Effect of ultrasonic root-end cavity preparation on dentinal microcrack formation: A micro-computed tomography study.

BACKGROUND: Root-end preparation during surgical endodontics may produce dentinal microcrack formation and/or propagation, leading to vertical root fractures. AIM: This study aimed to evaluate and compare the microcrack formation and propagation during ultrasonic root-end cavity preparation, with diamond- and zirconium nitride-coated tips using micro-computed tomography (CT).
METHODOLOGY: Twenty-eight extracted, mature, free-of-defect, mandibular premolars were decoronated and micro-CT scanned for the presence of preexisting microcracks. Samples were prepared till F4, were obturated, and were re-scanned to determine an increase in the number of microcracks. After 3 mm of root-end resection, the samples were randomly divided for ultrasonic root-end preparation into Group 1 (n = 12): preparation was done with ProUltra Surg tip, Group 2 (n = 12): preparation was done with KiS tip, and Group 3 (n = 4): no preparation was done. The samples were rescanned, and data collected were analyzed using ANOVA and Student's t-test. The level of significance was set at 5%.
RESULTS: Preexisting microcracks were observed in few samples. The mean percentage number increase of microcracks after root-end preparation in ProUltra Surg group was 1.25 and in KiS group, it was 0.5. The difference in mean percentage increase for the number of microcracks after root-end cavity preparation using ProUltra Surg tip and KiS tip was 0.75. However, this difference was not statistically significant (P = 0.34).
CONCLUSION: Both diamond- and zirconium nitride-coated ultrasonic tips resulted in more number of dentinal crack formation and propagation when preexisting cracks were present. Copyright:

INTRODUCTION

Nonsurgical retreatment of root canal-failed cases might not always be possible because of natural, restorative, or iatrogenic obstructions.[1] Surgical retreatment includes surgical debridement of pathological periradicular tissue, root-end resection, root-end cavity preparation, and filling to seal the root canal system effectively.

Conventionally, the root-end cavity was prepared with burs using a low-speed handpiece, but it carried some disadvantages such as preparation not being parallel to the canal, difficult access, and risk of perforation. The development of ultrasonic root-end preparation allowed deeper cavities that follow the original path of the root canal more closely and enhanced access. The more centered root-end preparation also lessens the risk of lateral perforation. To overcome the lesser cutting efficiency and longer preparation time of stainless steel root-end preparation tips, diamond- and zirconium nitride surface-coated tips were introduced.

The contact between the instrument and canal walls during preparation creates stress concentration in dentin and microcrack formation.[2] From a clinical standpoint, these microcracks are important because they may further develop into vertical root fractures. A recent study has shown that root fracture is not an instant event but rather a gradual propagation of tiny, less pronounced craze lines in the tooth structure.[3]

Diagnosing and locating crack lines are difficult in clinical trials even though cone-beam CT has improved the diagnosis.[45] In the last years, micro-computed tomography (CT) has opened up new possibilities for endodontic research by allowing nondestructive, volumetric, quantitative, and qualitative assessments before and after endodontic procedures.

Thus, the present study evaluated and compared the crack formation during root-end cavity preparation with diamond- and zirconium nitride-coated root-end preparation tips using micro-CT.

METHODOLOGY

Twenty-eight atraumatically extracted, intact, mature, nonrestored, mandibular premolars, with single-root and single-straight canal (curvature < 20°), free of external cracks and resorption, were selected and decoronated to a standardized root length of 17 mm. The roots were coated with a thin layer of polyethylene impression material and placed in acrylic resin as was described by Liu et al.[6] The specimens were sent for micro-CT imaging (Phoenix v | tome | x m, Germany) to determine preexisting dentinal cracks. Micro-CT scans were captured at 75 kV and 400 μA. The long axis of the roots was adjusted to be perpendicular to the beam to provide scans in the same sagittal positions. Each root was rotated 360°, with a total scan time of approximately 2000 s. The distance from the X-ray source to the sample was 124 mm and to the detector was 812 mm. Typically, 1000 CT slices (pixel pitch 200 μ and voxel resolution 30.5 μ) were obtained per root.

A #10 K file (M-Access Dentsply Maillefer, Ballaigues, Switzerland) was inserted into the root canal until the tip of the file was just visible at the apical foramen, and the working length was established by deducting 0.5 mm from this length. The samples were prepared with ProTaper Universal (PTU) rotary files (Dentsply, Maillefer Ballaigues, Switzerland) up to F4 following the manufacturer's instructions. Between the use of each file, the canals were irrigated with 5 ml of 3% NaOCl (Amble Healthcare Private Limited, Delhi, India) and at last 3 ml of 17% ethylenediaminetetraacetic acid (Prevest Dentpro Limited, Jammu, Jammu and Kashmir, India) for 1 min followed by 5-ml distilled water. Thereafter, the samples were obturated with Gutta Percha and AH Plus® sealer using single-cone technique. The roots were stored for 1 week at 37°C and 100% humidity to allow the sealer to set.

Root-end resection and root-end preparation

The apical 3 mm of each root was resected perpendicular to the long axis of the root using a straight flat bur (SF-12, Mani, Inc, Utsunomiya, Tochigi, Japan) in a high-speed handpiece with continuous water spray. All the samples were resent for micro-CT imaging for the determination of microcracks after root canal preparation and obturation.

Thereafter, the samples were randomly divided into:

Group 1 (n = 12): Class-1 root-end cavities were prepared to the depth of 3 mm with ProUltra SURG tip #1 (Dentsply, Maillefer Ballaigues, Switzerland) at medium power setting (6) of ultrasonic device (Satelec P5 Newtron Acteon, France) as recommended by the manufacturer

Group 2 (n = 12): Class-1 root-end cavities were prepared to the depth of 3 mm with a diamond-coated KIS tip #1 (Kavo Kerr Dental, France) at medium power setting (6) of ultrasonic device as recommended by the manufacturer

Group 3 (n = 4): Control group – No preparation was done.

The root-end preparation of each group was performed under internal-water cooling, and the samples were sent for third micro-CT imaging to determine dentinal microcracks after root-end preparation.

Each sample was observed at 0.25-mm interval for apical 5 mm that created 20 slices per sample. Only apical 5 mm was evaluated because root-end preparation was done only in apical 3 mm, and an additional 2 mm was evaluated to observe if at all any effect was caused coronal to the prepared area.

If there were microcracks on the external surface of the root and/or on the internal root canal wall, the slice was accepted as having “crack,” and the number was noted. As the number of microcracks observed preoperatively was different in each sample, the percentage of microcracks was calculated to standardize the procedure, and the increase in percentage number of microcracks was evaluated. ANOVA and Student's t-test analysis were performed using Statistical Package for the Social Sciences 13.0 software (SPSS Inc., Chicago, IL, USA). The level of statistical significance was set at 5%.

RESULTS

There were preexisting dentinal microcracks in few of the samples, and an increase in the number of dentinal microcracks was observed after root canal preparation and obturation and after root-end cavity preparation. However, this increase was statistically insignificant (P > 0.05) for both of the experimental groups [Table 1 and Figure 1]. The control group did not show any increase in the number or propagation of microcracks. Samples with preexisting microcracks showed more number of microcrack formation and/or propagation after obturation and after root-end cavity preparation as compared to those which were free of microcracks. Both ProUltra SURG (1.25 ± 2.26) and KiS (0.5 ± 1.446) root-end preparation tips caused generation and/or propagation of microcracks; however, the difference was statistically insignificant (P = 0.34) [Table 2].

Table 1

Comparison of mean of the percentage number of dentinal microcracks with confidence interval and P value observed at different stages with ProUltra Surg tip and KiS tip under micro-computed tomography imaging

Procedural stages	ProUltra Surg				KiS tip
	Mean±SD	CI		P	Mean±SD	CI		P
		Lower	Upper			Lower	Upper
After obturation - preoperative cracks	2.5±5.838	−1.2097	6.2097	0.166	0.25±0.866	−0.3002	0.8002	0.169
After retrocavity preparation - after obturation	1.25±2.26	−0.1867	2.6867	0.08	0.5±1.446	−0.4187	1.4187	0.25

CI: Confidence interval, SD: Standard deviation

Figure 1

Micro-computed tomography images (a) no dentinal microcrack, (b) preexisting dentinal microcrack, (c) microcracks after root canal obturation, (d) microcracks after root-end cavity preparation

Table 2

Mean percentage number of dentinal microcracks increased and standard deviation after the use of different tips for retrocavity preparation on observation under micro-computed tomography imaging

Groups (ProUltra Surg vs. KiS)	Mean percentage increase±SD	P	CI
			Lower	Upper
After obturation - preoperative	2.25±17.42	0.20	−1.28	5.78
After retrocavity preparation - after obturation	0.75±3.60	0.34	−2.3569	0.8569

CI: Confidence interval, SD: Standard deviation

DISCUSSION

The purpose of root-end cavity preparation is to remove irritants from the root canal system inaccessible to the operator via a coronal entry.

A great concern to the clinician is the formation of microcracks or microfractures following root-end resection or root-end preparation. The root-end microcracks increase susceptibility to root fracture, the inability to seal the root-end preparation properly, and the possibility of the additional site of bacterial contamination.[7] The development of microcracks and microfractures following sonic or ultrasonic root-end instrumentation is still controversial. Therefore, this study was conducted to evaluate the effect of root-end cavity preparation on the root dentin walls in terms of microcrack formation and propagation.

Standardization of the procedure was done to minimize the errors. Only freshly extracted, intact, noncarious, single-canal, same-length human mandibular premolars with single apical foramen were selected. Teeth with < 20° curvatures were included as an increase in the degree of canal curvature may result in the generation of more dentinal cracks which may lead to a strong generation of stress at the point of maximum canal curvature. In a study by Çapar et al.,[8] it was observed that single-cone technique applied minimal pressure compared to the other filling techniques that create compaction forces on the root canal walls, leading to dentinal microcracks. Therefore, this technique was chosen as an obturating technique.

Dentinal microcracks can be diagnosed in vitro utilizing microscopes, dyes, and/or transillumination. However, all these methods result in the destruction of samples. In the present study, samples were micro-CT scanned as this is a nondestructive, three-dimensional technique that provides quantitative as well as qualitative assessments.

Instead of the number of microcracks, the percentage number of microcracks was calculated to standardize the procedure. The micro-CT imaging revealed preexisting microcracks in some of the samples. This is in accordance with the studies of Barreto et al.,[9] Singh et al.,[10] and Arias et al.[11] These preexisting dentinal microcracks may be attributed to noniatrogenic factors such as age changes, the effect of functional stresses such as occlusal interference or parafunctional stresses, any excessive force on teeth during extraction, or environmental changes on dentin such as dehydration.[9]

It has been reported that instrumentation of root canals alone weakens the roots.[12] In the present study, there was a mean percentage increase in the number of dentinal cracks in the experimental group from preoperative (21.29 ± 49.41) to after root canal preparation and obturation (22.66 ± 53.55). It is in accordance with the studies by Jain et al.,[13] Karataş et al.,[14] and Harandi et al.[15] who concluded that there is an increase in the initiation of dentinal cracks after cleaning and shaping. Shemesh et al.[16] attributed it to nickel-titanium (NiTi) rotary kinematics. However, in the present study, this increase was found to be statistically insignificant (P > 0.05), which is in accordance with the study done by Beling et al.[7] On the other hand, Onnink et al.[17] found a statistically significant difference between prepared and unprepared teeth in terms of crack formation, which may be because the authors used lateral compaction technique for obturation, whereas in the present study, the single-cone obturation technique was used which results in less stress on the root canal wall, thereby resulting in lesser crack formation. Jamleh et al.[18] concluded that the incidence of microcracks after PTU instrumentation can be attributed to many factors. First, the continuous motion of the PTU system will have a greater threading effect. The use of a PTU instrument with taper up to 9% may also have contributed to the higher incidence of microcracks observed. Bier et al.[19] found that when NiTi rotary instruments with a taper of 0.06 or more are used, dentinal defects result.

In the present study, root-end cavity preparation was done using two different ultrasonic tips: ProUltra SURG and KiS tip. The micro-CT imaging revealed that both the groups showed increase in the percentage of cracks when compared to the preoperative and the postobturation groups, but this increase was statistically insignificant (P > 0.05). In addition, samples with preexisting cracks showed more development of microcracks after root-end cavity preparation than those which do not have.

Recent studies by Saunders et al.[20] and Layton et al.[21] demonstrated the presence of root-end cracks in extracted teeth after ultrasonic preparation. There were significantly more root ends with cracks after ultrasonic root-end preparation than after root-end resection only. In root ends that had any cracks, significantly more canal cracks per root occurred when the ultrasonic tip was used on the high-frequency setting for root-end preparation than when the ultrasonic tip was used on the low-power setting. In the present study, medium-frequency power setting was used for root-end preparation as per manufacturer's guidelines on the freshly extracted teeth, which could be the reason for significantly less number of microcracks.

On comparing both the groups, there was more increase in the mean percentage of microcrack formation in ProUltra SURG group than KiS group, but the difference among both the groups was statistically insignificant (P > 0.05). ProUltra SURG tip is a zirconium nitride-coated tip, whereas KiS is a diamond-coated tip. Diamond-coated tips have better cutting efficiency than zirconium nitride-coated tips, which may be the reason for lesser number of crack formation or propagation in KiS tips. Gunes and Aydinbelge also concluded in their study that diamond-coated root-end tips are more efficient in removing dentin and thus minimizing dentinal fractures.[22] Ishikawa et al.[23] and Taschieri et al.[24] reported no insignificant difference among zirconium nitride-coated and diamond-coated ultrasonic root-end tip groups in the number of microcracks in their in vitro studies. Along the similar lines, Gondim et al.[25] reported that no significant differences in chipping and cracking area were detected in treatments with diamond-coated, stainless steel, and zirconium nitride-coated tips, but stainless steel root-end tips showed a greater number of teeth with cracking and a larger chipping area, probably due to the longer preparation time needed. Both the KiS and ProUltra SURG tips are very efficient and provide good vision and have excellent cutting efficiency.

CONCLUSION

Preexisting dentinal microcracks result in more number of microcrack formation/propagation after root-end cavity preparation. Root-end cavity preparation with both diamond-coated and zirconium nitride-coated ultrasonic tips caused insignificant increase in dentinal microcracks. However, further in vivo and in vitro studies with more number of samples and in curved roots should be carried out to extrapolate the results clinically.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.