments have not observed any accidental canals' obliteration with the dentin debris [10]. Patency was maintained while preparation was also confirmed by other authors [19]. The used instruments were remained intact during preparation as a result of usage of new set for only one simulated canal. Own results are coherent with the same as obtained by [PERSON] et al., who compared the molar teeth canals' shaping with Mtwo, Reciproc, and K-files [4]. [PERSON] et al., who were preparing canals with rotary systems: K3, Mtwo, and Race, have not observed any separated files [20]. No deformation in files made of NiTi alloy was observed, however, it occurred in case of hand instruments. The frequency of deformation was rising with the instruments' diameter in apex, which can be explained with lower flexibility of instruments and higher susceptibility to deformation. Similarly, both [PERSON] et al. and [PERSON] et al. have not observed the instruments deformation, neither [4; 21] [PERSON] et al. [22] detected a very little deformation on the surface of WaveOne and ProTaper Universal (SEM assessment). In this study, the alterations of working length (WL), however not statistically significant, were observed. In the published research, the loss of WL was observed [3; 7; 8; 18; 23; 24; 25]. Some of these results has been reported as not statistically significant [3; 5; 10; 24], in the contrary to others. [7; 8; 18; 23]. No loss of WL was observed by [PERSON] et al. and [PERSON] et al. [19; 26]. [PERSON] et al., who instrumented \"L-shaped\" simulated canals, observed change of WL in ProTaper Next estimated at 0.22+/\(-\)0.082 mm [8]. Those results were more considerable than in our study (0.125+/\(-\)0.20 mm). [PERSON] et al. observed bigger change in WL in WaveOne (0.47 mm) and smaller in Mtwo (0.04 mm) [17]. [PERSON] et al. obtained lower WL loss in ProTaper Next than in WaveOne [25]. [PERSON] et al. obtained lower loss of WL while using with WaveOne Gold than in ProTaper Next. However, that loss was bigger than in own research [7]. In 5 out of 10 cases, while preparing the canal with hand K-files, elbow or ledge intracanal errors were created. Results obtained by [PERSON] et al. in endo-blocks negotiated with K-files showed that elbow and ledge occurred in 4 out of 10 cases [26]. They did not observe that type of deformations in simulated canals instrumented with NiTi S5. [PERSON] et al. more often observed elbow or zipp deformations in canals prepared with hand K-flex files (64.7%) than in case of NiTi Profile instruments (13.3%) [21]. [PERSON] et al. studying S-shaped canals preparation with Mtwo (single length and crowndown), ProTaper, WaveOne and Reciproc [17] observed a single case of aberration elbow or ledge type in case of WaveO\" and Mtwo (single-length technique). Instrumentation with ProTaper caused those types of aberrations in 3 out of 10 cases. No aberrations were observed in canals prepared with Reciproc and Mtwo (crown-down technique). [PERSON] et al. in 4 endo-blocks with \"L-shaped\" canals (out of 10) instrumented with ProTaper and 2 prepared with Hero 642 observed deformations of ledge or elbow type [18]. [PERSON] et al. observed: 3 case (out of 10) of ledge or elbow creation while preparing canals with WaveOne and 2 if instrumented with ProTaper or Reciproc [25]. Different methods allow assessing the canals' curve are described in the literature. The most popular and most frequently used one was presented in 1971 by [PERSON] [27]. Later [PERSON] et al.