# A practical transportation of Apical Foramen by Different NiTi Alloy Systems: A Systematic Review
## 3 Results and Discussion
The results of this study may also be influenced by procedural errors, as the authors prepared the teeth using instruments with different apical diameters (EdgeFile with #40, ProTaper Next with #30, and One Shape with #25). Finally, the authors concluded that the absence of differences in apical transportation could be due to the limited sample size of the study. The discrepancy in the results obtained from these two studies is likely attributable to various factors, and is not necessarily related to the alloy of the file used. Instead, the differences may be attributed to variations in the design and kinematics of the instruments, as well as the asymmetric or centralized motion and rotation axis of the instruments. Other factors, such as sample size, methodology, and procedural errors, may also contribute to the discrepant findings [55]. Only three of the studies included in this systematic review evaluated the apical transportation occurring during the root canal shaping phases in curved canals [35; 36; 39]. The apical curvature of the canal is one of the factors with the greatest effect on the occurrence of apical transport. In fact, the risk of transportation is linked to the degree and radius of canal curvature: the more severe the curvature and the smaller the radius of curvature, the more distinct the risk of straightening. Furthermore, it has also been shown that both the radius and the degree of canal curvature have an impact on the stress of the instruments used to prepare these canals [56; 57]. For these reasons, future scientific literature should carry out such evaluations on canals with apical curves. Only one of the studies included in this systematic review of the literature investigated apical transportation using resin blocks and no teeth [36]. In these resin blocks, the canals had precisely defined parameters, including length, taper, curvature, and degree of curvature. All resin blocks were the same, thus representing a sample with repeatable characteristics. This is essential to be able to exclude the bias that can affect the results of studies performed on non-homogeneous samples. In fact, it is possible that the characteristics of root canal anatomy, such as length, curvature and taper, can alter the results of the research. Furthermore, the resin blocks are transparent, making measurements easier for the researcher [36]. The authors concluded that martensitic endodontic instruments determined a lower apical transportation than austenitic instruments during the endodontic shaping phases [36]. However, it must be considered that the dentin and the resin used have different physical characteristics; the compressive resistance and elasticity of resin are lower than dentin [56]. Furthermore, during the shaping phase, the rotation of the tools in the resin blocks generates heat, which may contribute to the softening of the resin material, thus leading to inefficient shaping [58; 59]. However, it must be considered that most of the studies included in this systematic review of the literature compare different instruments not only in terms of the type of heat treatment undergone, but also with regard to characteristics such as taper, cross-sectional profile and blade design. Differences in mechanical properties such as flexibility are not only determined by heat treatment, but also by geometric factors such as pitch length, taper or conicity, and cross-sectional shape [60]. All of these characteristics affect clinical performance. For example, non-cutting tips are characterized by a reduction in the tip or the transition angle [61; 62]. The literature shows that passive (and therefore non-cutting) tips result in more uniform material removal on the outside and inside of the curve than instruments with active tips [63]. Some of the studies included in this review compared the apical transportation occurring during the shaping phase using instruments with the same design but using different heat treatments [32; 34; 35].