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Influencing factors and prevention strategies for separation of nickel titani...

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发表于 2018-9-16 16:50:20 | 显示全部楼层 |阅读模式
Influencing factors and prevention strategies for separation of nickel titanium instruments
The key to the success of modern root canal therapy lies in the cleaning and shaping of the root canal system. In recent years, with the continuous development of nickel-titanium alloy technology, nickel-titanium instruments have shown excellent performance in many aspects, such as shaping ability and root canal adaptability, and become the mainstream of root canal preparation machinery. Instrumentation is one of the most intractable complications in root canal therapy. The separation of nickel-titanium instruments has always been a hot topic in clinical discussion because of the obstruction of the cleaning and shaping of the root canal system, which affects the prognosis of root canal therapy. In this paper, the factors affecting the separation of nickel titanium instruments and their prevention and control strategies are introduced.
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 楼主| 发表于 2018-9-16 16:50:44 | 显示全部楼层
1. nickel titanium instruments separation mechanism

At present, it is believed that there are two main mechanisms for the separation of Ni-Ti devices: torsional fatigue and cyclic fatigue. Torsional fatigue is defined as mechanical separation when the tip of the instrument or any other part is bound to the bending or narrowing part of the root canal, and the stalk continues to rotate, when the torsional stress exceeds the elastic limit of the metal. Cyclic fatigue mainly occurs in curved root canals. The results showed that the instrument was subjected to tensile and compressive stresses at the curved root canal. Each rotation in the curved root canal experienced a complete tensile and compressive stress cycle. This periodic stress load would lead to metal fatigue and eventually mechanical separation.

The main difference between the two separation mechanisms is that plastic deformation such as unscrewing spiral and reverse bending can be seen on the surface of the instrument of torsional fatigue separation, while cyclic fatigue often has no visible permanent deformation which can be distinguished by naked eyes, and the degree of fatigue accumulation during root canal preparation can not be evaluated. Therefore, cyclic fatigue is unpredictable and is considered to be the most important reason for the separation of Ni-Ti mechanical devices.
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 楼主| 发表于 2018-9-16 19:30:17 | 显示全部楼层
2. influencing factors analysis

2.1 root canal anatomical factors

A large number of studies have pointed out that the anatomical morphology of the root canal is closely related to the separation of nickel-titanium instruments, including: the thickness of the root canal, the bending angle of the root canal, the bending radius, the arc length of the bending part of the root canal and the position of the bending part. Peters et al. found that the stress in the root canal was related to the thickness of the root canal, and the stress in the narrow root canal was significantly greater than that in the thick root canal. Therefore, the incidence of instrumental separation increases significantly in cases of narrow canals, calcification, occlusion and retreatment. The degree of root canal bending can be evaluated by bending angle and bending radius. The larger the bending angle and the smaller the radius, the more obvious the degree of bending.

The tensile and compressive stress of the NiTi instruments was positively correlated with the bending angle of the root canal, but negatively correlated with the bending radius. It was found that nickel-titanium instrumental separation occurred mostly in the middle and posterior part of the curved root canal when the bending angle of the root canal was greater than 30 degrees. Pruett et al. also confirmed that when the radius of root canal bending was reduced from 5 mm to 2 mm and the angle of root canal bending was increased to 30 degrees, the separation period of nickel-titanium instruments would be significantly reduced. Therefore, the greater the bending angle, the smaller the bending radius, the longer the bending arc length, and the closer the bending position to the apical region, the higher the risk of instrument separation, suggesting that we should be more cautious in operation. In addition, the position of the affected teeth is also one of the factors affecting the separation of nickel-titanium instruments. With the tooth position moving backward, the probability of instrument separation is also increasing. This may be related to the narrow root canal of the posterior teeth, the large degree of bending, the easy occurrence of calcification and occlusion, and in clinical practice, the posterior teeth have small mouth opening and unclear visual field. And so on.
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 楼主| 发表于 2018-9-16 19:30:40 | 显示全部楼层
2.2 nickel titanium device factors

Crystal structure of 2.2.1

Nickel-titanium alloy is a binary alloy composed of nickel and titanium. With the change of pressure and temperature, nickel-titanium alloy can show austenite (A phase), R phase and martensite (M phase). Austenite is a state of high temperature or load removal, and its alloy is rigid and stable in shape; martensite is a state of relatively low temperature or load, its alloy has good toughness, ductility, high flexural strength; R phase is the transition state of austenite and martensite, with high ductility. The traditional nickel-titanium file is made of austenite (A phase), which has higher hardness, poor elasticity and fracture resistance.

In recent years, with the development of processing and heat treatment technology, Ni-Ti files made of M-wire, R-phase and CM-wire have been developed. The typical ones are Wave One, Reciproc and ProTaperNext made of M-wire, TF, TFA and K3XF made of R-phase and CM-wire. Nickel-titanium files such as HayFlexCM have better flexibility, root canal adaptability and fatigue resistance than traditional nickel-titanium files. In addition, new nickel-titanium files, such as HyFlex EDM, WaveOne Gold and Reciproc Blue, have been introduced by researchers through innovative design concepts and improved manufacturing processes. HyFlex EDM is fabricated with CM-wire and has excellent flexibility. In addition, the latest EDM etching technology has been applied, and the bending resistance has been greatly improved.

The results showed that the wave One Gold and Reciproc Blue made by Goldwire and Bluewire were superior to those made by M-wire in fatigue resistance, elasticity and root canal adaptability.
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 楼主| 发表于 2018-9-16 19:31:00 | 显示全部楼层
2.2.2 morphological design

The results show that different diameters, tapers and cross-sectional designs of Ni-Ti devices will affect the fatigue resistance of Ni-Ti devices to varying degrees. The cross section design is different from the type of instrument, mainly including triangle, U shape, S shape and rectangle. The larger the cross-sectional area, the more contact points with the root canal wall, the more concentrated the stress, the greater the friction force on the root canal wall, the easier the mechanical separation. Therefore, the cross-section design of nickel-titanium instruments has an important impact on their fatigue resistance. The continuous improvement of cross-section design can optimize the performance and fatigue resistance of nickel-titanium instruments. It was also found that instruments with the same material, cross-sectional design and taper had better cyclic fatigue resistance than those with larger diameter. It was suggested that instruments with small diameter and small taper should be used in root canal dredging and smooth passage. However, the tip of instruments was more easily bound to the root canal and torsional fatigue occurred. The probability of separation should be emphasized.

Shen et al. believed that the nickel-titanium devices with variable taper design had higher separation risk than those with single taper design. With the increase of the diameter or taper of the instrument, the ability to resist torsional fatigue increases, but the tensile and compressive stress on the external surface of the instrument increases accordingly, so the large diameter and taper of the instrument is easier to produce cyclic fatigue. Therefore, we can not think that large instruments are safe. The key lies in how to choose and use them correctly. At present, scholars at home and abroad agree that nickel-titanium systems with different shapes and designs are suitable for different working conditions, and each has its advantages. Clinicians should select a more appropriate nickel-titanium system according to specific conditions, so as to achieve better clinical results and minimize the separation of instruments.
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 楼主| 发表于 2018-9-16 19:31:39 | 显示全部楼层
2.2.3 production process

Traditional nickel-titanium instruments are mostly manufactured by mechanical cutting. During the cutting process, some stress concentration defects (such as pits, metal strips, etc.) often occur, and machining grooves appear on the surface. These potential weaknesses easily lead to sudden and unpredictable separation of instruments. With the development of processing technology, many scholars have found that some special surface treatment technologies, such as ion implantation, electrolytic surface polishing, deoxidizing surface treatment and surface coating technology, can improve the surface properties and remove the defective surface layer, thereby increasing the fracture resistance of nickel-titanium instruments. In addition, special heat treatment is an important method to improve the mechanical properties of Ni-Ti alloys by optimizing their microstructure.

In recent years, R-phase, M-wire, Goldwire, Bluewire and CM-wire nickel-titanium alloy materials have been successfully developed according to the principle that the crystal structure of nickel-titanium alloy changes after special heat treatment. The nickel-titanium instruments show good superelasticity and fatigue resistance, and the safety of clinical use has been greatly improved. In addition, Ni-Ti instruments (such as TF, TFA, K3XF, etc.) made by a new twisting method combined with heat treatment have the advantages of high strength and good elasticity, avoiding the potential weakness caused by mechanical cutting and reducing the incidence of device separation.
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 楼主| 发表于 2018-9-16 19:31:53 | 显示全部楼层
2.2.4 mode of motion

Reciprocating motion is a new type of motion of Ni-Ti instruments designed according to the principle of balance force, i.e. counter-clockwise and clockwise alternate motion. The typical Ni-Ti instruments are Wave One, Wave One Gold, Reciproc and Reciproc Blue. This kind of movement can release the Ni-Ti instruments embedded in dentin, release the stress loaded on the instruments, thus reducing the occurrence of device separation. A large number of studies have shown that the fatigue resistance of Ni-Ti instruments can be significantly improved by root canal preparation in the previous complex motion mode compared with the continuous rotation. Even for the same equipment, the anti-fatigue fracture performance under reciprocating motion mode is significantly better than that under continuous rotation motion mode.

In addition, the motion mode of TFA is a unique combination of reciprocating motion and continuous rotating motion, i.e. continuous rotating motion without external force loading, and improved reciprocating motion when encountering external force loading, and automatically adjusting rotation angle with the change of external force size. Degree (-50~360 degrees). In addition, rotational speed and torque also affect the separation of nickel-titanium instruments. Research shows that rotational speed is positively correlated with the friction between the instruments and the root canal wall. Therefore, suitable rotational speed and torque (recommended parameters from the manufacturer) can not only improve the cutting efficiency, but also avoid the separation of instruments.
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 楼主| 发表于 2018-9-16 19:32:13 | 显示全部楼层
2.3 operator factors

Operator factors are also one of the important factors affecting the clinical separation of nickel-titanium instruments, including the experience of the operator, the proficiency of the instrument and the order and method of using the instrument. Studies have shown that experienced surgeons have a lower probability of instrument deformation and separation than inexperienced ones. Operators are not familiar with the anatomy of the root canal before the preparation, do not carefully examine the instruments, the preparation process in the tip of the instrument applied too much pressure, the incorrect use of root canal irrigation fluid and lubricant, jump using instruments or staying in the root canal for too long will lead to the separation of instruments. In addition, arbitrary use of violence, especially when there is resistance to narrowness, attempts to use violence through narrow areas will undoubtedly increase the risk of device separation. Therefore, clinicians should be familiar with the performance and correct use of nickel-titanium instruments, master different root canal preparation techniques, and participate in systematic high-quality root canal therapy training and assessment before clinical operation, and carry out a large number of repeated standardized operation training in vitro.

2.4 other factors

Some scholars have found that after repeated disinfection under high temperature and high pressure, the surface properties of nickel-titanium instruments will change, thus reducing their service life. Other studies have shown that the fatigue resistance of heat-treated Ni-Ti instruments increases after high temperature and high pressure disinfection. Bulem et al. believed that high temperature and high pressure disinfection would not cause the crystal phase change of nickel-titanium instruments and would not affect their fatigue resistance. In addition, some scholars found that nickel-titanium alloy electrochemical corrosion would occur in the root canal irrigation fluids, resulting in the decrease of fatigue resistance and acceleration of instrument separation. However, some studies have concluded that the flushing solution has no significant effect on the fatigue resistance of Ni-Ti instruments, which may be related to the experimental conditions and the types of Ni-Ti instruments.

To sum up, the effect of high temperature and high pressure disinfection and rinse solution on the device is still uncertain. As far as the current research is concerned, they are not the main factors affecting the separation of nickel-titanium instruments, and can be safely used clinically.
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 楼主| 发表于 2018-9-16 19:32:38 | 显示全部楼层
3. preventive measures

In the past, we were all keen on discussing how to deal with separation devices. As more and more clinicians realized the importance of prevention over treatment, the focus was gradually on how to prevent separation of nickel-titanium devices. Therefore, it is particularly important to know the preventive measures for separation of nickel titanium instruments.

3.1 strengthening preoperative assessment

Before root canal therapy, the surgeon should conduct a comprehensive clinical and imaging examination of the affected teeth, read the apical X-ray film carefully, preliminary understanding of the anatomical morphology of the root canal, if necessary, take CBCT examination. Comprehensively understand the three-dimensional anatomical characteristics of the root canal, correctly select the indications, and select the appropriate nickel-titanium instruments and preparatory techniques in combination with the performance characteristics of different instruments. If necessary, two or even a variety of nickel-titanium preparatory systems and preparatory techniques can be combined to avoid the separation of instruments due to improper selection of instruments. If encounter calcification, step or apical steep bend and other complex cases, it is necessary to use the trumpet hand with stainless steel instruments for dredging and preliminary preparation, careful use of nickel-titanium instruments, otherwise easy to mechanical separation. Therefore, it is very important to strengthen the understanding of root canal anatomy, grasp the performance characteristics of the instruments and select the correct indications.

3.2 establish a straight line and smooth channel.

Studies have shown that in the early stage of root canal preparation, the establishment of straight and smooth passages can reduce the torsional stress of the instruments, thus greatly reducing the risk of nickel-titanium instrument separation. Establishing a straight path is to form a continuous, smooth and smooth cone shape from the opening to the root canal mouth and even the crown of the root canal, including the shaping of the pith and the opening of the middle and upper part of the root canal. By removing the resistance of root canal crown, removing the shoulder collar of dentin, reducing or eliminating the crown bending degree of the root canal, the curvature of the whole root canal is relatively reduced, ensuring that the nickel-titanium instruments can enter the middle and lower part of the root canal in a straight line, reducing the crown resistance and the stress of the instruments. The glide path is a smooth passage from the orifice of the root canal to the physiological apical stop. It can reduce the torque and stress of the rotating NiTi instruments and reduce the possibility of instrument separation. Traditional smooth channels are often built by hand trumpet K file, which is time-consuming and laborious.

In recent years, pathfile, scout Race, Gfile, HyFlex GPF, Mtwo (04, 10), proglider, Gpliot, WaveOne Gold Glider and other machine-used nickel-titanium access files have appeared, which not only simplify the operation, but also can better maintain the original anatomical shape of root canal, better formability, smaller root canal deviation and no root canal deviation compared with hand-used K file. It is easy to form steps.
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 楼主| 发表于 2018-9-16 19:32:58 | 显示全部楼层
3.3 standard operation

Before using nickel-titanium devices, clinicians should attend high-quality professional training, be familiar with the performance and usage of related nickel-titanium devices, and conduct extensive training in vitro. Before root canal preparation with nickel-titanium instruments, stainless steel instruments should be used to clear the root canal by hand to ensure smooth and reproducible root canal path. The process of probing and dredging root canals with small-sized instruments is also the process of further evaluating and judging the anatomical morphology of root canals. In order to keep the NiTi instruments in and out of the root canal under rotation, a gentle guide technique should be used to make them passively enter the root canal. During operation, a stable hard tissue fulcrum should be used to move up and down smoothly and short distances along the original direction of the root canal. Do not apply excessive pressure cutting to the apex, do not stay too long in the same position, each instrument in each tube continuous working time should not exceed 5 seconds. In case of resistance, we should return in time and release stress to prevent the instrument from breaking.

During the preparatory process, the root canal system should be soaked in flushing fluid, and a large amount of replacement irrigation should be carried out, and the EDTA gel should be used to assist the lubrication of the root canal. Fatigue accumulation will occur during the repeated use of nickel-titanium instruments. When the equipment is accumulated to a certain extent, the mechanical separation will occur, and the surface before the separation will not be obviously deformed, so it is difficult to predict. Therefore, it is necessary to strictly control the number of times of use. If conditions permit, the equipment will be carefully examined under the microscope before and after use, and if wear is found to be deficient. Loss, slight deformation and thread release should be discarded in time. In addition, the damage of nickel-titanium files during the preparation of moderately and severely curved root canals will be greatly increased. Therefore, it is necessary not only to record the times of using nickel-titanium files, but also to record the difficulty of the previous cases. At present, the number of safe use is uncertain, which is related to the type of nickel-titanium instruments, root canal anatomical morphology and the operator's experience. It is generally recommended that 3 to 5 molars be prepared and then discarded, but it should be used once for the preparation of severe curved or complex root canals.

The author considers that the alloy composition of Ni-Ti instruments should also be considered as one of the factors for the number of times of use. For traditional austenitic alloys, the number of times of use should be less than that of new Ni-Ti alloys, such as M phase, R phase heat treatment, CM phase, Gold alloy, Blue alloy and so on. In addition, the single file system is not recommended to increase the number of times of use because of the greater stress on the equipment during the preparation process.
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