The separation of nickel-titanium instruments can be roughly divided into two ways, namely torsional fracture and cyclic fatigue fracture.
Torsion breaking refers to the breaking of the instrument caused by the tip or any part of the instrument 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 kind of breakage usually occurs at the tip of small instruments and instruments, because small instruments are mostly used for the preparation of the apical curvature, which is easy to get stuck and break. There are often visible deformations on the surface of the device that is torsionally broken, such as untwisting, irregular threads, or reverse tightening.
Fatigue fracture is caused by metal bending fatigue. Although the instrument is not stuck in the root canal, it is constantly stretched and compressed during the repeated rotation of the curved part to produce metal fatigue, which is far less than the elastic limit of the instrument itself. Break down. This type of device fracture often occurs without any signs, and is considered to be an important factor in the fracture of mechanical nickel-titanium devices.
1. Root canal bending angle and radius
The stress of the instrument in the root canal is positively correlated with the bending angle of the root canal. The greater the bending angle, the greater the stress, and the more prone to fatigue fracture of the instrument. Studies have shown that when using mechanical nickel-titanium instruments to prepare root canals with a curvature greater than 30°, there is a 12.5% fracture rate, and they all occur in the apical area of the root canal.
The bending radius represents the steepness of the root canal bend. The smaller the radius, the more bend the root canal. When the bending radius decreases, the stress and torsion of the device increase, the fatigue cycle is shortened, and it is prone to breakage.
2. The diameter and taper of the instrument
A small file is used for root canal enlargement. During the preparation of the root canal, the proximal tip tends to be trapped in the root canal, causing torsion and fracture. In curved root canals, large instruments are more susceptible to bending fatigue and have a shorter fatigue cycle than small instruments.
When the cross section of the file is the same, the greater the taper, the stronger the ability to resist torsion breaking, but the flexibility is poor and the ability to resist rotation fatigue is poor.
3. The speed of the phone
Different brands of machine-use nickel-titanium instruments have different speeds recommended by the manufacturers. A higher speed will shorten the service life of the device, and the probability of breaking the device will be higher.
Some scholars used 3 different speeds (150r/min, 250r/min, 350r/min) ProTaper for root canal preparation. The results showed that the 350r/min group was more prone to instrument breakage than the first two groups. A lower speed will reduce the operating efficiency of the instrument, but it will reduce the incidence of instrument breakage. 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.
4. Equipment design and metal surface treatment
The quenching strain band of nickel-titanium instruments is very narrow, that is, nickel-titanium instruments can be broken without noticing old deformation, and this deformation is the only indication that warns clinicians. Nitinol instruments can be broken without deformation.
5. The operator’s experience and the way the equipment is used
The survey found that people with less experience had more broken devices. In addition, improper use by the operator, violent operation, and excessive force are also important reasons for the fracture of the device in clinical practice. Therefore, the apical force should be avoided, the root canal should be passively entered, and the standard operation should be carried out according to the instructions as far as possible.
6. The number of times the equipment is used
The breakage of the device is directly affected by the number of times of use. The more times of use, the easier it is to cause fatigue and the worse the flexural resistance. Therefore, it is particularly necessary to record the number of times the device is used. Counting the use of nickel-titanium devices has become an important method for clinicians to save costs and reduce the risk of fracture.
The number of safe use and application time of Ni-Ti equipment for machine is still inconclusive. It is recommended that 5-6 teeth be prepared and discarded. Some scholars use 10 teeth. The anterior teeth will be used more often and the posterior teeth will be less. However, it is recommended to use it once for the preparation of severely curved or complex root canals.
7. The effect of autoclaving and washing fluid
Research found that autoclaving greatly increases the fatigue resistance of nickel-titanium instruments and can protect nickel-titanium instruments.
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