## 4 Discussion
observed that two root canals with the same angle can have a different curvature radius and updated the method providing the value of the radius made it possible to define the curvature of the canal [28]. Their invention is of great clinical importance because the bigger the canal curve and the lower the curvature radius; the risk of straightening the canal is higher. The curvature parameters influence on the instrument's overload and subsequently positively correlates with the risk of instrument separation. In 2005 the term CAA was introduced by [PERSON] et al. as well as two new parameters describing the coronal part of the curved canals: initial distance of curvature and curvature length [29,30]. [PERSON] et al. conducted research on 80 patent canals with diameter in apex \(\geq\)15 ISO and curvature angle 25-39\({}^{\circ}\), using Reciproc, WaveOne, Mtwo, as well as ProTaper instruments, T and observed a similar degree of natural curvature preservation in all instruments--no statistically significant difference between them [31]. Results acquired in our research differ from those presented by other authors. It may be due to the different technique of curvature angle measurement. Because the canal was prepared on the working length shortened for 0.5 mm, the simulated canals' apical foramen was not prepared and subsequently its location was not changed. Each instrument removed material from both inner and outer canals' curvature. As a result, the A-line was moved inside and crossing that line with the outer canals curvature was altered. That crossing was located in a bigger distance concerning canal orifice and more outer to the primary location. As a result, the curvature angle increased. Comparison of examined groups proved a statistically significant difference only for canals prepared with WaveOne and Mtwo (taper 7% and 6%). Mtwo and ProTaper Next has the same taper (6%) of the last instrument used in the study. However, while comparing WO group (taper 7%) and PN group (taper 6%) no statistically significant difference was observed. Similar results were observed when comparing PN and MT groups. On the basics of the obtained data, it cannot be clearly stated if the statistically significant difference was caused by the usage of instruments with different taper or by differences in alloys used for the manufacturing of the particular type of instruments. The increase of simulated canals volume after mechanical preparation was observed in each examined group. [PERSON] et al. investigating the changes in the volume of 5 single-rooted premolars prepared with hand ProTaper instruments till size F3 [32] by means of microcomputed tomography, observed the decrease in canals volume in each case. The decrease in volume was correlated with the type of canal according to Wein classification, and it was estimated between 4.8% and 150.2%. [PERSON] et al. assessed the changes of canals volume in 50 molar teeth using a microcomputed tomography [33]. They divided the material into two groups. In group I, canals were prepared with the step-back technique and hand K-files made out of NiTi alloy. In the second group, ProTaper instruments and the crown-down technique were used. They determined volume changes in group I: 56.2% to 71.9% and in II: 72.8-78.2%. [PERSON] et al. examined the changes in simulated canals volume prepared withPtoTaper Universal vs Reciproc and K3 XF, utilizing the MicroCT method. The least change in volume in each part was observed in canals instrumented with K3 XF and the highest in ones instrumented with Reciproc [34]. The volumetric changes should be correlated to different tapers of used instruments. WaveOne instruments are characterized by the biggest taper (7%) out of used instruments, that is why the volume change was the biggest. Mtow and PtoTaper Next have the same taper, so the volume changes in MT and PO groups did not have a statistically significant difference. The hand K-files with the smallest taper caused the lowest volume changes in K group.