One of the major problems during root canal preparation is instrument fracture. Instruments may fracture at different levels of stress, so various types of NiTi rotary endodontic instruments systems have been developed to overcome this problem. The geometrical design of the NiTi rotary instruments dictates their mechanical behavior and fracture tendency (Ha et al. 2016, 2017; Hamdy et al. 2019).
The fracture of rotary endodontic instruments during endodontic treatment of curved root canals is often considered an enigma for endodontic practitioner. Mainly, they arise on exceeding torsional or bending stresses during function (Rzhanov and Belyaeva 2012). Therefore, torsion and bending are considered as two essential parameters of the endodontic files to ensure good performance and safety (McGuigan et al. 2013).
Raw materials, design, and manufacturing process can have a significant impact on instrument fracture (Shen et al. 2016; Gavini et al. 2018). Fracture of the NiTi rotary endodontic instrument may occur due to repeated bending stresses (compressive and tensile) accumulated at the point of maximum stress of the file in a curved root canal. Torsional fracture arises when the tip of the file becomes locked in the root canal, while the shank of the rotary instrument continues to rotate (Kim et al. 2012).
The procedure of NiTi rotary instruments manufacture influences their functional quality. The geometrical design of NiTi rotary instruments affects the clinical performance and durability. Consequently, geometric optimization is a platform to improving the fracture resistance of NiTi rotary instruments. The geometrical optimization needs to study each variation in design separately to achieve the most promising design (Ruddle et al. 2013).
Although there have been considerable developments in the design and manufacturing techniques of NiTi rotary instruments, instrumental fracture and separation remains a concern, particularly in severely curved root canals. Several investigations have exposed that both torsional stiffness and bending flexibility of a rotary endodontic file depend on the geometrical design such as cross-sectional design, pitch, taper of radial lands rake angles tip and off-center cross section (Basheer Ahamed et al. 2018).
Finite element analysis study is a beneficial approach to evaluate the mechanical performance of endodontic files and stress distribution. Moreover, it creates a more controlled condition, allowing evaluation of each geometrical design parameter separately (Tsao et al. 2013; Jiang et al. 2018).
In terms of cross-sectional shape, various kinds of NiTi rotary system have been hosted in the market, each having a unique cross-sectional shape that has a different behavior on stress distribution and, hence fracture tendency. Four cross-sectional shapes may be achieved such as convex triangle, triangle, rectangular, and parallelogram (Medha et al. 2014). A cross-section shape of NiTi rotary instruments is the most critical parameter affecting the instrument fracture, hence lifespan (Cheung et al. 2011).
During rotary movement of endodontic instruments, contact between the instrument and root canal walls generates internal stresses in the instrument and on the root canal wall. While this force is essential for tooth cutting, it can cause a screw-in effect (an apical driving force) that may cause over extension of the instrument beyond the tooth apex (Ha et al. 2016). Geometrically, parallelogram (slender rectangular) and rectangular cross-sectional instruments (i.e., one- or two-point contacts) showed lower screw-in forces than triangular and convex triangular cross-sectional instruments (three-point symmetrical contacts) (Kwak et al. 2019). A previous study showed that triangular cross sections result in more flexibility than square designs for the same file taper and diameter (Versluis et al. 2012).
Pitch of the file refers to the number of flutes per unit length. It was revealed that reducing pitch (increasing threads) constantly reduced the rigidity (stiffness) of the files. Thus, increasing their flexibility with marked decrease in the internal stress of the file (Baek et al. 2011; Versluis et al. 2012). Taper is the increase in the diameter of file per millimeter increase in length. Taper is one more feature of file design. It has been shown that increased taper will increase file rigidity and may produce a higher screw-in effect (Kwak et al. 2019).
Recently, an off-centered cross-sectional design was introduced to some endodontic files such as Revo-S system and ProTaper Next. The new design depends on deviation of the rotational center from being in the center of the cross section when compared with conventional concentric rotary conventional Ni-Ti file. The modified off-centered rotational architecture features produces a bombastic motion of the instrument inside root canal (snake-like) that decreases the creation of the stress upon the rotational movement. Thus, reducing contact of the instrument with wall of canal. Moreover, it creates more room for easier debris removal (Pasqualini et al. 2015). There are no currently available clinical studies, until now, regarding the stress patterns of the off-centered design of the endodontic instruments.
There are demands to revolutionize the designs and metallurgy of the NiTi rotary instruments to overcome the problem of fracture of the instruments on use. Therefore, the aim of the current study was to evaluate imaginary innovative geometrical design modalities (cross-sectional geometry, pitch, taper, and off-center cross section) on the stress distribution in NiTi instruments under bending and torsion conditions using finite element analysis (FEA).