Comparative Study on the Effectiveness of Two Nickel-titanium Instruments in Removing Gutta-percha

Ye LU, Xing HUANG, Ting WANG, Hongwei LIU, Jing ZHAO, Jing ZHU , Xin LI, Shulan CHEN*

1. School of Stomatology, Qingdao University, Qingdao 266003, China; 2. Department of Stomatology, Qingdao Municipal Hospital, Qingdao 266011, China; 3. Medical College, Qingdao University, Qingdao 266003, China

Abstract [Objectives] This study is to investigate the difference in the comprehensive effects of the two types of machine-used nickel-titanium (ni-ti) instruments in removing root canal filling materials to provide guidance for clinical applications. [Methods] We selected 60 single-tube premolars and randomly divided them into a P group (ProTaper ni-ti file group) and a K group (K3 ni-ti file group). The root canal was prepared according to the uniform standard crown cutting and manual k-file stepwise regression method. The root canal was fully filled and the X-ray film was taken. X-rays were taken again after re-treatment. The removal rate of the root filler material was obtained using Auto CAD 2000 image analysis software. [Results] We found no statistical difference in the clearance and total time of the two nickel-titanium devices in removing the gum filling material. Both the ProTaper ni-ti file and the K3 ni-ti file were able to remove most of the root canal filling during root canal retreatment. [Conclusions] There was no significant difference between the two devices in clinical applications.

Key words Root canal retreatment, Ni-ti instrument, Residual filling material

1 Introduction

At present, root canal retreatment is a relatively conservative and effective treatment after failure of root canal treatment. With the continuous development of modern root canal treatment equipment, the success rate of root canal retreatment has been continuously improved. ProTaper ni-ti file and K3 ni-ti file are the commonly used nickel-titanium instruments used in root canal retreatment. Root canal therapy is an effective method for the treatment of pulp diseases and periapical diseases. However, root canal treatment failure often occurs in clinical practice. The success rate of root canal treatment given by the American association of root canal physicians is only 70%-95%[1]. Root canal retreatment can prevent further destruction of periapical tissues at the affected tooth and prevent aggravate clinical symptoms, ensure the affected tooth have a long-term repair effect and a good clinical prognosis[2]. The biological principles of root canal retreatment and root canal treatment are the same, by thoroughly removing the microorganisms in the root canal system and then closely filling the root canal system to achieve the effect of treating pulp and periapical diseases.

At present, the commonly used ni-ti file for the machine has ProTaper ni-ti file, K3, Mtwo, TF,etc., and each has its own characteristics. Compared with other machine ni-chin system, K3 machine ni-chin file has stronger anti-folding ability, and the design of the file tip is more safe than others, which is helpful to avoid side piercing. K3 is in radial contact with dentin in the root canal, and the forward cutting angle is conducive to cutting when rotating, the cutting edge is continuously changing and grooved, which is conducive to the removal of debris and the reduction of debris and infectious substances out of the apical hole[1]. ProTaper device is an commonly used in clinical ni-ti machine file currently[2], compared with other machine ni-ti device: its cross section is convex triangle, and the changes in the taper design is conducive to improve the cutting force, which improves the preparatory efficiency; in root canal preparation, the anatomical morphology of the root canal does not change much, and it has advantages in the preparation of curved root canal[3]. ProTaper re-treatment file is a special file for root canal re-treatment designed and developed by Densply company. It belongs to ProTaper upgrade product, including 3 re-treatment files. The unique design for root canal re-treatment of ProTaper re-treatment file is conducive to cleaning of root filling more thoroughly and shortening the re-treatment time.

Kustarcietal.[4]compared the amount of debris in the apical orifice of K3 and ProTaper used for retreatment, the results showed that the amount of debris in the ProTaper group was higher than that in the K3 group. Lopesetal.[5]found that ProTaper retreatment device had a slightly lower safety factor than other retherapeutic devices by observing the surface and spiral axis of the broken device during root canal retreatment. Chandrasekaretal.[6]used spiral CT to compare the volume of residual root filling after root canal retreatment by ProTaper re-treated files, RaCe, K3 and H files respectively. The results showed that the H file group had more residual root filling than the other groups. Saadetal.[7]applied K3 and ProTaper instruments for root canal retreatment of anterior teeth and premolars, the results showed that the operation time was less than H file by hand. Buenoetal.[8]compared the clearance rate of residual filling material in the root canal by k-file and h-file by hand respectively. The experimental results show that stainless steel hand instruments are superior to K3 in removing root canal fillings. The above studies indicate that K3 and ProTaper nickel-titanium file each has its own merits in the removal of root filling and root canal shaping, but the application effect of K3 and ProTaper in root canal retreatment is lack of further comprehensive research.

This clinical study selected ProTaper ni-ti file and K3 ni-ti file for cleaning and preparation of the root canal retreatment. By comparing the removal of the operation time of the root canal filling material, the damage of the instrument and the removal rate of the root canal filling between the ProTaper ni-ti file and K3 ni-ti file to explore the use of two kinds of ni-ti files in root canal retreatment, to help enrich and improve clinical theory, so as to better guide clinical work.

2 Statistical analysis

3 Results

Both ProTaper Universal and K3 used in this study left some filling materials inside the root canals. The buccolingual directions root canal filling material removal rate in P group and K group was (90.8±2.8)% and (90.2±4.4)%. The mesiodistal directions root canal filling material removal rate in P group and K group was (91.5±3.8)% and (91.2±2.6)%. The residual rate of total time in P group and K group was (308.1±15.6) and (316.4±22.7) s. There was no statistical differences between two nickel-titanium instruments regarding the removal of gutta-percha filling material and total time (P>0.05). P group of 1 D3, 1 F2 slight deformation, K group of 10.06 taper instruments slightly deformed. There was no case of instrument separation. There was no step, root canal blockage and root canal perforation in the root canal retreatment.

4 Discussions

Root canal therapy is an effective method to treat pulp diseases and periapical diseases in clinical work, but it often fails due to various reasons[9]. Root canal retreatment is a conservative method to solve failure cases of root canal treatment before root tip surgery and tooth extraction, and the cure rate is usually around 90%[10]. Root canal retreatment usually more difficult than root canal treatment. The most commonly used hand instruments for root canal retreatment in clinical include H file and K file, but the flexibility is relatively poor[11]. Hand instruments are prone to instrument dissociation. The machine instruments has stronger memory and flexibility than the hand instruments, and has strong ability to resist torsional deformation. The ni-ti file tip has no cutting action, ensuring that it is prepared with the original bending of the root canal, and the original shape of the root canal is kept to the utmost extent, at the same time the occurrence of the apical offset is reduced.

In addition, root canal morphology is also an important factor affecting the effect of root canal retreatment of filler removal. Compared with the circular root canal, the contents in the oval root canal are more difficult to remove. Hulsmannetal.[12]proposed that the root tip preparation diameter is larger than the initial preparation diameter during root canal retreatment, which can significantly reduce the amount of residual filling materials. Therefore, in this study, the root tip was expanded and prepared to #40. Dissolver is a common auxiliary method for removing the fillers, but some scholars believe that the dissolving and softening of the tooth gel will adhere to the root canal wall to form a stain layer, which increases the difficulty of cleaning[13], and the commonly used dissolver such as chloroform has potential Carcinogenic risk[14], so no dissolver were used in each group in this experiment.

So far, a large number of studies at home and abroad have shown that no root canal treatment device has been found to completely and effectively remove the filling material in the root canal[6, 15-16]. Currently, the most commonly used mechanical ni-ti instruments for root canal retreatment are ProTaper ni-ti file and K3 ni-ti file. According to related research, the two nickel-titanium instruments can not completely remove the root canal filling in the root canal retreatment, and the efficiency is higher than that of the manual instruments, but there is no comparative study on the effect of retreatment of the two instruments. In this study, the effects of the two ni-ti instruments on root canal retreatment were evaluated by comparing the removal rate of root filling, operation time and the damage of the instruments.

Through this study, there was no statistically significant difference in the clearance rate of root canal filling materials after root canal retreatment with two kinds of mechanical ni-ti instruments. This result is consistent with the results of other studies. In the experiment, in order to prevent the experimental results from being affected by the difference in the amount of apex preparation in the study, group P combines 3 ProTaper retreatment instruments with ProTaper modification file F2, the K group is prepared to the root tip with a 0.04 taper and then to the root tip with a 0.06 taper. This method ensures that the root tip diameter of each group is 0.25 mm, and the taper diameter of the root canal is not much different after preparation. Chandrasekaretal.[7]compared the volume of the remaining root canal filling material after retreatment of ProTaper with K3 for root canal retreatment by spiral CT scan. The results showed that the ProTaper retreatment group had fewer residual roots than the K3 group. However, the difference was not statistically significant and was the same as the results of our study.

In other studies, there are several methods for evaluating the residual amount of root filler after retreatment of root canal preparation. These methods include longitudinal tooth splitting,invitrotooth transparency, Auto CAD 2000 image analysis software analysis, and micro-focus computed tomography (Micro-CT). Every method has its own merits. The advantage of the longitudinal tooth splitting method[17-18]is that the operation is simple and easy, but the force of the tooth splitting is not easy to grasp, the sample is easily damaged, and some root canal filling materials may be lost, which may have an impact on the research results. And it is only by visual observation and score, which is easy to produce bias. The microfocus computed tomography (MSCT) method is more intuitive and accurate, and can be accurately evaluated without damaging the teeth. It is an ideal method for evaluating the root canal clearance rate, but the operation is complicated and expensive. Nielsenetal.[19]applied Micro-CT scanning root canal treatment, root canal volume and surface morphology before and after root canal retreatment, can accurately display the internal and external anatomy of the teeth and without damaging the tooth tissue.

Da Rosa[14]used Micro-CT scanning technology to evaluate the effect of ProTaper retreatment file for root canal retreatment. The results showed that ProTaper retreatment file could not completely remove the root canal filling material. The specific operation of the transparent method of the vitro tooth transparent method is to perform decalcification, dehydration and transparency operation on the vitro tooth after the root canal retreatment, Therefore, when the isolated tooth is observed with a stereo microscope, the image of the residual root filling in the root canal can be observed. Then, the vitro teeth were photographed, and the root filling image data was analyzed by the corresponding software, and then the residual rate of the root canal filling material was calculated by the ratio of the root filling to the root canal area. The Auto CAD 2000 image analysis software method evaluates the residual rate of the root filling by the information provided by the X-ray film. This method avoids the problem that the tooth mediastinum will lose some of the residual gelatin and root paste. In this experiment, in order to avoid the incompleteness of the two-dimensional image information, the X-ray film was taken from the buccal tongue and the nearly far direction, and the image software was analyzed to ensure the accurate result. This evaluation method is also simpler than the operation of the vitro tooth transparency method, but the result is not as accurate as the microfocus computed tomography, moreover, it is impossible to distinguish whether the remaining root filling in the root canal is filled with dental gum or paste during the evaluation. In order to avoid the judgment errors caused by the doctor’s subjective impression, in this study, we selected a doctor who did not understand the content and significance of the experiment to statistically evaluate the experiment to ensure a more comprehensive, objective and accurate comparison of the use of ProTaper ni-ti file and K3 in root canal retreatment.

There is a slight difference between the two instruments in terms of damage to the instruments. The evaluation of the injury condition of the experimental device was carried out by a doctor who did not understand the significance of the experiment, thus preventing the doctor from subjectively understanding the influence on the experimental results. After using five root canals, each nickel-titanium instrument was placed under a microscope that could be magnified 10 times to observe the deformation and separation of the instrument, and whether root canal blockage, steps, and perforations were formed during the retreatment. If the device is found to be deformed or broken during the treatment, it should replace the device immediately. Both nickel-titanium instruments are progressive multi-taper design, which causes the device to break, so both devices have suffered damage in this experiment. The root canal treatment device works at the tip of the stress concentration zone, and the cross-sectional area is inversely proportional to the instrument tip stress, so the small instrument is more prone to instrument deformation and separation at the same speed and torque. Yaredetal.[20]have shown that the tip area of root canal therapy instrument is a stress concentration area during its operation, and the cross-sectional area is inversely proportional to the stress at the tip of the instrument, so smaller instruments are more likely to deform and separate under the same speed and torque.

Instrumental separation is a very common complication in root canal retreatment[21]. Canetal.[22]found that root canal retreatment devices are generally more likely to be damaged after three uses. Now with the application of reciprocating motion nickel-titanium file, it has been found that the probability of device separation of reciprocating motion nickel-titanium file is smaller than that of continuous rotation system nickel-titanium file[23]. This suggests that we should reasonably choose the machine nickel-titanium instrument in clinical operation and use it according to the corresponding requirements to reduce the separation of the instrument.

5 Conclusions

ProTaper ni-ti file and K3 ni-ti file can remove most root canal fillings in retreatment of root canal preparation. There is no statistical difference between the two instruments. Both nickel-titanium instruments can shorten the retreatment operation time, and the operation time of the two instruments is not statistically different. When using the machine nickel-titanium instrument, it is necessary to pay attention to the number of uses to prevent the breakage of the instrument.