by TOM C. PAGONIS, DDS, MS
The introduction and incorporation of nickel-titanium (NiTi) alloy in engine-driven endodontic files some three decades ago launched unprecedented advances and improvement in the structure and mechanical strength of endodontic rotary and reciprocating file systems. With recent technologies various heat treatment modalities have been developed that have enhanced the cyclic fatigue lifespan of engine driven files while simultaneously decreasing their torsional resistance.1
Role of temperature in phase structures
Endodontic NiTi alloy files, contain approximately 56% nickel (Ni) and 44% titanium (Ti) as a measure of weight percentage. This approximate equal distribution of Ni and Ti creates what is termed a 1:1 or equiatomic ratio.2 Typically, the NiTi alloy is present in two kinds of crystalline forms, which are temperature dependent.3 A third or intermediate phase called the R-phase (‘rhombohedral phase”) occurs on cooling and before the transformation to martensitic phase.
- Austenite phase (austenitic) : Contains the B2 crystal structure, a parent phase existing at high temperature.
- Martensite phase (martensitic): Contains the B190 crystal structure, existing at low temperature.4
The martensite phase consists of two main properties; shape memory effect (SME) and superelasticity. The austenite phase is characterized by more rigidity. These properties are caused due to the transition of austenite phase to martensite phase in response to temperature or stress.3
Mechanical properties of the endodontic files are significantly dependent on the application of temperature. For example, phase composition changes as the alloys are heated or cooled.3
- Austenite state occurs when the temperature is maintained beyond the austenite finish temperature (Af). Resultant structure presents with superior superelasticity, stiffness and hardness.5
- Martensitic state occurs when the temperature is maintained lower than the martensite finish temperature (Mf). Resultant structure is ductile, soft, and exhibits shape memory properties.5
Martensite has twinned phase design that results in superior cyclic fatigue resistance allowing easy reorientation compared to austenite.4
Heat-treated versus non-heated treated (conventional) NiTi endodontic files
The main aim of heat treatment is to modify the composition of NiTi alloys through temperature transition, thereby altering the fatigue resistance.6 Heat treatment promotes phase transition of NiTi alloys, reduces internal stress, leading to the formation of more martensite phase. Change in composition of the heat-treated instrument increases its fatigue resistance and flexibility compared to conventional NiTi files.6
Performance of non-heated conventional
NiTi endodontic files exist in the austenite phase at room temperature. Due to their stiffness and low fatigue resistance, it is difficult to use conventional files in curved root canals.6 Additionally, conventional instruments undergo grinding rather than twisting.2 Due to the grinding process, files have surface defects that causes various other problems such as defects related to corrosion resistance, cutting efficiency, and fracture resistance.
Types of heat treatments for endodontic files
M-wire technology utilizes a novel thermomechanical processing mechanism for improving the mechanical properties (cyclic fatigue resistance) of the rotary NiTi instruments. Compared to conventional NiTi wire, M wire has greater wear resistance, flexibility and strength due to the presence of a martensitic microstructure that has specific nanocrystalline form.7
It has martensite and R-phase whose elastic moduli is low compared to austenite, thus contributing to improved flexibility when compared to conventional instruments.8
Shortly after the introduction of M-wire, another manufacturing method developed by SybronEndo (Orange, CA, USA) created a twisted rotary NiTi file (TF). TF are manufactured by three novel approaches: heat treatment of R phase, metal wire twisting, and unique surface conditioning.9
Transformation of NiTi wire occurs through the twisting process, wherein the austenitic phase undergoes phase transition to the R-phase by a thermal process. R-phase has various other properties including lesser transformation strain than martensite phase, and low shear moduli. Studies have shown that instruments based on R-phase have higher cyclic fatigue resistance, and better flexibility than conventional rotatory files.10
Martensite NiTi alloy
Martensite NiTi alloy as mentioned earlier is comparatively softer than austenite. Upon heating it demonstrates shape memory effect and superior resistance to fatigue.11
CM wire was developed in 2010, as a new type of NiTi alloy. It has greater flexibility than conventional rotary files. These CM files are developed through a unique thermomechanical procedure, controlling the shape memory characteristic of the alloy. Therefore, they lack the shape memory properties which are seen in traditional superelastic NiTi files. CM NiTi files once unloaded do not rebound, thereby shape restoration is possible after heat treatment.6
Hyflex CM wires have higher cutting efficiency, fatigue resistance and flexibility than conventional endodontic files.3
Gold treated and blue treated instruments
A new method has been introduced that alters the crystalline phase in order to overcome the challenges encountered during traditional machining methods. This process is called as post-machining heat treatment.
After grinding process, the endodontic files are processed through thermomechanical heat treatment.12 Commercially, two types of instruments are available; gold-treated and blue-treated.Benefits of heat treatment
Heat treatment has several advantages compared to the non-heat treated NiTi alloy instruments.6
- Mechanical properties are superior than traditional endodontic files
- Heat treated files have superior ductility, greater cyclic fatigue resistance
- Reduced file fracture during instrumentation
- Greater safety factor as heat treated instruments have more ductility that allows more scope for angular distortion. Distorted cutting spirals can be easily identified, making it easier for the dentist to discard them before breakage.
- Greater flexibility
- Novel endodontic NiTi alloys that undergo heat treatment have higher flexibility compared to non-heated treated instruments.7
- Heat treatment of NiTi endodontic files enhances the flexibility, therefore additional care should be taken to apply less pressure during canal preparation to minimize files breakage.
- Visible alternation on the heat-treated files is helpful in alerting the dentist to discard them before they break.13
- Literature suggests that no statistically significant difference with respect to the torsional resistance has been observed between heat-treated instruments and non-heat-treated instruments (p < 0.05). 14 The main topic of concern with heat treated NiTi instruments is that they exhibit similar torsional properties to conventional NiTi files.
Heat treatment of endodontic rotary and reciprocating files has proved advantageous in terms of flexibility, cyclic resistance, and resistance to fracture. However, data around torsional failure is still unclear. Thermomechanical treatment has the potential to improve the design, composition, performance and safety characteristics of the endodontic files. Future file systems can be created with this method and have the potential to offer successful treatment outcome.
- Silva, E.J.N.L.; Giraldes, J.F.N.; de Lima, C.O.; Vieira, V.T.L.; Elias, C.N.; Antunes, H.S. Influence of heat treatment on torsional resistance and surface roughness of nickel-titanium instruments. Int. Endod. J. 2019, 52, 1645–1651.
- Thompson SA (2000) An overview of nickel-titanium alloys used in dentistry. International Endodontic Journal 33, 297– 310.
- Zupanc, New thermomechanically treated NiTi alloys – a review, International Endodontic Journal, 51, 1088–1103, 2018
- Shen Y, Zhou HM, Zheng YF, Peng B, Haapasalo M (2013) Current challenges and concepts of the thermomechanical treatment of nickel-titanium instruments. Journal of Endodontics 39, 163–72.
- Zhou H, Peng B, Zheng YF (2013) An overview of the mechanical properties of nickel-titanium endodontic instruments. Endodontic Topics 29, 42–54.
- Kwak SW, Shen Y, Liu H, Wang Z, Kim HC and Haapasalo M (2021) Heat Treatment and Surface Treatment of Nickel–Titanium Endodontic Instruments. Front. Dent. Med. 2:769977
- Ye J, Gao Y. Metallurgical characterization of M-Wire nickel-titanium shape memory alloy used for endodontic rotary instruments during low-cycle fatigue. J Endod 2012;38(1):105–107.
- Pereira ES, Gomes RO, Leroy AM, et al. (2013) Mechanical behavior of M-Wire and conventional NiTi wire used to manufacture rotary endodontic instruments. Dental Materials 29, e318–2
- Larsen CM, Watanabe I, Glickman GN, He J (2009) Cyclic fatigue analysis of a new generation of nickel titanium rotary instruments. Journal of Endodontics 35, 401–3.
- Choi J, Oh S, Kim YC, Jee KK, Kum K, Chang S (2016) Fracture resistance of K3 nickel-titanium files made from different thermal treatments. Bioinorganic Chemistry and Applications 2016, 6374721.
- McKelvey AL, Ritchie RO (2001) Fatigue-crack growth behavior in the superelastic and shape-memory material nitinol. Metallurgical and Materials Transactions A 32A, 731–43.
- Pereira ES, Peixoto IF, Viana AC, et al. Physical and mechanical properties of a thermomechanically treated NiTi wire used in the manufacture of rotary endodontic instruments. Int Endod J 2012;45(5):469–474.
- Peng B, Shen Y, Cheung GS, et al. Defects in ProTaper S1 instruments after clinical use: longitudinal examination. Int Endod J 2005;38:550–7.
- Gambarini et al., Influence of Different Heat Treatments on Torsional and Cyclic Fatigue Resistance of Nickel–Titanium Rotary Files: A Comparative Study, Appl. Sci. 2020, 10, 5604.