It has been well established that nickel–titanium (NiTi) alloy files are able to maintain the original canal shape during the instrumentation procedure. However, they do come with one important disadvantage: a higher risk of file separation or breakage compared with stainless steel hand files. If a file breaks inside a canal, it is usually very difficult, if not impossible, to retrieve it, and it is not possible to further instrument and irrigate beyond it. The likelihood of a successful therapy can thereby be greatly diminished.
NiTi files principally break for 2 reasons:
- Cyclic fatigue occurs when the file has been bent too many times; repeated tension and compression stresses cause fatigue crack propagation, and the file simply breaks.
- Torsional failure occurs when the tip of the file engages inside the canal such that it does not rotate anymore, but the motor continues to rotate the rest of the file. If the torque control of the motor does not sense this, the file will break, leaving the tip firmly engaged inside the canal.
Many manufacturers of NiTi files have, with some success, actively developed new file designs and heat treatments of the NiTi alloy to improve cyclic fatigue and flexibility of the NiTi files, thereby reducing the risk of file breakage when they are used inside the canals. These improvements, however, have come at some cost to the torsional strength.
Figure 1. (A) Top, ProGlider; bottom, WaveOne Gold. (Image courtesy of Dr. Noah Chivian).
Over the years, research has discovered that one way to reduce the risk of torsional failure is to establish a “glide path” down to the apical area of the tooth prior to using the motor-driven shaping NiTi files. Creating a glide path of sufficient size
before introducing the initial rotary NiTi file into the canal has been shown to significantly reduce the risk of file breakage. It is not clear, however, whether the new and improved NiTi files, particularly those that reciprocate within the canals rather than rotate 360°, need a glide path prepared as do the older NiTi files.
Kwak et al from Pusan National University, South Korea, investigated the effect of glide paths on new reciprocating NiTi files by establishing glide paths in 15 resin endodontic training blocks using rotating files specifically designed to create such a path (ProGlider, Dentsply/Maillefer; Figure 1). An additional 15 blocks did not have glide paths.
To compare the newer file design and heat-treated alloy, the authors measured 2 types of WaveOne files (Dentsply/Maillefer): the older version WaveOne and the newer version, WaveOne Gold (Figure 1). The blocks were instrumented with the files while the authors carefully monitored the torque applied to each file. They found that WaveOne Gold had significantly reduced torque created if a glide path had been established; the older version generated a higher maximum torque than did WaveOne Gold regardless of the establishment of a glide path (Tables 1 and 2).
Based on this study, it is clear that, in order to reduce the risk of file breakage due to torsional failure, endodontists should create a sufficient glide path in the apical area of the canal prior to using these new, highly flexible NiTi files.
Kwak SW, Ha J-H, Cheung GS-P, et al. Effect of the glide path establishment on the torque generation to the files during instrumentation: an in vitro measurement. J Endod 2017;doi:10.1016/j.joen.2017.09.016.