# A practical transportation of Apical Foramen by Different NiTi Alloy Systems: A Systematic Review
## 3 Results and Discussion
This method is important, as it can be used in in vitro studies to improve the distribution in the study sample, using established morphological parameters to provide a consistent baseline [41]. For example, it is possible to select radicular systems with similar morphologies to eliminate potential anatomical bias that would alter the research results, using established morphological parameters to provide a consistent baseline [42]. The authors of the studies included in this review agree on the results, stating that the use of instruments with martensitic alloys results in less apical transportation during endodontic treatment [22; 31; 32; 34; 35; 36; 37; 38]. The NiTi alloys used in endodontic instruments contain approximately 56% by weight of nickel and 44% of titanium, in an equiatomic ratio of 1:1 [41]. In some nickel-titanium alloys, 2% of the nickel is converted to cobalt [43]. Due to the possibility of changing the type of atomic bonds by means of the nickel and titanium included in the alloy, the mechanical properties of NiTi alloy and its crystal structure can change. The alloy can exist in two different microstructural phases--austenite and martensite--with distinct mechanical properties [8]. In the austenitic phase, the alloy is rigid and exhibits shape memory, meaning that when the instrument is bent, it tends to return to its original shape. On the other hand, the martensitic phase is more flexible, and does not have shape memory [21]. The phase transition can occur either through the application of force or through temperature variation. * Application of force: This results in the transition from the austenitic phase to the martensitic phase. Force is applied when the canal walls compress the rotating instrument, particularly in curved canals [21; 44; 45]. * Variation in temperature: High temperatures induce the austenitic phase, while low temperatures induce the martensitic phase [21; 44; 45]. The heat treatments applied to NiTi alloys by companies are aimed at modifying the alloy's transition temperature, which is the temperature at which the alloy transitions from one phase to another. This allows the creation of instruments with varying percentages of martensite and austenite, resulting in instruments with different levels of flexibility [21; 44; 45]. The flexibility of the instrument enables it to rotate centrally within the canal, reducing alterations to the endodontic anatomy. When an endodontic instrument passes through a curved canal, it tends to work more on the outer portion of the curve, particularly if the instrument is rigid [45; 46; 47]. Since it is rare to find a perfectly straight canal in nature, with most canals having varying degrees of curvature, especially in the apical portion, a rigid instrument is more likely to cause apical transportation [22; 31; 32; 34; 35; 36; 37; 34; 37; 38; 39; 48; 50; 51; 52]. Two the studies included in this systematic review focused on endodontically treated teeth [32; 35]. Both studies concluded that the use of martensitic alloys resulted in less apical transportation, even in cases of retreatment. Two of the articles included in this review did not find any differences in apical transportation when comparing austenitic and martensitic instruments [33; 39]. 1. [PERSON] et al. suggested that the absence of differences in apical transportation could be attributed to the specific characteristics of the iRaCe instruments analyzed, such as their small cross-sectional areas, which increase flexibility and allow for better debris removal, as well as the design of their working parts, which feature alternating cutting edges. This design enables the preparation of curved root canals with larger apical diameters to be achieved with minimal apical transportation compared to when using other rotary NiTi instruments [39; 53; 54]. 2. [PERSON] et al. stated that there were no differences in apical transportation among the tested instruments, but attributed this result to the similar behavior of the ProTaper Next and EdgeFile alloys [33].