# A practical transportation of Apical Foramen by Different NiTi Alloy Systems: A Systematic Review
## 1 Introduction  
Shaping the root canal system is essential for achieving clinical success in endodontic treatment [1]. Canal shaping should aim to preserve the canal and apical anatomy as much as possible [2]. Numerous NiTi instruments have been introduced to reduce alterations to and deformations of the canal during endodontic shaping [3, 4, 5]. However, due to various factors, such as the geometry, alloy, and movement of the instruments, as well as the characteristics of the canal (dentin hardness, canal curvature radius and angle, and complex anatomies) and operator experience, it is still possible to alter this anatomy, leading to consequences such as stripping, canal transportation, or apical transportation [6, 7]. Apical transportation refers to the deviation of the physiological apical position from theiatrogenic position, generally located on the outer surface of the curvature, which can compromise the outcome of root canal treatment, and once transportation has occurred, it is impossible to return the canal to its original shape, particularly in curved canals [8; 9]. Apical transportation occurs due to the leaning of the rotary instrument in order to return to the original position when subjected to deformation. Consequently, rotary instruments tend to remove more of the dentin on the outermost side of the canal curve. Apical transportation can be classified into three types, depending on the degree [10]:  
Type I: minor movement of the position of the foramen, leading to its iatrogenic relocation. Type II: moderate movement of the physiological position of the foramen, with consequent iatrogenic displacement on the external root surface. In this type of apical transport, a wider communication with the periapical space is achieved. As a result, type II apical transportation can lead to a weak or perforated root. Type III: severe movement of the physiologic position of the canal, resulting in a significant iatrogenic relocation of the physiologic foramen. Regarding the treatment of different types of apical transportation, type I transportation is the only type manageable by nonsurgical endodontics. Type II and III require surgical endodontics in order to properly complete the endodontic therapy [11]. Apical transportation may result in inadequately cleaned root canals, root weakening, and improper debridement of the canal in the apical region, and can also lead to the zipping or perforation of the apical foramen [12]. Moreover, it may result in the extrusion of irrigation solution, debris, and filling materials [13; 14]. Furthermore, although type I does not require surgical endodontic treatment, it can still make orthograde endodontic therapy more difficult, sometimes making it necessary to use mineral trioxide aggregate (M.T.A.) or new biocarcanic materials in order to obtain a correct apical seal [15; 16]. In recent decades, there has been a revolution in the production of endodontic instruments with different mechanical characteristics. Technological advancements in the production and treatment of NiTi alloys for use in endodontic files have allowed the development of alloys with different phase transformation behaviors, improving their flexibility, centering ability, fatigue, and torsional resistance [17; 18; 19; 20; 21]. Companies have devised various methods for altering the manufacturing process or modifying the physical properties of NiTi instruments, including the use of complex heating-cooling phases to vary the temperature of NiTi alloy's transition between austenitic and martensitic phases. While austenitic NiTi is strong and hard, the material becomes soft, ductile, and flexible in its martensitic form [22; 23; 24; 25; 26]. This has led to the creation of alloys such as M-Wire(tm) (Dentsply-Tulsa & WDV), R-Phase(tm) (Kerr), CM-Wire(tm) (Coltene Whaledent), Blue(tm) (Dentsply-Tulsa), Gold(tm) (Dentsply-Tulsa), and Max-Wire(tm) (FKG), characterized by their ability to transition from the martensitic to austenitic phase at different temperatures.