Rotary Nickel-Titanium Instruments: Comparison
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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[5][6],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[11][15],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.

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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
Blum et al., 1999 [23]Blum et al., 1999 [16] In vitro Mandibular incisors ProFile Step-back & crown-down Students > Endodontist (T)

Students < Endodontist (F)

Step-back > crown-down
Blum et al., 1999 [32]Blum et al., 1999 [17] In vitro Mandibular incisors ProFile Step-back & crown-down Contact area- 10 mm (Step-back) and 5 mm (crown-down) from the tip

Step-back > crown-down (T & F)
Sattapan et al., 2000 [28]Sattapan et al., 2000 [18] In vitro Maxillary/mandibular central and lateral incisors Quantec Series 2000 Single file Small canal > medium canal
Peters et al., 2002 [17]Peters et al., 2002 [19] In vitro Extracted human teeth/plastic blocks Profile Crown-down Straight canal blocks > curved canal blocks > natural teeth (T)

Curved canal blocks > natural teeth > straight canal blocks (F)
Peters et al., 2003 [29]Peters et al., 2003 [20] In vitro Maxillary molars ProTaper Single-length F3 > F2 > F1 > S1 > S2 (T)

F3 > S1 > F2 > F1 > S2 (F)
Blum et al., 2003 [30]Blum et al., 2003 [21] In vitro Maxillary central incisors/ mandibular central or lateral incisors ProTaper Single-Length Narrow canal > large canal (T)

Large canal > narrow canal (F)
Hübscher et al., 2003 [31]Hübscher et al., 2003 [22] In vitro Maxillary molars FlexMaster Crown-down-like modified sequence Constricted canal > wide canal (T & F)
Diemer et al., 2004 [13] In vitro Resin blocks Hero Single file Shorter pitch length > longer pitch length (T & F)
Da Silva et al., 2005 [34]Da Silva et al., 2005 [23] In vitro Maxillary/mandibular central and lateral incisors Race 720, Race 721, Profile Single-length Profile > Race 720 > Race 721 (T & F)
Schrader et al., 2005 [33]Schrader et al., 2005 [24] In vitro Plastic blocks Profile Crown-down 35/0.04 had peak T and F in 4% taper sequence

40/0.06 and 35/0.04 had peak T and F in combination of 4% and 6% taper, respectively
Peters et al., 2005 [54]Peters et al., 2005 [25] In vitro Dentin discs ProFile, ProTaper Single file Glyde > Control > EDTA > H2O for ProFile (T)

Glyde > H		2O > EDTA > Control for ProTaper (T)

Glyde > H		2O > Control > EDTA for ProFile (F)

Control > H		2O > Glyde > EDTA for ProTaper (F)

ProTaper > ProFile (T)

ProFile > ProTaper (F)
Boessler et al., 2007 [55]Boessler et al., 2007 [26] In vitro Dentin discs ProFile Single file Dry Control > NaOCL 1% > H2O > HEPB 18% (T)

Dry Control > H		2O > NaOCL 1% > HEPB 18% (F)
Boessler et al., 2009 [56]Boessler et al., 2009 [27] In vitro Dentin discs ProTaper Single-length Electropolished > machined (T)

Machined > electropolished (F)
Diop et al., 2009 [40]Diop et al., 2009 [28] In vitro Human cadaveric mandivular central/lateral incisors ProTaper Single file Apical > coronal (T & F)

Right > left (F)

Posterior > anterior (F)
Ha et al., 2010 [15] In vitro Resin blocks K3, Mtwo, NRT, ProFile, ProTaper Single file ProTaper > K3 > NRT-safe tip > NRT-active tip > Mtwo > ProFile (F)
Son et al., 2010 [18]Son et al., 2010 [29] In vitro Resin blocks ProTaper, ProFile Single-length 0° > 10° > 20° > 30° canal curvature (F)
Sung et al., 2010 [24]Sung et al., 2010 [30] In vitro Resin blocks ProFile, GT rotary, K3 Single-length Greater taper > smaller taper (F)
Bardsley et al., 2011 [36]Bardsley et al., 2011 [31] In vitro Plastic blocks Vortex Crown-down 200 rpm > 400 rpm > 600 rpm (T & F)
Peters et al., 2012 [37]Peters et al., 2012 [32] In vitro Plastic blocks Hyflex CM Single-length & crown-down Single-length > crown-down (T & F)
Ha et al., 2012 [8] In vitro Endo-training blocks PathFile, NiTiFlex, ProTaper Single file #13 > #15 > #18 > #20 (T & F)
Diemer et al., 2013 [19]Diemer et al., 2013 [33] In vitro Resin blocks HeroShaper, Prototypes Single file H6 > H0 > H4 (T)

H0 > H6 > H4 (F)
Pereira et al., 2013 [41]Pereira et al., 2013 [34] In vitro Plastic blocks ProTaper Next Single-length 250 rpm > 300 rpm > 350 rpm (T & F)
Arias et al., 2014 [27]Arias et al., 2014 [35] In vitro Maxillary incisors/mandibular molars ProTaper Next, ProTaper Universal Single-length Small canals > large canals (T & F)
Pereira et al., 2015 [43]Pereira et al., 2015 [36] In vitro Plastic blocks ProTaper Universal, Profile Vortex, Vortex Blue, Typhoon Infinite Flex Single-length Typhoon > ProTaper Universal > Vortex Blue > ProFile Vortex (T)

ProTaper Universal > ProFile Vortex > Vortex Blue > Typhoon (F)
Ha et al., 2015 [9] In vitro Resin block G-1, G-2, uG glide path files Single file G-2 > uG > G-1 (F)
Peixoto et al., 2015 [11] In vitro Acrylic blocks Mtwo, Race, ProTaper Universal Single file ProTaper Universal > Race > Mtwo (T)

ProTaper Universal > Mtwo > Race (F)
Arias et al., 2016 [35]Arias et al., 2016 [37] In vitro Mandibular molars PathFile, ProGlider Single-length & Single file PathFile 1 > PathFile 2 > ProGlider (T & F)

ProGlider (16/.02, single file) > PathFile (16/.02, Sequence) (T & F)
Moreinos et al., 2016 [59]Moreinos et al., 2016 [38] In vitro Simulated metal block canal Gentlefile, ProTaper Next, Revo-S Single-length ProTaper X1 > Revo-S SC2 > Gentlefile 1 (F)

Revo-S SC3 > ProTaper X2 > Gentlefile 2 (F)
Kwak et al., 2016 [14] In vitro Resin blocks OneG, pG, OneG heat-treated, pG heat-treated glide path files Single file pG > OneG > OneG heat-treated > pG heat-treated (F)
Ha et al., 2016 [4] In vitro Resin blocks Mtwo, Reciproc 25, ProTaper Universal, ProTaper Next Single file Reciproc 25 > ProTaper Universal > ProTaper Next > Mtwo (F)
Jamleh et al., 2016 [51]Jamleh et al., 2016 [39] In vitro Premolar teeth Twisted File, Twisted File Adaptive, ProTaper Universal, ProTaper Next Single-length ProTaper Universal > ProTaper Next > Twisted File > Twisted File Adaptive (L)
Arias et al., 2017 [26]Arias et al., 2017 [40] In vitro Mandibular molars PathFile, ProGlider, ProTaper Gold Single-length Glide path reduced the torque of shaping files
Tokita et al., 2017 [47]Tokita et al., 2017 [41] In vitro Resin canal models Twisted File Adaptive Single-length CR > torque-sensitive reciprocation > time-dependent reciprocation (T)

Time-dependent reciprocation > CR > torque-sensitive reciprocation (F)
Ha et al., 2017 [45]Ha et al., 2017 [42] In vitro Resin Canals One G Single-length 4/6 pecking depath > 2/4 pecking depath (F)
Fukumori et al., 2018 [22]Fukumori et al., 2018 [43] In vitro Resin canals EndoWave Single file EndoWave (30/0.06) > EndoWave (30/0.04) (T & F)
Kwak et al., 2018 [25]Kwak et al., 2018 [44] In vitro Resin blocks WaveOne, WaveOne Gold Single file WaveOne > WaveOne Gold (T)

Without Glide Path > with Glide Path (T)
Jamleh et al., 2018 [58]Jamleh et al., 2018 [45] In vitro Maxillary premolar teeth WaveOne, WaveOne Gold Single file WaveOne > WaveOne Gold (F)
Nishijo et al., 2018 [50]Nishijo et al., 2018 [46] In vitro Endo training blocks Hyflex EDM Glide Path File (EDM), Hyflex GPF, Scout Race (Race) Single file Hyflex EDM Glide Path File > GPF > Race (CR) (F)

Hyflex EDM Glide Path File > Race > Hyflex GPF (Reciprocation) (F)
Gambarini et al., 2019 [53]Gambarini et al., 2019 [47] In vitro Maxillary anterior teeth Twisted File Single file Inward pecking motion > outward brushing motion (T)
Abu-Tahun et al., 2019 [46]Abu-Tahun et al., 2019 [48] In vitro Resin canals One G, Hyflex EDM Single file No glide path > 5 insertions > 10 insertions > 15 insertions > 20 insertions (T)
Kwak et al., 2019 [20]Kwak et al., 2019 [49] In vitro Resin blocks ProTaper Universal, ProTaper Gold, WaveOne, WaveOne Gold Single-length & Single file ProTaper Universal > WaveOne > ProTaper Gold > WaveOne Gold (F)
Nayak et al., 2019 [52]Nayak et al., 2019 [50] In vitro Resin blocks WaveOne Gold, Self-adjusting file, 2Shape Single-length &single file WaveOne Gold > 2Shape 2 > 2Shape 1 > self-adjusting file (F)
Kwak et al., 2019 [49]Kwak et al., 2019 [51] In vitro Resin blocks K3XF, Twisted File Adaptive Single-length K3XF (CR) > K3XF (adaptive motion) > TFA (adaptive motion) (T)
Maki et al., 2019 [42]Maki et al., 2019 [52] In vitro Resin canal blocks ProTaper Next Single-length High and/or medium-speed > low-speed (clockwise T)

High-speed > medium-speed > low-speed (F)
Gambarini et al., 2019 [21]Gambarini et al., 2019 [53] In vivo Double-rooted maxillary premolars ProTaper Next, EdgeFile Single-length ProTaper Next > EdgeFile (T & preparation time)
Bürklein et al., 2019 [39]Bürklein et al., 2019 [54] In vitro Maxillary incisors K-flexofile stainless steel, F6 SkyTaper & EndoPilot, DentaPort ZX OTR, VDW.silver Balanced-force, single-length Balanced-force > rotary (F)

Rotary > balanced-force (T)

No significant differences among 3 motors (T)
Almeida et al., 2020 [12] In vitro Acrylic blocks ProTaper Next, ProTaper Universal Single-length ProTaper Next X2 > ProTaper Next X1 > ProTaper Universal S1 > ProTaper Universal F1 (T)

ProTaper Next X1 > ProTaper Universal S2 > ProTaper Next X2 > ProTaper Universal F1 (F)
Maki et al., 2020 [57]Maki et al., 2020 [55] In vitro Resin blocks Reciproc, Reciproc Blue Single file Reciproc > Reciproc Blue (T & F)
Lee et al., 2020 [3] In vitro Molars ProTaper Next, One Curve, Hyflex EDM, Twisted File Adaptive Single-length CR > adaptive motion (T)

Hyflex EDM > One Curve > ProTaper Next > Twisted File Adaptive (T)
Htun et al., 2020 [48]Htun et al., 2020 [56] In vitro Mandibular incisors Hyflex EDM glide path file, stainless steel K-file Single file CR > OGP > stainless steel manual (T in positive domain)

CR > stainless steel manual > OGP (T in negative domain)

OGP > stainless steel manual > CR (F in positive domain)

OGP > CR > stainless steel manual (F in negative domain)
Kimura et al., 2020 [38]Kimura et al., 2020 [57] In vitro Resin blocks Endowave Single file, crown-down Single file (CR) > single file (OTR) (clockwise & counterclockwise T)

crown-down (CR) > crown-down (OTR) (clockwise T)

crown-down (OTR) > crown-down (CR) (counterclockwise T)
Peters et al., 2020 [44]Peters et al., 2020 [58] In vitro Plastic blocks TruNatomy, ProTaper Next Single-length ProTaper Next X2 > ProTaper Next X3 > ProTaper Next X1 > TruNatomy 36 > TruNatomy 26 > TruNatomy 20 (T)

ProTaper Next X1 > ProTaper Next X2 > ProTaper Next X3 > TruNatomy 36 > TruNatomy 26 > TruNatomy 20 (F)

From these studies, we identified the following 26 factors that influence Ni-Ti rotary instrument torque and force: type of sample [17][19], canal curvature [17[19][29],18], cross-sectional design [11[11][15][33][49][53],15,19,20,21], taper [21][53], blade [15], pitch length [11,12[11][12][13][14][15],13,14,15], helix angle [13[13][15][49],15,20], rake angle [11,12,15][11][12][15], cutting efficiency [11[11][12],12], instrument size [9,11,12[9][11][12][43][16][30],22,23,24], glide-path preparation [25[44][40],26], canal size [27,28,29[35][18][20][21][22],30,31], contact area [32[17][24],33], preparation technique [8[8][16][17][24][23][37][31][32][57],23,32,33,34,35,36,37,38], preparation time [21,39][53][54], insertion depth [17[19][16][18][20][21][22][17][24][23][37][31][32][57][28][34][52][36][58][42],23,28,29,30,31,32,33,34,35,36,37,38,40,41,42,43,44,45], insertion rate [41,46][34][48], displacement [40][28], motor [39][54], kinematics [3,4,20,38,47,48,49,50[3][4][49][57][41][56][51][46][39][50],51,52], operative motion [53][47], rotational speed [36[31][34],41], pecking speed [42][52], lubricant [54[25][26],55], experience of the operators [23][16], and metallurgy [14,20,21,43,44,50,56,57,58,59][14][49][53][36][58][46][27][55][45][38].

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).

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Figure 2. Instrument systems used in the studies included in the review.

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Figure 3. Publication year of the articles included in the review.

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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.

References

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  42. Ha, J.-H.; Kwak, S.W.; Sigurdsson, A.; Chang, S.W.; Kim, S.K.; Kim, H.-C. Stress generation during pecking motion of rotary nickel-titanium instruments with different pecking depth. J. Endod. 2017, 43, 1688–1691.
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  45. Jamleh, A.; Alfadley, A.; Alfouzan, K. Vertical force induced with WaveOne and WaveOne Gold systems during canal shaping. J. Endod. 2018, 44, 1412–1415.
  46. Nishijo, M.; Ebihara, A.; Tokita, D.; Doi, H.; Hanawa, T.; Okiji, T. Evaluation of selected mechanical properties of niti rotary glide path files manufactured from controlled memory wires. Dent. Mater. J. 2018, 37, 549–554.
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  48. Tahun, I.H.A.; Kwak, S.W.; Ha, J.-H.; Sigurdsson, A.; Kayahan, M.B.; Kim, H.-C. Effective establishment of glide-path to reduce torsional stress during nickel-titanium rotary instrumentation. Materials 2019, 12, 493.
  49. Kwak, S.W.; Lee, C.-J.; Kim, S.K.; Kim, H.-C.; Ha, J.-H. Comparison of screw-in forces during movement of endodontic files with different geometries, alloys, and kinetics. Materials 2019, 12, 1506.
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