The breakage of nickel-titanium root canal file can be roughly divided into two types: torsional fracture and cyclic fatigue fracture.
Torsion fracture refers to the fracture of the dental file caused by the tip or any part of the root canal file being stuck in the narrow part of the root canal while the handle is still rotating, exceeding the maximum elastic deformation limit of the nickel-titanium alloy. This fracture often occurs in small root canal files and the tip of the root canal file, because small root canal files are mostly used to prepare the curved part of the apex and are easily stuck and broken. The surface of the root canal file that has undergone torsion fracture often has visible deformation, such as unthreading, irregular threads or reverse tightness.
Fatigue fracture is caused by metal bending fatigue. Although the dental file is not stuck in the root canal, it is constantly stretched and compressed during repeated rotation at the bending part, causing metal fatigue, and thus breaks under conditions far less than the elastic limit of the root canal file itself. This type of root canal file fracture often occurs without any signs and is considered an important factor in the fracture of machine-used nickel-titanium root canal files.
1. Root canal bending angle and radius
The stress of the dental file in the root canal is positively correlated with the bending angle of the root canal. The larger the bending angle, the greater the stress, and the more likely the root canal file is to break due to fatigue. Studies have shown that when preparing root canals with a curvature of >30°, the machine-used nickel-titanium root canal file has a 12.5% breakage rate, and all of them occur in the apical area of the root canal. The bending radius represents the steepness of the root canal curvature. The smaller the radius, the more curved the root canal. When the bending radius decreases, the stress and torque of the dental file increase, the fatigue cycle shortens, and it is easy to break.
2. Diameter and taper of root canal files
When using small dental file for root canal enlargement, the near tip is easily bound in the root canal during root canal preparation, resulting in torsion fracture. In curved root canals, large root canal files are subject to greater stress and are more susceptible to bending fatigue, with a shorter fatigue cycle than small dental file.
When the cross-section of the file is the same, the larger the taper, the stronger the ability to resist torsion fracture, but the poorer the flexibility and the poorer the ability to resist rotation fatigue.
3. Rotation speed of the handpiece
Different brands of machine-used nickel-titanium root canal files have different recommended rotation speeds. A higher rotation speed will shorten the service life of the dental file and increase the chance of the dental file breaking. Some scholars used ProTaper with three different rotation speeds (150r/min, 250r/min, and 350r/min) for root canal preparation. The results showed that the 350r/min group was more likely to have a root canal file break than the first two groups. A lower rotation speed will reduce the operating efficiency of the dental file, but will reduce the incidence of dental file breakage.
4. Design of dental file and metal surface treatment
The quenching strain band of nickel-titanium root canal files is very narrow. It is recommended to choose a lower speed within the speed range recommended by the manufacturer, because preventing breakage is more important than improving efficiency. That is, nickel-titanium root canal files can break without noticing old deformation, and this deformation is precisely the only indication to warn clinicians. Nickel-titanium root canal files can break even without deformation.
5. Operator experience and root canal file usage
The survey found that those with less experience had more root canal file breakage. In addition, improper use, violent operation, and excessive force by the operator are also important causes of root canal file breakage in clinical practice. Therefore, force should be avoided towards the apex, passive entry into the root canal should be used, and standardized operation should be performed as much as possible according to the instructions.
6. Number of times the root canal file is used
The fracture of the dental file is directly affected by the number of times it is used. The more times it is used, the more fatigue it will produce and the worse its fracture resistance will be. Therefore, it is particularly necessary to record the number of times the dental file is used. Counting the use of nickel-titanium root canal files has become an important method for clinicians to save costs and reduce the risk of fracture. There is no consensus on the safe number of times and application time of machine-used nickel-titanium dental file. It is recommended to discard it after preparing 5 to 6 teeth, and some scholars have used it for 10 teeth. The number of times the front teeth are used will be more, and the number of times the back teeth are used will be less. However, when preparing severely curved or complex root canals, it is recommended to use it once.
7. Effects of high-pressure sterilization and flushing fluid
Studies have found that high-pressure sterilization greatly increases the fatigue resistance of nickel-titanium dental file and can protect them.