Nickel–titanium alloy (Ni-Ti) rotary instruments must exert torque to cut and eradicate septic dentin during canal preparation; torsional stress, associated with friction between the instrument and dentin wall, accumulates in the instruments.
- dynamic torsional test
- force
- nickel-titanium rotary instrument
- root canal preparation
- screw-in tendency
- torque
1. Introduction
Nickel–titanium alloy (Ni-Ti) rotary instruments must exert torque to cut and eradicate septic dentin during canal preparation; torsional stress, associated with friction between the instrument and dentin wall, accumulates in the instruments [1][2][1,2]. The accumulated stress can be retained as residual stress—plastic deformation—after withdrawal of the instrument from the canal if the stress exceeds the elastic limit [3]. The engagement of rotary instruments, especially those with spiral-shaped active cutting edges, with the dentin wall can generate apically-directed screw-in forces, causing the instrument to become locked in the canal [4]. When this occurs, additional torque is required for the instrument to continue rotating. Thus, torsional stress is instantly accumulated in the instrument, leading to torsional fracture [5][6][5,6], which is dissimilar to cyclic fatigue fracture caused by the repeated tension/compression stresses at the curvature [7]. Furthermore, screw-in forces may cause the instrument to engage beyond the apical foramen [8] and result in the extrusion of microbes into periapical tissue [9], root weakening, and cracks in the apical area [10].
Numerous studies have been conducted to examine the dynamic torque and force characteristics of Ni-Ti instrument systems to identify factors having an impact on the stress generated within the rotary instruments. Though potential influencing factors, such as instrument pitch length [11][12][13][14][11,12,13,14] and instrument rake angle [11][15][11,15], have been discussed and debated, no single-most important factor has been identified. Thus, there continues to be debate on how the stress generated within Ni-Ti instruments during root canal instrumentation can be limited to a level at which clinical safety is ensured.
2. Data, Model, Applications and Influences
As shown in Figure 1, 4096 articles were identified. After duplicates were removed and preliminary screening was conducted, 75 articles underwent full-text review. Fifty-two studies (Table 1) were eligible for inclusion.
Figure 1. Summary of the search processs.
Table 1. Summary of reviewed studies analyzing torque and force generated by nickel-titanium rotary instruments during root canal preparation.
| Author/Year | Type of Study | Type of Sample | Instrument | Preparation Technique | Result |
|---|
From these studies, we identified the following 26 factors that influence Ni-Ti rotary instrument torque and force: type of sample [19][17], canal curvature [19][29][17,18], cross-sectional design [11][15][33][49][53][11,15,19,20,21], taper [53][21], blade [15], pitch length [11][12][13][14][15][11,12,13,14,15], helix angle [13][15][49][13,15,20], rake angle [11][12][15][11,12,15], cutting efficiency [11][12][11,12], instrument size [9][11][12][43][16][30][9,11,12,22,23,24], glide-path preparation [44][40][25,26], canal size [35][18][20][21][22][27,28,29,30,31], contact area [17][24][32,33], preparation technique [8][16][17][24][23][37][31][32][57][8,23,32,33,34,35,36,37,38], preparation time [53][54][21,39], insertion depth [19][16][18][20][21][22][17][24][23][37][31][32][57][28][34][52][36][58][42][17,23,28,29,30,31,32,33,34,35,36,37,38,40,41,42,43,44,45], insertion rate [34][48][41,46], displacement [28][40], motor [54][39], kinematics [3][4][49][57][41][56][51][46][39][50][3,4,20,38,47,48,49,50,51,52], operative motion [47][53], rotational speed [31][34][36,41], pecking speed [52][42], lubricant [25][26][54,55], experience of the operators [16][23], and metallurgy [14][49][53][36][58][46][27][55][45][38][14,20,21,43,44,50,56,57,58,59].
Pro-branded systems, such as ProFile, ProTaper Next, ProTaper and ProTaper Universal, were most frequently investigated (Figure 2). The highest numbers of articles were published in 2019 and in the first half of 2020 (Figure 3), and 48% of the articles included in this review were published in Journal of Endodontics (Figure 4).
Figure 2. Instrument systems used in the studies included in the review.
Figure 3. Publication year of the articles included in the review.
Figure 4. Journals in which articles included in the review were published (by percentage). “Proc. Inst. Mech. Eng. H” refers to “Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine”.
The main findings obtained from the present systematic review can be summarized as: Higher torque or force generation was related to convex triangle cross-sectional design, regressive taper, short pitch length, large instrument size, small canal size, single-length preparation technique, long preparation time, deep insertion depth, low rate of insertion, continuous rotation (torque), reciprocating motion (force), lower rotational speed and conventional alloy.